side being sixty feet, and of large wings in the form of round towers.
The whole was executed in a style of great magnificence, and Tycho, who
was a nobleman by descent, gratified his taste for splendor and
ornament, by giving to every part of the structure an air of the most
finished elegance. Nor were the instruments with which it was furnished
less magnificent than the buildings. They were vastly larger than had
before been employed in the survey of the heavens, and many of them were
adorned with costly ornaments. The cut on page 46, Fig. 6, represents
one of Tycho's large and splendid instruments, (an astronomical
quadrant,) on one side of which was figured a representation of the
astronomer and his assistants, in the midst of their instruments, and
intently engaged in making and recording observations. It conveys to us
a striking idea of the magnificence of his arrangements, and of the
extent of his operations.
Here Tycho sat in state, clad in the robes of nobility, and supported
throughout his establishment the etiquette due to his rank. His
observations were more numerous than all that had ever been made before,
and they were carried to a degree of accuracy that is astonishing, when
we consider that they were made without the use of the telescope, which
was not yet invented.
Tycho carried on his observations at Uraniburg for about twenty years,
during which time he accumulated an immense store of accurate and
valuable _facts_, which afforded the groundwork of the discovery of the
great laws of the solar system established by Kepler, of whom I shall
tell you more hereafter.
But the high marks of distinction which Tycho enjoyed, not only from his
own Sovereign, but also from foreign potentates, provoked the envy of
the courtiers of his royal patron. They did not indeed venture to make
their attacks upon him while his generous patron was living; but the
King was no sooner dead, and succeeded by a young monarch, who did not
feel the same [Illustration Fig. 6.] interest in protecting and
encouraging this great ornament of the kingdom, than his envious foes
carried into execution their long-meditated plot for his ruin. They
represented to the young King, that the treasury was exhausted, and that
it was necessary to retrench a number of pensions, which had been
granted for useless purposes, and in particular that of Tycho, which,
they maintained, ought to be conferred upon some person capable of
rendering greater services to the state. By these means, they succeeded
in depriving him of his support, and he was compelled to retreat under
the hospitable mansion of a friend in Germany. Here he became known to
the Emperor, who invited him to Prague, where, with an ample stipend, he
resumed his labors. But, though surrounded with affectionate friends and
admiring disciples, he was still an exile in a foreign land. Although
his country had been base in its ingratitude, it was yet the land which
he loved; the scene of his earliest affection; the theatre of his
scientific glory. These feelings continually preyed upon his mind, and
his unsettled spirit was ever hovering among his native mountains. In
this condition he was attacked by a disease of the most painful kind,
and, though its agonizing paroxysms had lengthened intermissions, yet he
saw that death was approaching. He implored his pupils to persevere in
their scientific labors; he conversed with Kepler on some of the
profoundest points of astronomy; and with these secular occupations he
mingled frequent acts of piety and devotion. In this happy condition he
expired, without pain, at the age of fifty-five.[3]
The observatory at Greenwich was not built until a hundred years after
that of Tycho Brahe, namely, in 1676. The great interests of the British
nation, which are involved in navigation, constituted the ruling motive
with the government to lend their aid in erecting and maintaining this
observatory.
The site of the observatory at Greenwich is on a commanding eminence
facing the River Thames, five miles east of the central parts of London.
Being part of a royal park, the neighboring grounds are in no danger of
being occupied by buildings, so as to obstruct the view. It is also in
full view of the shipping on the Thames; and, according to a standing
regulation of the observatory, at the instant of one o'clock, every day,
a huge ball is dropped from over the house, as a signal to the
commanders of vessels for regulating their chronometers.
The buildings comprise a series of rooms, of sufficient number and
extent to accommodate the different instruments, the inmates of the
establishment, and the library; and on the top is a celebrated camera
obscura, exhibiting a most distinct and perfect picture of the grand and
unrivalled scenery which this eminence commands.
This establishment, by the accuracy and extent of its observations, has
contributed more than all other institutions to perfect the science of
astronomy.
To preside over and direct this great institution, a man of the highest
eminence in the science is appointed by the government, with the title
of _Astronomer Royal_. He is paid an ample salary, with the
understanding that he is to devote himself exclusively to the business
of the observatory. The astronomers royal of the Greenwich observatory,
from the time of its first establishment, in 1676, to the present time,
have constituted a series of the proudest names of which British science
can boast. A more detailed sketch of their interesting history will be
given towards the close of these Letters.
Six assistants, besides inferior laborers, are constantly in attendance;
and the business of making and recording observations is conducted with
the utmost system and order.
The great objects to be attained in the construction of an observatory
are, a commanding and unobstructed view of the heavens; freedom from
causes that affect the transparency and uniform state of the
atmosphere, such as fires, smoke, or marshy grounds; mechanical
facilities for the management of instruments, and, especially, every
precaution that is necessary to secure perfect steadiness. This last
consideration is one of the greatest importance, particularly in the use
of very large magnifiers; for we must recollect, that any motion in the
instrument is magnified by the full power of the glass, and gives a
proportional unsteadiness to the object. A situation is therefore
selected as remote as possible from public roads, (for even the passing
of carriages would give a tremulous motion to the ground, which would be
sensible in large instruments,) and structures of solid masonry are
commenced deep enough in the ground to be unaffected by frost, and built
up to the height required, without any connexion with the other parts of
the building. Many observatories are furnished with a movable dome for a
roof, capable of revolving on rollers, so that instruments penetrating
through the roof may be easily brought to bear upon any point at or near
the zenith.
You will not perhaps desire me to go into a minute description of all
the various instruments that are used in a well-constructed observatory.
Nor is this necessary, since a very large proportion of all astronomical
observations are taken on the meridian, by means of the transit
instrument and clock. When a body, in its diurnal revolution, comes to
the meridian, it is at its highest point above the horizon, and is then
least affected by refraction and parallax. This, then, is the most
favorable position for taking observations upon it. Moreover, it is
peculiarly easy to take observations on a body when in this situation.
Hence the transit instrument and clock are the most important members of
an astronomical observatory. You will, therefore, expect me to give you
some account of these instruments.
[Illustration Fig. 7.]
The _transit instrument_ is a telescope which is fixed permanently in
the meridian, and moves only in that plane. The accompanying diagram,
Fig. 7, represents a side view of a portable transit instrument,
exhibiting the telescope supported on a firm horizontal axis, on which
it turns in the plane of the meridian, from the south point of the
horizon through the zenith to the north point. It can therefore be so
directed as to observe the passage of a star across the meridian at any
altitude. The accompanying graduated circle enables the observer to set
the instrument at any required altitude, corresponding to the known
altitude at which the body to be observed crosses the meridian. Or it
may be used to measure the altitude of a body, or its zenith distance,
at the time of its meridian passage. Near the circle may be seen a
spirit-level, which serves to show when the axis is exactly on a level
with the horizon. The framework is made of solid metal, (usually brass,)
every thing being arranged with reference to keeping the instrument
perfectly steady. It stands on screws, which not only afford a steady
support, but are useful for adjusting the instrument to a perfect
level. The transit instrument is sometimes fixed immovably to a solid
foundation, as a pillar of stone, which is built up from a depth in the
ground below the reach of frost. When enclosed in a building, as in an
observatory, the stone pillar is carried up separate from the walls and
floors of the building, so as to be entirely free from the agitations to
which they are liable.
The use of the transit instrument is to show the precise instant when a
heavenly body is on the meridian, or to measure the time it occupies in
crossing the meridian. The _astronomical clock_ is the constant
companion of the transit instrument. This clock is so regulated as to
keep exact pace with the stars, and of course with the revolution of the
earth on its axis; that is, it is regulated to _sidereal_ time. It
measures the progress of a star, indicating an hour for every fifteen
degrees, and twenty-four hours for the whole period of the revolution of
the star. Sidereal time commences when the vernal equinox is on the
meridian, just as solar time commences when the sun is on the meridian.
Hence the hour by the sidereal clock has no correspondence with the hour
of the day, but simply indicates how long it is since the equinoctial
point crossed the meridian. For example, the clock of an observatory
points to three hours and twenty minutes; this may be in the morning, at
noon, or any other time of the day,--for it merely shows that it is
three hours and twenty minutes since the equinox was on the meridian.
Hence, when a star is on the meridian, the clock itself shows its right
ascension, which you will recollect is the angular distance measured on
the equinoctial, from the point of intersection of the ecliptic and
equinoctial, called the vernal equinox, reckoning fifteen degrees for
every hour, and a proportional number of degrees and minutes for a less
period. I have before remarked, that a very large portion of all
astronomical observations are taken when the bodies are on the meridian,
by means of the transit instrument and clock.
Having now described these instruments, I will next explain the manner
of using them for different observations. Any thing becomes a measure of
time, which divides duration equally. The equinoctial, therefore, is
peculiarly adapted to this purpose, since, in the daily revolution of
the heavens, equal portions of the equinoctial pass under the meridian
in equal times. The only difficulty is, to ascertain the amount of these
portions for given intervals. Now, the clock shows us exactly this
amount; for, when regulated to sidereal time, (as it easily may be,) the
hour-hand keeps exact pace with the equator, revolving once on the
dial-plate of the clock while the equator turns once by the revolution
of the earth. The same is true, also, of all the small circles of
diurnal revolution; they all turn exactly at the same rate as the
equinoctial, and a star situated any where between the equator and the
pole will move in its diurnal circle along with the clock, in the same
manner as though it were in the equinoctial. Hence, if we note the
interval of time between the passage of any two stars, as shown by the
clock, we have a measure of the number of degrees by which they are
distant from each other in right ascension. Hence we see how easy it is
to take arcs of right ascension: the transit instrument shows us when a
body is on the meridian; the clock indicates how long it is since the
vernal equinox passed it, which is the right ascension itself; or it
tells us the difference of right ascension between any two bodies,
simply by indicating the difference in time between their periods of
passing the meridian. Again, it is easy to take the _declination_ of a
body when on the meridian. By declination, you will recollect, is meant
the distance of a heavenly body from the equinoctial; the same, indeed,
as latitude on the earth. When a star is passing the meridian, if, on
the instant of crossing the meridian wire of the telescope, we take its
distance from the north pole, (which may readily be done, because the
position of the pole is always known, being equal to the latitude of the
place,) and subtract this distance from ninety degrees, the remainder
will be the distance from the equator, which is the declination. You
will ask, why we take this indirect method of finding the declination?
Why we do not rather take the distance of the star from the equinoctial,
at once? I answer, that it is easy to point an instrument to the north
pole, and to ascertain its exact position, and of course to measure any
distance from it on the meridian, while, as there is nothing to mark the
exact situation of the equinoctial, it is not so easy to take direct
measurements from it. When we have thus determined the situation of a
heavenly body, with respect to two great circles at right angles with
each other, as in the present case, the distance of a body from the
equator and from the equinoctial colure, or that meridian which passes
though the vernal equinox, we know its relative position in the heavens;
and when we have thus determined the relative positions of all the
stars, we may lay them down on a map or a globe, exactly as we do places
on the earth, by means of their latitude and longitude.
The foregoing is only a _specimen_ of the various uses of the transit
instrument, in finding the relative places of the heavenly bodies.
Another use of this excellent instrument is, to regulate our clocks and
watches. By an observation with the transit instrument, we find when the
sun's centre is on the meridian. This is the exact time of _apparent_
noon. But watches and clocks usually keep _mean_ time, and therefore, in
order to set our timepiece by the transit instrument, we must apply to
the apparent time of noon the equation of time, as will be explained in
my next Letter.
A _noon-mark_ may easily be made by the aid of the transit instrument. A
window sill is frequently selected as a suitable place for the mark,
advantage being taken of the shadow projected upon it by the
perpendicular casing of the window. Let an assistant stand, with a rule
laid on the line of shadow, and with a knife ready to make the mark, the
instant when the observer at the transit instrument announces that the
centre of the sun is on the meridian. By a concerted signal, as the
stroke of a bell, the inhabitants of a town may all fix a noon-mark from
the same observation. If the signal be given on one of the days when
apparent time and mean time become equal to each other, as on the
twenty-fourth of December, no equation of time is required.
As a noon-mark is convenient for regulating timepieces, I will point out
a method of making one, which may be practised without the aid of the
telescope. Upon a smooth, level plane, freely exposed to the sun, with a
pair of compasses describe a circle. In the centre, where the leg of the
compasses stood, erect a perpendicular wire of such a length, that the
termination of its shadow shall fall upon the circumference of the
circle at some hour before noon, as about ten o'clock. Make a small dot
at the point where the end of the shadow falls upon the circle, and do
the same where it falls upon it again in the afternoon. Take a point
half-way between these two points, and from it draw a line to the
centre, and it will be a true meridian line. The direction of this line
would be the same, whether it were made in the Summer or in the Winter;
but it is expedient to draw it about the fifteenth of June, for then the
shadow alters its length most rapidly, and the moment of its crossing
the wire will be more definite, than in the Winter. At this time of
year, also, the sun and clock agree, or are together, as will be more
fully explained in my next Letter; whereas, at other times of the year,
the time of noon, as indicated by a common clock, would not agree with
that indicated by the sun. If the upper end of the wire is flattened,
and a small hole is made in it, through which the sun may shine, the
instant when this bright spot falls upon the circle will be better
defined than the termination of the shadow.
Another important instrument of the observatory is the _mural circle_.
It is a graduated circle, usually of very large size, fixed permanently
in the plane of the meridian, and attached firmly to a perpendicular
wall; and on its centre is a telescope, which revolves along with it,
and is easily brought to bear on any object in any point in the
meridian. It is made of large size, sometimes twenty feet in diameter,
in order that very small angles may be measured on its limb; for it is
obvious that a small angle, as one second, will be a larger space on the
limb of an instrument, in proportion as the instrument itself is larger.
The vertical circle usually connected with the transit instrument, as in
Fig. 7, may indeed be employed for the same purposes as the mural
circle, namely, to measure arcs of the meridian, as meridian altitudes,
zenith distances, north polar distances, and declinations; but as that
circle must necessarily be small, and therefore incapable of measuring
very minute angles, the mural circle is particularly useful in measuring
these important arcs. It is very difficult to keep so large an
instrument perfectly steady; and therefore it is attached to a massive
wall of solid masonry, and is hence called a _mural_ circle, from a
Latin word, (_murus_,) which signifies a wall.
The diagram, Fig. 8, page 56, represents a mural circle fixed to its
wall, and ready for observations. It will be seen, that every expedient
is employed to give the instrument firmness of parts and steadiness of
position. The circle is of solid metal, usually of brass, and it is
strengthened by numerous radii, which keep it from warping or bending;
and these are made in the form of hollow cones, because that is the
figure which unites in the highest degree lightness and strength. On the
rim of the instrument, at A, you may observe a microscope. This is
attached to a micrometer,--a delicate piece of apparatus, used for
reading the minute subdivisions of angles; for, after dividing the limb
of the instrument as minutely as possible, it will then be necessary to
magnify those divisions with the microscope, and subdivide each of these
parts with the micrometer. Thus, if we have a mural circle twenty feet
in diameter, and of course nearly sixty-three feet in circumference,
since there are twenty-one thousand and six hundred minutes in the
whole circle, we shall find, by calculation, that one minute would
occupy, on the limb of such an instrument, only about one thirtieth of
an inch, and a second, only one eighteen hundredth of an inch. We could
not, therefore, hope to carry the actual divisions to a greater degree
of minuteness than minutes; but each of these spaces may again be
subdivided into seconds by the micrometer.
[Illustration Fig. 8.]
From these statements, you will acquire some faint idea of the extreme
difficulty of making perfect astronomical instruments, especially where
they are intended to measure such minute angles as one second. Indeed,
the art of constructing astronomical instruments is one which requires
such refined mechanical genius,--so superior a mind to devise, and so
delicate a hand to execute,--that the most celebrated instrument-makers
take rank with the most distinguished astronomers; supplying, as they
do, the means by which only the latter are enabled to make these great
discoveries. Astronomers have sometimes made their own telescopes; but
they have seldom, if ever, possessed the refined manual skill which is
requisite for graduating delicate instruments.
The _sextant_ is also one of the most valuable instruments for taking
celestial arcs, or the distance between any two points on the celestial
sphere, being applicable to a much greater number of purposes than the
instruments already described. It is particularly valuable for measuring
celestial arcs at sea, because it is not, like most astronomical
instruments, affected by the motion of the ship. The principle of the
sextant may be briefly described, as follows: it gives the angular
distance between any two bodies on the celestial sphere, by reflecting
the image of one of the bodies so as to coincide with the other body, as
seen directly. The arc through which the reflector is turned, to bring
the reflected body to coincide with the other body, becomes a measure of
the angular distance between them. By keeping this principle in view,
you will be able to understand the use of the several parts of the
instrument, as they are exhibited in the diagram, Fig. 9, page 58.
It is, you observe, of a triangular shape, and it is made strong and
firm by metallic cross-bars. It has two reflectors, I and H, called,
respectively, the index glass and the horizon glass, both of which are
firmly fixed perpendicular to the plane of the instrument. The index
glass is attached to the movable arm, ID, and turns as this is moved
along the graduated limb, EF. This arm also carries a _vernier_, at D, a
contrivance which, like the micrometer, enables us to take off minute
parts of the spaces into which the limb is divided. The horizon glass,
H, consists of two parts; the upper part being transparent or open, so
that the eye, looking through the telescope, T, can see through it a
distant body, as a star at S, while the lower part is a reflector.
[Illustration Fig. 9.]
Suppose it were required to measure the angular distance between the
moon and a certain star,--the moon being at M, and the star at S. The
instrument is held firmly in the hand, so that the eye, looking through
the telescope, sees the star, S, through the transparent part of the
horizon glass. Then the movable arm, ID, is moved from F towards E,
until the image of M is reflected down to S, when the number of degrees
and parts of a degree reckoned on the limb, from F to the index at D,
will show the angular distance between the two bodies.
FOOTNOTE:
[3] Brewster's Life of Newton
LETTER VI.
TIME AND THE CALENDAR.
"From old Eternity's mysterious orb
Was Time cut off, and cast beneath the skies."--_Young._
HAVING hitherto been conversant only with the many fine and sentimental
things which the poets have sung respecting Old Time, perhaps you will
find some difficulty in bringing down your mind to the calmer
consideration of what time really is, and according to what different
standards it is measured for different purposes. You will not, however,
I think, find the subject even in our matter-of-fact and unpoetical way
of treating it, altogether uninteresting. What, then, is time? _Time is
a measured portion of indefinite duration._ It consists of equal
portions cut off from eternity, as a line on the surface of the earth is
separated from its contiguous portions that constitute a great circle of
the sphere, by applying to it a two-foot scale; or as a few yards of
cloth are measured off from a piece of unknown or indefinite extent.
Any thing, or any event which takes place at equal intervals, may become
a measure of time. Thus, the pulsations of the wrist, the flowing of a
given quantity of sand from one vessel to another, as in the hourglass,
the beating of a pendulum, and the revolution of a star, have been
severally employed as measures of time. But the great standard of time
is the period of the revolution of the earth on its axis, which, by the
most exact observations, is found to be always the same. I have
anticipated a little of this subject, in giving an account of the
transit instrument and clock, but I propose, in this letter, to enter
into it more at large.