ASTRONOMY IN THE SEVENTEENTH CENTURY
The seventeenth century embraces the most remarkable epoch in the whole
history of astronomy. It was during this period that those wonderful
discoveries were made which have been the means of raising astronomy to
the lofty position which it now occupies among the sciences. The
unrivalled genius and patient labours of the illustrious men whose names
stand out in such prominence on the written pages of the history of this
era have rendered it one of the most interesting and elevating of
studies. Though Copernicus lived in the preceding century, yet the names
of Tycho Brahe, Kepler, Galileo, and Newton, testify to the greatness of
the discoveries that were made during this period, which have surrounded
the memories of those men with a lustre of undying fame.
Foremost among astronomers of less conspicuous eminence who made
important discoveries in this century we find the name of Huygens.
CHRISTIAN HUYGENS was born at The Hague in 1629. He was the second son
of Constantine Huygens, an eminent diplomatist, and secretary to the
Prince of Orange. Huygens studied at Leyden and Breda, and became
highly distinguished as a geometrician and scientist. He made important
investigations relative to the figure of the Earth, and wrote a learned
treatise on the cause of gravity; he also determined with greater
accuracy investigations made by Galileo regarding the accelerated motion
of bodies when subjected to the influence of that force.
Huygens admitted that the planets and their satellites attracted each
other with a force varying according to the inverse ratio of the squares
of their distances, but rejected the mutual attraction of the molecules
of matter, believing that they possessed gravity towards a central point
only, to which they were attracted. This supposition was at variance
with the Newtonian theory, which, however, was universally regarded as
the correct one.
Huygens originated the theory by which it is believed that light is
produced by the undulatory vibration of the ether; he also discovered
polarization.
Up to this time the method adopted in the construction of clocks was not
capable of producing a mechanism which measured time with sufficient
accuracy to satisfy the requirements of astronomers. Huygens endeavoured
to supply this want, and applied his mechanical ingenuity in
constructing a clock that could be relied upon to keep accurate time.
Though the pendulum motion was first adopted by Galileo, he was unable
to arrange its mechanism so that it should keep up a continuous
movement. The oscillation of the pendulum ceased after a time, and a
fresh impulse had to be applied to set it in motion. Consequently,
Galileo's clock was of no service as a timekeeper.
Huygens overcame this difficulty by so arranging the mechanism of his
clock that the balance, instead of being horizontal, was directed
perpendicularly, and prolonged downwards to form a pendulum, the
oscillations of which regulated the downward motion of the weight. This
invention, which was highly applauded, proved to be of great service
everywhere, and was especially valuable for astronomical purposes.
Huygens next directed his attention to the construction of telescopes,
and displayed much skill in the grinding and polishing of lenses. He
made several instruments superior in power and accuracy to any that
existed previously, and with one of these made some remarkable
discoveries when observing the planet Saturn.
The telescopic appearance of Saturn is one of the most beautiful in the
heavens. The planet, surrounded by two brilliant rings, and accompanied
by eight attendant moons, surpasses all the other orbs of the firmament
as an object of interest and admiration. To the naked eye, Saturn is
visible as a star of the first magnitude, and was known to the ancients
as the most remote of the planets. Travelling in space at a distance of
nearly one thousand millions of miles from the Sun, the planet
accomplishes a revolution of its mighty orbit in twenty-nine and a half
years.
Galileo was the first astronomer who directed a telescope to Saturn. He
observed that the planet presented a triform appearance, and that on
each side of the central globe there were two objects, in close contact
with it, which caused it to assume an ovoid shape. After further
observation, Galileo perceived that the lateral bodies gradually
decreased in size, until they became invisible. At the expiration of a
certain period of time they reappeared, and were observed to go through
a certain cycle of changes. By the application of increased telescopic
power it was discovered that the appendages were not of a rounded form,
but appeared as two small crescents, having their concave surfaces
directed towards the planet and their extremities in contact with it,
resembling the manner in which the handles are attached to a cup.
These objects were observed to go through a series of periodic changes.
After having become invisible, they reappeared as two luminous straight
bands, projecting from each side of the planet; during the next seven or
eight years they gradually opened out, and assumed a crescentic form;
they afterwards began to contract, and on the expiration of a similar
period, during which time they gradually decreased in size, they again
became invisible. It was perceived that the appendages completed a cycle
of their changes in about fifteen years.
In 1656, Huygens, with a telescope constructed by himself, was enabled
to solve the enigma which for so many years baffled the efforts of the
ablest astronomers. He announced his discovery in the form of a Latin
cryptograph which, when deciphered, read as follows:--
'Annulo cingitur, tenui plano, nusquam cohaerente, ad eclipticam
inclinatio.'
'The planet is surrounded by a slender flat ring everywhere distinct
from its surface, and inclined to the ecliptic.'
Huygens perceived the shadow of the ring thrown on the planet, and was
able to account in a satisfactory manner for all the phenomena observed
in connection with its variable appearance.
The true form of the ring is circular, but by us it is seen
foreshortened; consequently, when the Earth is above or below its plane,
it appears of an elliptical shape. When the position of the planet is
such that the plane of the ring passes through the Sun, the edge of the
ring only is illumined, and then it becomes invisible for a short
period. In the same manner, when the plane of the ring passes through
the Earth, the illumined edge of the ring is not of sufficient magnitude
to appear visible, but as the enlightened side of the plane becomes more
inclined towards the Earth, the ring comes again into view. When the
plane of the ring passes between the Earth and the Sun, the unillumined
side of the ring is turned towards the Earth, and during the time it
remains in this position it is invisible.
Huygens discovered the sixth satellite of Saturn (Titan), and also the
Great Nebula in Orion.
JOHANN HEVELIUS, a celebrated Prussian astronomer, was born at Dantzig
in 1611, and died in that city in 1687. He was a man of wealth, and
erected an observatory at his residence, where, for a period of forty
years, he carried out a series of astronomical observations.
He constructed a chart of the stars, and in order to complete his work,
formed nine new constellations in those spaces in the celestial vault
which were previously un-named. They are known by the names
Camelopardus, Canes Venatici, Coma Bernices, Lacerta, Leo Minor, Lynx,
Monoceros, Sextans, and Vulpecula. He also executed a chart of the
Moon's surface, wrote a description of the lunar spots, and discovered
the Libration of the Moon in Longitude.
On May 30, 1661, Hevelius observed a transit of Mercury, a description
of which he published, and included with it Horrox's treatise on the
first-recorded transit of Venus. This work, after having passed through
several hands, became the property of Hevelius, who was capable of
appreciating its merits. The manuscript was sent to him by Huygens, and
in acknowledging it he writes: 'How greatly does my Mercury exult in the
joyous prospect that he may shortly fold within his arms Horrox's long
looked-for and beloved Venus! He renders you unfeigned thanks that by
your permission this much-desired union is about to be celebrated, and
that the writer is able, with your concurrence, to introduce them both
together to the public.'
Hevelius made numerous researches on comets, and suggested that the
form of their paths might be a parabola.
GIOVANNI DOMENICO CASSINI was born at Perinaldo, near Nice, in 1625. He
studied at Genoa and Bologna, and was afterwards appointed to the Chair
of Astronomy at the latter University. He was a man of high scientific
attainments, and made many important astronomical discoveries.
In 1671 he became Director of the Royal Observatory at Paris, and
devoted a long life to trying and difficult observations, which in his
later years deprived him of his eyesight.
In 1644 Cassini proved beyond doubt that Jupiter rotated on his axis,
and also assigned his period of rotation with considerable accuracy. He
published tables of the planet's satellites, and determined their
motions from observations of their eclipses. He ascertained the periods
of rotation of Venus and Mars; executed a chart of the lunar surface,
and observed an occultation of Jupiter by the Moon.
Cassini discovered the dual nature of Saturn's ring, having perceived
that instead of one there are two concentric rings separated by a dark
space. He also discovered four of the planet's satellites--viz. Japetus,
Rhea, Dione, and Tethys. He made a near approximation to the solar
parallax by means of researches on the parallax of Mars, and
investigated some irregularities of the Moon's motion. Cassini
discovered the belts of Jupiter, and also the Zodiacal Light, and
established the coincidence of the nodes of the lunar equator and orbit.
JAQUES CASSINI, son of Giovanni, was born at Paris in 1677. He followed
in his father's footsteps, and wrote several treatises on astronomical
subjects. He investigated the period of the rotation of Venus on her
axis, and upheld the results arrived at by his father, which were
afterwards confirmed by observations made by Schroeter. Cassini made
some valuable researches with regard to the proper motion of the stars,
and demonstrated that their change of position on the celestial vault
was real, and not caused by a displacement of the ecliptic. He attempted
to ascertain the apparent diameter of Sirius, and made observations with
regard to the visibility of the stars. The Cassini family produced
several generations of eminent astronomers, whose discoveries and
investigations were of much value in advancing the science of astronomy.
OLAUS ROEMER, an eminent Danish astronomer, was born at Copenhagen
September 25, 1644. When Picard, a French astronomer, visited Denmark in
1671, for the purpose of ascertaining the exact position of
'Uranienburg,' the site of Tycho Brahe's observatory, he made the
acquaintance of Roemer, who was engaged in studying mathematics and
astronomy under Erasmus Bartolinus. Having perceived that the young man
was gifted with no ordinary degree of talent, he secured his services to
assist him in his observations, and, on the conclusion of his labours,
Picard was so much impressed with the ability displayed by Roemer, that
he invited him to accompany him to France. This invitation he accepted,
and took up his residence in the French capital, where he continued to
prosecute his astronomical studies.
In 1675 Roemer communicated to the Academy of Sciences a paper, in which
he announced his discovery of the progressive transmission of light. It
was believed that light travelled instantaneously, but Roemer was able
to demonstrate the inaccuracy of this conclusion, and determined that
light travels through space with a measurable velocity.
By diligently observing the eclipses of Jupiter's satellites, Roemer
perceived that sometimes they occurred before, and sometimes after their
predicted times. This irregularity, he discovered, depended upon the
position of the Earth with regard to Jupiter. When the Earth, in
traversing her orbit, moved round to the opposite side of the Sun,
thereby bringing Jupiter into conjunction, an eclipse occurred sixteen
minutes twenty-six seconds later than it did when Jupiter was in
opposition or nearest to the Earth. As there existed an impression that
light travelled instantaneously, it was believed that an eclipse
occurred at the moment it was perceived in the telescope. This, however,
was not so. Roemer, after a long series of observations, concluded that
the discrepancies were due to the fact that light travels with a
measurable velocity, and that it requires a greater length of time,
upwards of sixteen minutes, to traverse the additional distance--the
diameter of the Earth's orbit--which intervenes between the Earth and
Jupiter, when the planet is in conjunction, as compared with the
distance between the Earth and Jupiter, when the latter is in
opposition. This discovery of Roemer's was the means of enabling the
velocity of light to be ascertained, which, according to recent
calculations, is about 187,000 miles a second. As an acknowledgment of
the importance of his communication, Roemer was awarded a seat in the
Academy, and apartments were assigned to him at the Royal Observatory,
where he carried on his astronomical studies.
In 1681 Roemer returned to Denmark, and was appointed Professor of
Mathematics in the University of Copenhagen; he was also entrusted with
the care of the city observatory--a duty which his reputation as an
astronomer eminently qualified him to undertake. The transit
instrument--a mechanism of much importance to astronomers--was invented
by Roemer in 1690; it consists of a telescope fixed to a horizontal
axis, and adjusted so as to revolve in the plane of the meridian. It is
employed in observing the passage of the heavenly bodies across the
observer's meridian. To note accurately by means of the astronomical
clock the exact instant of time at which a celestial body crosses the
centre of the field of view is the essential part of a transit
observation. Small transit instruments are employed for taking the time
and for regulating the observatory clock, but large instruments are
used for delicate and exact observations of Right Ascensions and
Declinations of stars of different magnitudes. Meridian, and altitude
and azimuth circles, are important astronomical appliances, which owe
their existence to the inventive skill of this distinguished astronomer.
Roemer resided for many years at the observatory in the city of
Copenhagen, where he pursued his astronomical studies until the time of
his death, which occurred in 1710. He meritoriously attempted to
determine the parallax of the fixed stars; and it is said that the
astronomical calculations and observations which he left behind him were
so voluminous as to equal in number those made by Tycho Brahe, nearly
all of which perished in a great conflagration that destroyed the
observatory and a large portion of the city of Copenhagen in 1728.
Among other astronomers of this century whose names deserve recording
were Descartes and Gassendi, whose mathematical researches in their
application to astronomy were of much value; Fabricius, Torricelli, and
Maraldi, who by their observations and investigations added many facts
to the general knowledge of the science; and Bayer, to whom belongs the
distinction of having constructed the first star-atlas.
In our own country during this period astronomy was cultivated by a few
enthusiastic men, who devoted their time and talents to promoting the
advancement of the science. It, however, received no recognition as a
subject of study at any of the Universities, and no public observatory
existed in Great Britain.
Though it was not until towards the close of the century that the
attention of all Europe was directed to England in admiration of the
discoveries of the illustrious Newton, yet astronomy had its humble
votaries, and chief among those was a young clergyman of the name of
Horrox.
JEREMIAH HORROX was born at Toxteth, near Liverpool, in 1619--close on
three centuries ago. Little is known of his family. His parents have
been described as persons who occupied a humble position in life, but,
as they were able to give their son a classical education which fitted
him for one of the learned professions, it is probable they were not so
obscure as they have been represented to be.
Having received his early education at Toxteth, Horrox afterwards
proceeded to Cambridge, and was entered as a student at Emmanuel College
on May 18, 1632, when in his fourteenth year.
At the University he devoted himself to the study of classics,
especially Latin, which in those days was the language adopted by men of
learning, when engaged in writing works of a philosophical and
scientific character.
After having remained at Cambridge for three years, Horrox returned to
his native county, and was appointed curate of Hoole, a place about
eight miles distant from Preston. Hoole is described as a narrow
low-lying strip of land consisting largely of moss, and almost converted
into an island by the waters of Martin Mere on the south, and the Ribble
on the north; and, though doubtless an open and favourable situation for
astronomical observation, it could not have been attractive as a place
of residence. Yet it was here on November 24, 1639, that Horrox made his
famous observation of the first recorded transit of Venus, an occurrence
with which his name will be for ever associated.
It was while at Cambridge that Horrox first turned his attention to the
study of astronomy. His love of the sublime, and the captivating
influence exerted on his mind by the contemplation of the heavenly
bodies, induced him to adopt astronomy as a pursuit congenial to his
tastes, and capable of exercising his highest mental powers. Having this
object in view, he applied himself with much earnestness to the study of
mathematics; he had, however, to rely mainly upon his own exertions, for
at that time no branch of physical or mathematical science was taught at
Cambridge, and consequently he obtained no professional instruction.
It was so also with astronomy, which, as a science, was scarcely known
in this country; no regular record of astronomical observations was kept
by any individual observer, and no public observatory existed in England
or in France.
The disadvantages and obstacles which Horrox had to encounter may be
best described by quoting his own words. He writes: 'There were many
hindrances. The abstruse nature of the study, my inexperience and want
of means dispirited me. I was much pained not to have any one to whom I
could look for guidance, or indeed for the sympathy of companionship in
my endeavours, and I was assailed by the languor and weariness which are
inseparable from every great undertaking. What then was to be done? I
could not make the pursuit an easy one, much less increase my fortune,
and least of all imbue others with a love for astronomy; and yet to
complain of philosophy on account of its difficulties would be foolish
and unworthy. I determined, therefore, that the tediousness of study
should be overcome by industry; my poverty--failing a better method--by
patience; and that instead of a master I would use astronomical books.
Armed with these weapons I would contend successfully; and, having heard
of others acquiring knowledge without greater help, I would blush that
any one should be able to do more than I, always remembering that word
of Virgil's--
Totidem nobis animaeque manusque.
Having heard much praise bestowed upon the works of Lansberg, a Flemish
astronomer, Horrox thought it would be to his advantage to procure a
copy of his writings. This he succeeded in obtaining after some
difficulty, and devoted a considerable time to calculating Ephemerides,
based upon the Lansberg Tables, but after making a number of
computations he discovered that they were unreliable and inaccurate.
In the year 1636 Horrox made the acquaintance of William Crabtree, a
devoted astronomer, who lived at Broughton, a suburb of Manchester. A
close friendship soon existed between the two men, and they carried on
an active correspondence about matters relating to the science which
they both loved so well.
Crabtree, who was an unbeliever in Lansberg, urged Horrox to discard the
Flemish astronomer's works, and devote his talents to the study of Tycho
Brahe and Kepler. This advice led Horrox to make a more rigorous
examination of the Lansberg Tables, and after comparing them with the
observations made by Crabtree, which coincided with his own, he resolved
to renounce them. Acting on the advice of his friend, Horrox directed
his attention to the writings of Kepler. The youthful astronomer soon
realised their value, and was charmed with the accuracy of observation
and inductive reasoning displayed in the elucidation of those general
laws which constituted a new era in the history of astronomy.
The Rudolphine Tables, which were the astronomical calculations
commenced by Tycho Brahe, and completed by Kepler, were regarded by
Horrox as much superior to those of Lansberg; but it occurred to him
that they might be improved by changing some of the numbers, and yet
retaining the hypotheses. To this task he applied himself with much
earnestness and assiduity, and after close application and laborious
study he accomplished the arduous undertaking of bringing those tables
to a high state of perfection.
In his investigation of the Lunar theory, Horrox outstripped all his
predecessors, and Sir Isaac Newton distinctly affirms he was the first
to discover that the Moon's motion round the Earth is in the form of an
ellipse with the centre in the lower focus. Besides having made this
discovery, Horrox was able to explain the causes of the inequalities of
the Moon's motion, which render the exact computation of her elements so
difficult.
The Annual Equation, an irregularity discovered by Tycho Brahe, which is
produced by the increase and decrease of the Sun's disturbing force as
the Earth approaches or recedes from him in her orbit, had its value
first assigned by Horrox. This he calculated to be eleven minutes
sixteen seconds, which is within four seconds of what it has since been
proved to be by the most recent observations.
The Evection, an irregular motion of the Moon discovered by Ptolemy,
whereby her mean longitude is increased or diminished, was explained by
Horrox as depending upon the libratory motion of the apsides, and the
change which takes place in the eccentricity of the lunar orbit.
These discoveries were made by Horrox before he attained the age of
twenty years, and if his reputation had alone rested upon them his name
would have been honourably associated with those who have attained to
the highest eminence in astronomy.
Another achievement which adds lustre to Horrox's name consists in his
detection of the inequality in the mean motions of Jupiter and Saturn.
He also directed his attention to the study of cometary bodies, and
arrived at certain conclusions with regard to the nature of their
movements. At first, he believed like Kepler that comets were projected
in straight lines from the Sun; this supposition having been upheld on
account of the great elongation of their orbits. He next perceived that
their velocity increased as they approached the Sun, and decreased as
they receded from him. Afterwards he says, 'They move in an elliptic
figure or near it,' and finally he arrived at the conclusion that
'comets move in elliptical orbits, being carried round the Sun with a
velocity which is probably variable.' This theory has been verified by
numerous observations, and is now generally accepted by astronomers.
Horrox also made a series of observations on the tides. He notified the
extent of their rise and fall at different periods, and investigated
other phenomena associated with their ebb and flow. After having
continued his observations for some time, he wrote to his friend
Crabtree, and informed him that he had perceived many interesting
details which had not been previously described, and he hoped to be
able to arrive at some important conclusions with regard to their nature
and cause. Unfortunately, Horrox's writings on this subject, along with
many other important papers, have been lost or destroyed. We are
therefore ignorant of the result of his researches, which were the first
undertaken by any person for the purpose of scientific inquiry.
From his study of the Lansberg and Rudolphine Tables, Horrox arrived at
the conclusion that a transit of Venus would occur on November 24, 1639.
This transit was for some unaccountable reason overlooked by Kepler, who
predicted one in 1631, and the next not until 1761. The transit of 1631
was not visible in Europe.
We are indebted to Horrox for a description of the transit of 1639--the
first that was ever observed of which there is any record; and were it
not for the accuracy of his calculations, the occurrence of the
phenomenon would have been unperceived, and no history of the
conjunction would have been handed down to posterity. As soon as Horrox
had assured himself of the time when the transit would take place, he
wrote to Crabtree to inform him of the date, and asked him to make
observations with his telescope, and especially to examine the diameter
of the planet, which he thought had been over-estimated. He also
requested him to write to Dr. Foster of Cambridge, and inform him of the
expected event, as it was desirable that the transit should be observed
from several places in consequence of the possibility of failure, owing
to an overcast sky. His letter is dated October 26, 1639. He says: 'My
reason for now writing is to advise you of a remarkable conjunction of
the Sun and Venus on the 24th of November, when there will be a transit.
As such a thing has not happened for many years past, and will not occur
again in this century, I earnestly entreat you to watch attentively with
your telescope in order to observe it as well as you can.
'Notice particularly the diameter of Venus, which is stated by Kepler to
be seven minutes, and by Lansberg to be eleven, but which I believe to
be scarcely greater than one minute.'
In describing the method which he adopted for observing the transit,
Horrox writes as follows: 'Having attentively examined Venus with my
instrument, I described on a sheet of paper a circle, whose diameter was
nearly equal to six inches--the narrowness of the apartment not
permitting me conveniently to use a larger size. I divided the
circumference of this circle into 360 degrees in the usual manner, and
its diameter into thirty equal parts, which gives about as many minutes
as are equivalent to the Sun's apparent diameter. Each of these thirty
parts was again divided into four equal portions, making in all one
hundred and twenty; and these, if necessary, may be more minutely
subdivided. The rest I left to ocular computation, which, in such small
sections, is quite as certain as any mechanical division. Suppose,
then, each of these thirty parts to be divided into sixty seconds,
according to the practice of astronomers. When the time of the
observation approached, I retired to my apartment, and, having closed
the windows against the light, I directed my telescope--previously
adjusted to a focus--through the aperture towards the Sun, and received
his rays at right angles upon the paper already mentioned. The Sun's
image exactly filled the circle, and I watched carefully and unceasingly
for any dark body that might enter upon the disc of light.
'Although the corrected computation of Venus' motions which I had before
prepared, and on the accuracy of which I implicitly relied, forbade me
to expect anything before three o'clock in the afternoon of the 24th,
yet since, according to the calculations of most astronomers, the
conjunction should take place sooner--by some even on the 23rd--I was
unwilling to depend entirely on my own opinion, which was not
sufficiently confirmed, lest by too much self-confidence I might
endanger the observation. Anxiously intent, therefore, on the
undertaking through the greater part of the 23rd, and on the whole of
the 24th, I omitted no available opportunity of observing her ingress. I
watched carefully on the 24th from sunrise to nine o'clock, and from a
little before ten until noon, and at one in the afternoon, being called
away in the intervals by business of the highest importance, which for
these ornamental pursuits I could not with propriety neglect.[3] But
during all this time I saw nothing in the Sun except a small and common
spot, consisting as it were of three points at a distance from the
centre towards the left, which I noticed on the preceding and following
days. This evidently had nothing to do with Venus. About fifteen minutes
past three in the afternoon, when I was again at liberty to continue my
labours, the clouds, as if by divine interposition, were entirely
dispersed, and I was once more invited to the grateful task of repeating
my observations. I then beheld a most agreeable spectacle--the object of
my sanguine wishes; a spot of unusual magnitude and of a perfectly
circular shape, which had already fully entered upon the Sun's disc on
the left, so that the limbs of the Sun and Venus precisely coincided,
forming an angle of contact. Not doubting that this was really the
shadow of the planet, I immediately applied myself sedulously to observe
it.
'In the first place, with respect to the inclination, the line of the
diameter of the circle being perpendicular to the horizon, although its
plane was somewhat inclined on account of the Sun's altitude, I found
that the shadow of Venus at the aforesaid hour--namely, fifteen minutes
past three--had entered the Sun's disc about 62 deg. 30', certainly between
60 deg. and 65 deg., from the top towards the right. This was the appearance in
the dark apartment; therefore, out of doors, beneath the open sky,
according to the laws of optics, the contrary would be the case, and
Venus would be below the centre of the Sun, distant 62 deg. 30' from the
lower limbs or the nadir, as the Arabians term it. The inclination
remained to all appearances the same until sunset, when the observation
was concluded.
'In the second place, the distance between the centres of Venus and the
Sun I found by three observations to be as follows:--
The Hour. | Distance of the Centres.
|
At 3.15 by the clock | 14' 24''
" 3.35 " | 13' 30''
" 3.45 " | 13' 0''
" 3.50 the apparent sunset. |
The true setting being 3.45, and the apparent about 5 minutes later, the
difference being caused by refraction. The clock therefore was
sufficiently correct.
'In the third place I found after careful and repeated observation that
the diameter of Venus, as her shadow was depicted on the paper, was
larger indeed than the thirtieth part of the solar diameter, though not
more so than the sixth, or at the utmost the fifth of such a part.
Therefore let the diameter of the Sun be to the diameter of Venus as 30'
to 1' 12''. Certainly her diameter never equalled 1' 30'', scarcely
perhaps 1' 20'', and this was evident as well when the planet was near
the Sun's limb as when far distant from it.
[Illustration: VENUS ON THE SUN'S DISC.]
'This observation was made in an obscure village where I have long been
in the habit of observing, about fifteen miles to the north of
Liverpool, the latitude of which I believe to be 53 deg. 20', although by
common maps it is stated at 54 deg. 12', therefore the latitude of the
village will be 53 deg. 35', and longitude of both 22 deg. 30' from the
Fortunate Islands, now called the Canaries. This is 14 deg. 15' to the west
of Uraniburg in Denmark, the longitude of which is stated by Brahe, a
native of the place, to be 36 deg. 45' from these islands.
'This is all I could observe respecting this celebrated conjunction
during the short time the Sun remained in the horizon: for although
Venus continued on his disc for several hours, she was not visible to me
longer than half an hour on account of his so quickly setting.
Nevertheless, all the observations which could possibly be made in so
short a time I was enabled by Divine Providence to complete so
effectually that I could scarcely have wished for a more extended
period. The inclination was the only point upon which I failed to attain
the utmost precision; for, owing to the rapid motion of the Sun it was
difficult to observe with certainty to a single degree, and I frankly
confess that I neither did nor could ascertain it. But all the rest is
sufficiently accurate, and as exact as I could desire.'
Besides having ascertained that the diameter of Venus subtends an angle
not much greater than one minute of arc, Horrox reduced the horizontal
solar parallax from fifty-seven seconds as stated by Kepler to fourteen
seconds, a calculation within one and a half second of the value
assigned to it by Halley sixty years after. He also reduced the Sun's
semi-diameter.
Crabtree, to whom Horrox refers as 'his most esteemed friend and a
person who has few superiors in mathematical learning,' made
preparations to observe the transit similar to those already described.
But the day was unfavourable, dark clouds obscured the sky and rendered
the Sun invisible. Crabtree was in despair, and relinquished all hope of
being able to witness the conjunction. However, just before sunset there
was a break in the clouds, and the Sun shone brilliantly for a short
interval. Crabtree at once seized his opportunity, and to his intense
delight observed the planet fully entered upon the Sun's disc. Instead
of proceeding to take observations, he was so overcome with emotion at
the sight of the phenomenon, that he continued to gaze upon it with rapt
attention, nor did he recover his self-possession until the clouds again
hid from his view the setting Sun.[4]
Crabtree's observation of the transit was, however, not a fruitless one.
He drew from memory a diagram showing the exact position of Venus on the
Sun's disc, which corresponded in every respect with Horrox's
observation; he also estimated the diameter of the planet to be 7/200
that of the Sun, which when calculated gives one minute three seconds;
Horrox having found it to be one minute twelve seconds. This transit of
Venus is remarkable as having been the first ever observed of which
there is any record, and for this we are indebted to the genius of
Horrox, who by a series of calculations, displaying a wonderfully
accurate knowledge of mathematics, was enabled to predict the occurrence
of the phenomenon on the very day, and almost at the hour it appeared,
and of which he and his friend Crabtree were the only observers.
Having thought it desirable to write an account of the transit, Horrox
prepared an elegant Latin treatise, entitled 'Venus in Sole
Visa'--'Venus seen in the Sun;' but not knowing what steps to take with
regard to its publication, he requested Crabtree to communicate with his
bookseller and obtain his advice on the matter.
In the meantime Horrox returned to Toxteth, and arranged to fulfil a
long-promised visit to Crabtree, which he looked forward to with much
pleasure, as it would afford him an opportunity of discussing with his
friend many matters of interest to both. This visit was frustrated in a
manner altogether unexpected. For we read that Horrox was seized with a
sudden and severe illness, the nature of which is not known, and that
his death occurred on the day previous to that of his intended visit to
his friend at Broughton. He expired on January 3, 1641, when in the 23rd
year of his age.
His death was a great grief to Crabtree, who, in one of his letters,
describes it as 'an irreparable loss:' and it is believed that he only
survived him a few years.[5] Of the papers left by Horrox, only a few
have been preserved, and these were discovered in Crabtree's house after
his death. Among them was his treatise on the transit of Venus which,
with other papers, was purchased by Dr. Worthington, Fellow of Emmanuel
College, Cambridge, a man of learning, who was capable of appreciating
their value. Ultimately, the treatise fell into the possession of
Hevelius, a celebrated German astronomer, who published it along with a
dissertation of his own, describing a transit of Mercury.
Horrox did not live to see any of his writings published, nor was any
monument erected to his memory until nearly two hundred years after his
death. But his name, though long forgotten except by astronomers, is now
engraved on marble in Westminster Abbey. Had his life been spared, it
would have been difficult to foretell to what eminence and fame he might
have risen, or what further discoveries his genius might have enabled
him to make. Few among English astronomers will hesitate to rank him
next with the illustrious Newton, and all will agree with Herschel, who
called him 'the pride and the boast of British Astronomy.'
WILLIAM GASCOIGNE was born in 1612, in the parish of Rothwell, in the
county of York, and afterwards resided at Middleton, near Leeds.
He was a man of an inventive turn of mind, and possessed good abilities,
which he devoted to improving the methods of telescopic observation.
At an early age he was occupied in observing celestial objects, making
researches in optics, and acquiring a proficient knowledge of astronomy.
Among his acquaintances were Crabtree and Horrox, with whom he carried
on a correspondence on matters appertaining to their favourite study.
The measurement of small angles was found at all times to be one of the
greatest difficulties which astronomers had to contend with. Tycho Brahe
was so misled by his measurements of the apparent diameters of the Sun
and Moon, that he concluded a total eclipse of the Sun was impossible.
Gascoigne overcame this difficulty by his invention of the micrometer.
This instrument, when applied to a telescope, was found to be of great
service in the correct measurement of minute angles and distances, and
was the means of greatly advancing the progress of practical astronomy
in the seventeenth century. A micrometer consists of a short tube,
across the opening of which are stretched two parallel wires; these
being intersected at right angles by a third. The wires are moved to or
from each other by delicately constructed screws, to which they are
attached. Each revolution, or part of a revolution, of a screw indicates
the distance by which the wires are moved.
This apparatus, when placed in the focus of a lens, gives very accurate
measurements of the diameters of celestial objects. It was successfully
used by Gascoigne in determining the apparent diameters of the Sun,
Moon, and several of the planets, and the mutual distances of the stars
which form the Pleiades.
Crabtree, after having paid Gascoigne a visit in 1639, describes in a
letter to Horrox the impression created on his mind by the micrometer.
He writes: 'The first thing Mr. Gascoigne showed me was a large
telescope, amplified and adorned with new inventions of his own, whereby
he can take the diameters of the Sun or Moon, or any small angle in the
heavens or upon the earth, most exactly through the glass to a second.'
The micrometer is now regarded as an indispensable appliance in the
observatory; the use of a spider web reticule instead of wire having
improved its efficiency. Gascoigne was one of the earliest astronomers
who recognised the value of the Keplerian telescope for observational
purposes, and Sherburn affirms that he was the first to construct an
instrument of this description having two convex lenses. Whether this be
true or not, it is certain that he applied the micrometer to the
telescope, and was the first to use telescopic sights, by means of which
he was able to fix the optical axis of his telescope, and ascertain by
observation the apparent positions of the heavenly bodies.
Crabtree, in a letter to Gascoigne, says: 'Could I purchase it with
travel, or procure it with gold, I would not be without a telescope for
observing small angles in the heavens; or want the use of your device of
a glass in a cane upon the movable ruler of your sextant, as I remember
for helping to the exact point of the Sun's rays.'
It was not known until the beginning of the eighteenth century that
Gascoigne had invented and used telescopic sights for the purpose of
making accurate astronomical observations. The accidental discovery of
some documents which contained a description of his appliances was the
means by which this became known.
Townley states that Gascoigne had completed a treatise on optics, which
was ready for publication, but that no trace of the manuscript could be
discovered after his death. Having embraced the Royalist cause, William
Gascoigne joined the forces of Charles I., and fell in the battle of
Marston Moor on July 2, 1644.
The early death of this young and remarkably clever man was a severe
blow to the science of astronomy in England.
The invention of logarithms, by Baron Napier, of Merchistoun, was found
to be of inestimable value to astronomers in facilitating and
abbreviating the methods of astronomical calculation.
By the use of logarithms, arithmetical computations which necessitated
laborious application for several months could with ease be completed in
as many days. It was remarked by Laplace that this invention was the
means of doubling the life of an astronomer, besides enabling him to
avoid errors and the tediousness associated with long and abstruse
calculations.
THOMAS HARRIOT, an eminent mathematician, and an assiduous astronomer,
made some valuable observations of the comet of 1607. He was one of the
earliest observers who made use of the telescope, and it was claimed on
his behalf that he discovered Jupiter's satellites, and the spots on the
Sun, independently of Galileo. Other astronomers have been desirous of
sharing this honour, but it has been conclusively proved that Galileo
was the first who made those discoveries.
The investigations of Norwood and Gilbert, the mechanical genius of
Hooke, and the patient researches of Flamsteed--the first Astronomer
Royal--were of much value in perfecting many details associated with the
study of astronomy.
The Royal Observatory at Greenwich was founded in 1675. The building was
erected under a warrant from Charles II. It announces the desire of the
Sovereign to build a small observatory in the park at Greenwich, 'in
order to the finding out of the longitude for perfecting the art of
navigation and astronomy.' This action on the part of the King may be
regarded as the first public acknowledgment of the usefulness of
astronomy for national purposes.
Since its erection, the observatory has been presided over by a
succession of talented men, who have raised it to a position of eminence
and usefulness unsurpassed by any similar institution in this or any
other country. The well-known names of Flamsteed, Halley, Bradley, and
Airy, testify to the valuable services rendered by those past directors
of the Greenwich Observatory in the cause of astronomical science.
If we take a general survey of the science of astronomy as it existed
from 1608 to 1674--a period that embraced the time in which Milton
lived--we shall find that it was still compassed by ignorance,
superstition, and mystery. Astrology was zealously cultivated; most
persons of rank and position had their nativity or horoscope cast, and
the belief in the ruling of the planets, and their influence on human
and terrestrial affairs, was through long usage firmly established in
the public mind. Indeed, at this time, astronomy was regarded as a
handmaid to astrology; for, with the aid of astronomical calculation,
the professors of this occult science were enabled to predict the
positions of the planets, and by this means practised their art with an
apparent degree of truthfulness.
Although over one hundred years had elapsed since the death of
Copernicus, his theory of the solar system did not find many supporters,
and the old forms of astronomical belief still retained their hold on
the minds of the majority of philosophic thinkers. This can be partly
accounted for, as many of the Ptolemaic doctrines were at first
associated with the Copernican theory, nor was it until a later period
that they were eliminated from the system.
Though Copernicus deserved the credit of having transferred the centre
of our system from the Earth to the Sun, yet his theory was imperfect in
its details, and contained many inaccuracies. He believed that the
planets could only move round the Sun in circular paths, nor was he
capable of conceiving of any other form of orbit in which they could
perform their revolutions. He was therefore compelled to retain the use
of cycles and epicycles, in order to account for irregularities in the
uniformly circular motions of those bodies.
We are indebted to the genius of Kepler for having placed the Copernican
system upon a sure and irremovable basis, and for having raised
astronomy to the position of a true physical science. By his discovery
that the planets travel round the Sun in elliptical orbits, he was
enabled to abolish cycles and epicycles, which created such confusion
and entanglement in the system, and to explain many apparent
irregularities of motion by ascribing to the Sun his true position with
regard to the motions of the planets.
After the death of Kepler, which occurred in 1630, the most eminent
supporter of the Copernican theory was the illustrious Galileo, whose
belief in its accuracy and truthfulness was confirmed by his own
discoveries.
Five of the planets were known at this time--viz. Mercury, Venus, Mars,
Jupiter, and Saturn; the latter, which revolves in its orbit at a
profound distance from the Sun, formed what at that time was believed to
be the boundary of the planetary system. The distance of the Earth from
the Sun was approximately known, and the orb was observed to rotate on
his axis.
It was also ascertained that the Moon shone by reflected light, and that
her surface was varied by inequalities resembling those of our Earth.
The elliptical form of her orbit had been discovered by Horrox, and her
elements were computed with a certain degree of accuracy.
The cloudy luminosity of the Milky Way had been resolved into a
multitude of separate stars, disclosing the immensity of the stellar
universe.
The crescent form of the planet Venus, the satellites of Jupiter and of
Saturn, and the progressive motion and measurement of light, had also
been discovered. Observations were made of transits of Mercury and
Venus, and refracting and reflecting telescopes were invented.
The law of universal gravitation, a power which retains the Earth and
planets in their orbits, causing them year after year to describe with
unerring regularity their oval paths round the Sun, was not known at
this time. Though Newton was born in 1642, he did not disclose the
results of his philosophic investigations until 1687--thirteen years
after the death of Milton--when, in the 'Principia,' he announced his
discovery of the great law of universal gravitation.
Kepler, though he discovered the laws of planetary motion, was unable to
determine the motive force which guided and retained those bodies in
their orbits. It was reserved for the genius of Newton to solve this
wonderful problem. This great philosopher was able to prove 'that every
particle of matter in the universe attracts every other particle with a
force proportioned to the mass of the attracting body, and inversely as
the square of the distance between them.' Newton was capable of
demonstrating that the force which guides and retains the Earth and
planets in their orbits resides in the Sun, and by the application of
this law of gravitation he was able to explain the motions of all
celestial bodies entering into the structure of the solar system.
This discovery may be regarded as the crowning point of the science of
astronomy, for, upon the unfailing energy of this mysterious power
depend the order and stability of the universe, extending as it does to
all material bodies existing in space, guiding, controlling, and
retaining them in their several paths and orbits, whether it be a tiny
meteor, a circling planet, or a mighty sun.
The nature of cometary bodies and the laws which govern their motions
were at this time still enshrouded in mystery, and when one of those
erratic wanderers made its appearance in the sky it was beheld by the
majority of mankind with feelings of awe and superstitious dread, and
regarded as a harbinger of evil and disaster, the precursor of war, of
famine, or the overthrow of an empire.
Newton, however, was able to divest those bodies of the mystery with
which they were surrounded by proving that any conic section may be
described about the Sun, consistent with the law of gravitation, and
that comets, notwithstanding the eccentricity of their orbits, obey the
laws of planetary motion.
Beyond the confines of our solar system, little was known of the
magnitude and extent of the sidereal universe which occupies the
infinitude of space by which we are surrounded. The stars were
recognised as self-luminous bodies, inconceivably remote, and although
they excited the curiosity of observers, and conjectures were made as to
their origin, yet no conclusive opinions were arrived at with regard to
their nature and constitution, and except that they were regarded as
glittering points of light which illumine the firmament, all else
appertaining to them remained an unravelled mystery. Even Copernicus had
no notion of a universe of stars.
Galileo, by his discovery that the galaxy consists of a multitude of
separate stars too remote to be defined by ordinary vision, demonstrated
how vast are the dimensions of the starry heavens, and on what a
stupendous scale the universe is constructed. But at this time it had
not occurred to astronomers, nor was it known until many years after,
that the stars are suns which shine with a splendour resembling that of
our Sun, and in many instances surpassing it. It was not until this
truth became known that the glories of the sidereal heavens were fully
comprehended, and their magnificence revealed. It was then ascertained
that the minute points of light which crowd the fields of our largest
telescopes, in their aggregations forming systems, clusters, galaxies,
and universes of stars, are shining orbs of light, among the countless
multitudes of which our Sun may be numbered as one.