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[Illustration: SPECTRA OF VARIOUS SOURCES OF LIGHT.]
The Heavens Above: A Popular Handbook of Astronomy
THE HEAVENS ABOVE:
A Popular Handbook of Astronomy.
by
J. A. GILLET,
Professor of Physics in the Normal College of the City of New York,
and
W. J. ROLFE,
Formerly Head Master of the High School, Cambridge, Mass.
With Six Lithographic Plates and Four Hundred
and Sixty Wood Engravings.
Potter, Ainsworth, & Co.,
New York and Chicago.
1882.
Copyright by
J. A. Gillet and W. J. Rolfe,
1882.
Franklin Press:
Rand, Avery, and Company,
Boston.
PREFACE.
It has been the aim of the authors to give in this little book a brief,
simple, and accurate account of the heavens as they are known to
astronomers of the present day. It is believed that there is nothing in
the book beyond the comprehension of readers of ordinary intelligence,
and that it contains all the information on the subject of astronomy
that is needful to a person of ordinary culture. The authors have
carefully avoided dry and abstruse mathematical calculations, yet they
have sought to make clear the methods by which astronomers have gained
their knowledge of the heavens. The various kinds of telescopes and
spectroscopes have been described, and their use in the study of the
heavens has been fully explained.
The cuts with which the book is illustrated have been drawn from all
available sources; and it is believed that they excel in number,
freshness, beauty, and accuracy those to be found in any similar work.
The lithographic plates are, with a single exception, reductions of the
plates prepared at the Observatory at Cambridge, Mass. The remaining
lithographic plate is a reduced copy of Professor Langley's celebrated
sun-spot engraving. Many of the views of the moon are from drawings made
from the photographs in Carpenter and Nasmyth's work on the moon. The
majority of the cuts illustrating the solar system are copied from the
French edition of Guillemin's "Heavens." Most of the remainder are from
Lockyer's "Solar Physics," Young's "Sun," and other recent authorities.
The cuts illustrating comets, meteors, and nebulæ, are nearly all taken
from the French editions of Guillemin's "Comets" and Guillemin's
"Heavens."
CONTENTS.
I. THE CELESTIAL SPHERE 3
II. THE SOLAR SYSTEM 41
I. THEORY OF THE SOLAR SYSTEM 41
The Ptolemaic System 41
The Copernican System 44
Tycho Brahe's System 44
Kepler's System 44
The Newtonian System 48
II. THE SUN AND PLANETS 53
I. The Earth 53
Form and Size 53
Day and Night 57
The Seasons 64
Tides 68
The Day and Time 74
The Year 78
Weight of the Earth and Precession 83
II. The Moon 86
Distance, Size, and Motions 86
The Atmosphere of the Moon 109
The Surface of the Moon 114
III. Inferior and Superior Planets 130
Inferior Planets 130
Superior Planets 134
IV. The Sun 140
I. Magnitude and Distance of the Sun 140
II. Physical and Chemical Condition of the Sun 149
Physical Condition of the Sun 149
The Spectroscope 152
Spectra 158
Chemical Constitution of the Sun 164
Motion at the Surface of the Sun 168
III. The Photosphere and Sun-Spots 175
The Photosphere 175
Sun-Spots 179
IV. The Chromosphere and Prominences 196
V. The Corona 204
V. Eclipses 210
VI. The Three Groups of Planets 221
I. General Characteristics of the Groups 221
II. The Inner Group of Planets 225
Mercury 225
Venus 230
Mars 235
III. The Asteroids 241
IV. Outer Group of Planets 244
Jupiter 244
The Satellites of Jupiter 250
Saturn 255
The Planet and his Moons 255
The Rings of Saturn 261
Uranus 269
Neptune 271
VII. Comets and Meteors 274
I. Comets 274
General Phenomena of Comets 274
Motion and Origin of Comets 281
Remarkable Comets 290
Connection between Meteors and Comets, 300
Physical and Chemical Constitution of Comets 314
II. The Zodiacal Light 318
III. THE STELLAR UNIVERSE 322
I. General Aspect of the Heavens 322
II. The Stars 330
The Constellations 330
Clusters 350
Double and Multiple Stars 355
New and Variable Stars 358
Distance of the Stars 364
Proper Motion of the Stars 365
Chemical and Physical Constitution of the Stars 371
III. Nebulæ 373
Classification of Nebulæ 373
Irregular Nebulæ 376
Spiral Nebulæ 384
The Nebular Hypothesis 391
IV. The Structure of the Stellar Universe 396
I.
THE CELESTIAL SPHERE.
I. _The Sphere._--A _sphere_ is a solid figure bounded by a surface
which curves equally in all directions at every point. The rate at which
the surface curves is called the _curvature_ of the sphere. The smaller
the sphere, the greater is its curvature. Every point on the surface of
a sphere is equally distant from a point within, called the _centre_ of
the sphere. The _circumference_ of a sphere is the distance around its
centre. The _diameter_ of a sphere is the distance through its centre.
The _radius_ of a sphere is the distance from the surface to the centre.
The surfaces of two spheres are to each other as the squares of their
radii or diameters; and the volumes of two spheres are to each other as
the cubes of their radii or diameters.
Distances on the surface of a sphere are usually denoted in _degrees_. A
degree is 1/360 of the circumference of the sphere. The larger a sphere,
the longer are the degrees on it.
A curve described about any point on the surface of a sphere, with a
radius of uniform length, will be a circle. As the radius of a circle
described on a sphere is a curved line, its length is usually denoted in
degrees. The circle described on the surface of a sphere increases with
the length of the radius, until the radius becomes 90°, in which case
the circle is the largest that can possibly be described on the sphere.
The largest circles that can be described on the surface of a sphere are
called _great circles_, and all other circles _small circles_.
Any number of great circles may be described on the surface of a
sphere, since any point on the sphere may be used for the centre of
the circle. The plane of every great circle passes through the
centre of the sphere, while the planes of all the small circles pass
through the sphere away from the centre. All great circles on the
same sphere are of the same size, while the small circles differ in
size according to the distance of their planes from the centre of
the sphere. The farther the plane of a circle is from the centre of
the sphere, the smaller is the circle.
By a _section_ of a sphere we usually mean the figure of the surface
formed by the cutting; by a _plane section_ we mean one whose
surface is plane. Every plane section of a sphere is a circle. When
the section passes through the centre of the sphere, it is a great
circle; in every other case the section is a small circle. Thus,
_AN_ and _SB_ (Fig. 1) are small circles, and _MM'_ and _SN_ are
large circles.
[Illustration: Fig. 1.]
In a diagram representing a sphere in section, all the circles whose
planes cut the section are represented by straight lines. Thus, in
Fig. 2, we have a diagram representing in section the sphere of Fig.
1. The straight lines _AN_, _SB_, _MM'_, and _SN_, represent the
corresponding circles of Fig. 1.
The _axis_ of a sphere is the diameter on which it rotates. The _poles_
of a sphere are the ends of its axis. Thus, supposing the spheres of
Figs. 1 and 2 to rotate on the diameter _PP'_, this line would be called
the axis of the sphere, and the points _P_ and _P'_ the poles of the
sphere. A great circle, MM', situated half way between the poles of a
sphere, is called the _equator_ of the sphere.
Every great circle of a sphere has two poles. These are the two points
on the surface of the sphere which lie 90° away from the circle. The
poles of a sphere are the poles of its equator.
[Illustration: Fig. 2.]
2. _The Celestial Sphere._--The heavens appear to have the form of a
sphere, whose centre is at the eye of the observer; and all the stars
seem to lie on the surface of this sphere. This form of the heavens is a
mere matter of perspective. The stars are really at very unequal
distances from us; but they are all seen projected upon the celestial
sphere in the direction in which they happen to lie. Thus, suppose an
observer situated at _C_ (Fig. 3), stars situated at _a_, _b_, _d_, _e_,
_f_, and _g_, would be projected upon the sphere at _A_, _B_, _D_, _E_,
_F_, and _G_, and would appear to lie on the surface of the heavens.
[Illustration: Fig. 3.]
3. _The Horizon._--Only half of the celestial sphere is visible at a
time. The plane that separates the visible from the invisible portion is
called the _horizon_. This plane is tangent to the earth at the point of
observation, and extends indefinitely into space in every direction. In
Fig. 4, _E_ represents the earth, _O_ the point of observation, and _SN_
the horizon. The points on the celestial sphere directly above and below
the observer are the poles of the horizon. They are called respectively
the _zenith_ and the _nadir_. No two observers in different parts of the
earth have the same horizon; and as a person moves over the earth he
carries his horizon with him.
[Illustration: Fig. 4.]
The dome of the heavens appears to rest on the earth, as shown in Fig.
5. This is because distant objects on the earth appear projected against
the heavens in the direction of the horizon.
[Illustration: Fig. 5.]
The _sensible_ horizon is a plane tangent to the earth at the point of
observation. The _rational_ horizon is a plane parallel with the
sensible horizon, and passing through the centre of the earth. As it
cuts the celestial sphere through the centre, it forms a great circle.
_SN_ (Fig. 6) represents the sensible horizon, and _S'N'_ the rational
horizon. Although these two horizons are really four thousand miles
apart, they appear to meet at the distance of the celestial sphere; a
line four thousand miles long at the distance of the celestial sphere
becoming a mere point, far too small to be detected with the most
powerful telescope.
[Illustration: Fig. 6.]
[Illustration: Fig. 7.]
4. _Rotation of the Celestial Sphere._--It is well known that the sun
and the majority of the stars rise in the east, and set in the west. In
our latitude there are certain stars in the north which never disappear
below the horizon. These stars are called the _circumpolar_ stars. A
close watch, however, reveals the fact that these all appear to revolve
around one of their number called the _pole star_, in the direction
indicated by the arrows in Fig. 7. In a word, the whole heavens appear
to rotate once a day, from east to west, about an axis, which is the
prolongation of the axis of the earth. The ends of this axis are called
the _poles_ of the heavens; and the great circle of the heavens, midway
between these poles, is called the _celestial equator_, or the
_equinoctial_. This rotation of the heavens is apparent only, being due
to the rotation of the earth from west to east.
5. _Diurnal Circles._--In this rotation of the heavens, the stars appear
to describe circles which are perpendicular to the celestial axis, and
parallel with the celestial equator. These circles are called _diurnal
circles_. The position of the poles in the heavens and the direction of
the diurnal circles with reference to the horizon, change with the
position of the observer on the earth. This is owing to the fact that
the horizon changes with the position of the observer.
[Illustration: Fig. 8.]
When the observer is north of the equator, the north pole of the heavens
is _elevated_ above the horizon, and the south pole is _depressed_ below
it, and the diurnal circles are _oblique_ to the horizon, leaning to the
south. This case is represented in Fig. 8, in which _PP'_ represents the
celestial axis, _EQ_ the celestial equator, _SN_ the horizon, and _ab_,
_cN_, _de_, _fg_, _Sh_, _kl_, diurnal circles. _O_ is the point of
observation, _Z_ the zenith, and _Z'_ the nadir.
[Illustration: Fig. 9.]
When the observer is south of the equator, as at _O_ in Fig. 9, the
south pole is _elevated_, the north pole _depressed_, and the diurnal
circles are _oblique_ to the horizon, leaning to the north. When the
diurnal circles are oblique to the horizon, as in Figs. 8 and 9, the
celestial sphere is called an _oblique sphere_.
When the observer is at the equator, as in Fig. 10, the poles of the
heavens are on the horizon, and the diurnal circles are _perpendicular_
to the horizon.
When the observer is at one of the poles, as in Fig. 11, the poles of
the heavens are in the zenith and the nadir, and the diurnal circles are
_parallel_ with the horizon.
[Illustration: Fig. 10.]
[Illustration: Fig. 11.]
6. _Elevation of the Pole and of the Equinoctial._--At the equator the
poles of the heavens lie on the horizon, and the celestial equator
passes through the zenith. As a person moves north from the equator, his
zenith moves north from the celestial equator, and his horizon moves
down from the north pole, and up from the south pole. The distance of
the zenith from the equinoctial, and of the horizon from the celestial
poles, will always be equal to the distance of the observer from the
equator. In other words, the elevation of the pole is equal to the
latitude of the place. In Fig. 12, _O_ is the point of observation, _Z_
the zenith, and _SN_ the horizon. _NP_, the elevation of the pole, is
equal to _ZE_, the distance of the zenith from the equinoctial, and to
the distance of _O_ from the equator, or the latitude of the place.
Two angles, or two arcs, which together equal 90°, are said to be
_complements_ of each other. _ZE_ and _ES_ in Fig. 12 are together equal
to 90°: hence they are complements of each other. _ZE_ is equal to the
latitude of the place, and _ES_ is the _elevation_ of the equinoctial
above the horizon: hence the elevation of the equinoctial is equal to
the complement of the latitude of the place.
[Illustration: Fig. 12.]
Were the observer south of the equator, the zenith would be south of the
equinoctial, and the south pole of the heavens would be the elevated
pole.
[Illustration: Fig. 13.]
_7. Four Sets of Stars._--At most points of observation there are four
sets of stars. These four sets are shown in Fig. 13.
(1) The stars in the neighborhood of the elevated pole _never set_. It
will be seen from Fig. 13, that if the distance of a star from the
elevated pole does not exceed the elevation of the pole, or the latitude
of the place, its diurnal circle will be wholly above the horizon. As
the observer approaches the equator, the elevation of the pole becomes
less and less, and the belt of circumpolar stars becomes narrower and
narrower: at the equator it disappears entirely. As the observer
approaches the pole, the elevation of the pole increases, and the belt
of circumpolar stars becomes broader and broader, until at the pole it
includes half of the heavens. At the poles, no stars rise or set, and
only half of the stars are ever seen at all.
(2) The stars in the neighborhood of the depressed pole _never rise_.
The breadth of this belt also increases as the observer approaches the
pole, and decreases as he approaches the equator, to vanish entirely
when he reaches the equator. The distance from the depressed pole to the
margin of this belt is always equal to the latitude of the place.
(3) The stars in the neighborhood of the equinoctial, on the side of the
elevated pole, _set, but are above the horizon longer than they are
below it_. This belt of stars extends from the equinoctial to a point
whose distance from the elevated pole is equal to the latitude of the
place: in other words, the breadth of this third belt of stars is equal
to the complement of the latitude of the place. Hence this belt of stars
becomes broader and broader as the observer approaches the equator, and
narrower and narrower as he approaches the pole. However, as the
observer approaches the equator, the horizon comes nearer and nearer the
celestial axis, and the time a star is below the horizon becomes more
nearly equal to the time it is above it. As the observer approaches the
pole, the horizon moves farther and farther from the axis, and the time
any star of this belt is below the horizon becomes more and more unequal
to the time it is above it. The farther any star of this belt is from
the equinoctial, the longer the time it is above the horizon, and the
shorter the time it is below it.
(4) The stars which are in the neighborhood of the equinoctial, on the
side of the depressed pole, _rise, but are below the horizon longer than
they are above it_. The width of this belt is also equal to the
complement of the latitude of the place. The farther any star of this
belt is from the equinoctial, the longer time it is below the horizon,
and the shorter time it is above it; and, the farther the place from the
equator, the longer every star of this belt is below the horizon, and
the shorter the time it is above it.
At the equator every star is above the horizon just half of the time;
and any star on the equinoctial is above the horizon just half of the
time in every part of the earth, since the equinoctial and horizon,
being great circles, bisect each other.
8. _Vertical Circles._--Great circles perpendicular to the horizon are
called _vertical circles_. All vertical circles pass through the zenith
and nadir. A number of these circles are shown in Fig. 14, in which
_SENW_ represents the horizon, and _Z_ the zenith.
[Illustration: Fig. 14.]
The vertical circle which passes through the north and south points of
the horizon is called the _meridian_; and the one which passes through
the east and west points, the _prime vertical_. These two circles are
shown in Fig. 15; _SZN_ being the meridian, and _EZW_ the prime
vertical. These two circles are at right angles to each other, or 90°
apart; and consequently they divide the horizon into four quadrants.
[Illustration: Fig. 15.]
9. _Altitude and Zenith Distance._--The _altitude_ of a heavenly body is
its distance above the horizon, and its _zenith distance_ is its
distance from the zenith. Both the altitude and the zenith distance of a
body are measured on the vertical circle which passes through the body.
The altitude and zenith distance of a heavenly body are complements of
each other.
10. _Azimuth and Amplitude.--Azimuth_ is distance measured east or west
from the meridian. When a heavenly body lies north of the prime
vertical, its azimuth is measured from the meridian on the north; and,
when it lies south of the prime vertical, its azimuth is measured from
the meridian on the south. The azimuth of a body can, therefore, never
exceed 90°. The azimuth of a body is the angle which the plane of the
vertical circle passing through it makes with that of the meridian.
The _amplitude_ of a body is its distance measured north or south from
the prime vertical. The amplitude and azimuth of a body are complements
of each other.
11. _Alt-azimuth Instrument._--An instrument for measuring the altitude
and azimuth of a heavenly body is called an _alt-azimuth_ instrument.
One form of this instrument is shown in Fig. 16. It consists essentially
of a telescope mounted on a vertical circle, and capable of turning on a
horizontal axis, which, in turn, is mounted on the vertical axis of a
horizontal circle. Both the horizontal and the vertical circles are
graduated, and the horizontal circle is placed exactly parallel with the
plane of the horizon.
When the instrument is properly adjusted, the axis of the telescope will
describe a vertical circle when the telescope is turned on the
horizontal axis, no matter to what part of the heavens it has been
pointed.
The horizontal and vertical axes carry each a pointer. These pointers
move over the graduated circles, and mark how far each axis turns.
To find the _azimuth_ of a star, the instrument is turned on its
vertical axis till its vertical circle is brought into the plane of the
meridian, and the reading of the horizontal circle noted. The telescope
is then directed to the star by turning it on both its vertical and
horizontal axes. The reading of the horizontal circle is again noted.
The difference between these two readings of the horizontal circle will
be the azimuth of the star.
[Illustration: Fig. 16.]
To find the _altitude_ of a star, the reading of the vertical circle is
first ascertained when the telescope is pointed horizontally, and again
when the telescope is pointed at the star. The difference between these
two readings of the vertical circle will be the altitude of the star.
12. _The Vernier._--To enable the observer to read the fractions of the
divisions on the circles, a device called a _vernier_ is often employed.
It consists of a short, graduated arc, attached to the end of an arm _c_
(Fig. 17), which is carried by the axis, and turns with the telescope.
This arc is of the length of _nine_ divisions on the circle, and it is
divided into _ten_ equal parts. If 0 of the vernier coincides with any
division, say 6, of the circle, 1 of the vernier will be 1/10 of a
division to the left of 7, 2 will be 2/10 of a division to the left of
8, 3 will be 3/10, of a division to the left of 9, etc. Hence, when 1
coincides with 7, 0 will be at 6-1/10; when 2 coincides with 8, 0 will
be at 6-2/10; when 3 coincides with 9, 0 will be at 6-3/10, etc.
[Illustration: Fig. 17.]
To ascertain the reading of the circle by means of the vernier, we first
notice the zero line. If it exactly coincides with any division of the
circle, the number of that division will be the reading of the circle.
If there is not an exact coincidence of the zero line with any division
of the circle, we run the eye along the vernier, and note which of its
divisions does coincide with a division of the circle. The reading of
the circle will then be the number of the first division on the circle
behind the 0 of the vernier, and a number of tenths equal to the number
of the division of the vernier, which coincides with a division of the
circle. For instance, suppose 0 of the vernier beyond 6 of the circle,
and 7 of the vernier to coincide with 13 of the circle. The reading of
the circle will then be 6-7/10.
13. _Hour Circles._--Great circles perpendicular to the celestial
equator are called _hour circles_. These circles all pass through the
poles of the heavens, as shown in Fig. 18. _EQ_ is the celestial
equator, and _P_ and _P'_ are the poles of the heavens.
The point _A_ on the equinoctial (Fig. 19) is called the _vernal
equinox_, or the _first point of Aries_. The hour circle, _APP'_, which
passes through it, is called the _equinoctial colure_.
[Illustration: Fig. 18.]
14. _Declination and Right Ascension._--The _declination_ of a heavenly
body is its distance north or south of the celestial equator. The _polar
distance_ of a heavenly body is its distance from the nearer pole.
Declination and polar distance are measured on hour circles, and for the
same heavenly body they are complements of each other.
[Illustration: Fig. 19.]
The _right ascension_ of a heavenly body is its distance eastward from
the first point of Aries, measured from the equinoctial colure. It is
equal to the arc of the celestial equator included between the first
point of Aries and the hour circle which passes through the heavenly
body. As right ascension is measured eastward entirely around the
celestial sphere, it may have any value from 0° up to 360°. Right
ascension corresponds to longitude on the earth, and declination to
latitude.
15. _The Meridian Circle._--The right ascension and declination of a
heavenly body are ascertained by means of an instrument called the
_meridian circle_, or _transit instrument_. A side-view of this
instrument is shown in Fig. 20.
[Illustration: Fig. 20.]
It consists essentially of a telescope mounted between two piers, so as
to turn in the plane of the meridian, and carrying a graduated circle.
The readings of this circle are ascertained by means of fixed
microscopes, under which it turns. A heavenly body can be observed with
this instrument, only when it is crossing the meridian. For this reason
it is often called the _transit circle_.
To find the declination of a star with this instrument, we first
ascertain the reading of the circle when the telescope is pointed to the
pole, and then the reading of the circle when pointed to the star on its
passage across the meridian. The difference between these two readings
will be the polar distance of the star, and the complement of them the
declination of the star.
To ascertain the reading of the circle when the telescope is pointed to
the pole, we must select one of the circumpolar stars near the pole, and
then point the telescope to it when it crosses the meridian, both above
and below the pole, and note the reading of the circle in each case. The
mean of these two readings will be the reading of the circle when the
telescope is pointed to the pole.
16. _Astronomical Clock._--An _astronomical clock_, or _sidereal clock_
as it is often called, is a clock arranged so as to mark hours from 1 to
24, instead of from 1 to 12, as in the case of an ordinary clock, and so
adjusted as to mark 0 when the vernal equinox, or first point of Aries,
is on the meridian.
As the first point of Aries makes a complete circuit of the heavens in
twenty-four hours, it must move at the rate of 15° an hour, or of 1° in
four minutes: hence, when the astronomical clock marks 1, the first
point of Aries must be 15° west of the meridian, and when it marks 2,
30° west of the meridian, etc. That is to say, by observing an accurate
astronomical clock, one can always tell how far the meridian at any time
is from the first point of Aries.
17. _How to find Right Ascension with the Meridian Circle._--To find the
right ascension of a heavenly body, we have merely to ascertain the
exact time, by the astronomical clock, at which the body crosses the
meridian. If a star crosses the meridian at 1 hour 20 minutes by the
astronomical clock, its right ascension must be 19°; if at 20 hours, its
right ascension must be 300°.
To enable the observer to ascertain with great exactness the time at
which a star crosses the meridian, a number of equidistant and parallel
spider-lines are stretched across the focus of the telescope, as shown
in Fig. 21. The observer notes the time when the star crosses each
spider-line; and the mean of all of these times will be the time when
the star crosses the meridian. The mean of several observations is
likely to be more nearly exact than any single observation.
[Illustration: Fig. 21.]
[Illustration: Fig. 22.]
18. _The Equatorial Telescope._--The _equatorial_ telescope is mounted
on two axes,--one parallel with the axis of the earth, and the other at
right angles to this, and therefore parallel with the plane of the
earth's equator. The former is called the _polar axis_, and the latter
the _declination axis_. Each axis carries a graduated circle. These
circles are called respectively the _hour circle_ and the _declination
circle_. The telescope is attached directly to the declination axis.
When the telescope is fixed in any declination, and then turned on its
polar axis, the line of sight will describe a diurnal circle; so that,
when the tube is once directed to a star, it can be made to follow the
star by simply turning the telescope on its polar axis.
In the case of large instruments of this class, the polar axis is
usually turned by clock-work at the rate at which the heavens rotate; so
that, when the telescope has once been pointed to a planet or other
heavenly body, it will continue to follow the body and keep it steadily
in the field of view without further trouble on the part of the
observer.
The great Washington Equatorial is shown in Fig. 22. Its object-glass is
26 inches in diameter, and its focal length is 32-1/2 feet. It was
constructed by Alvan Clark & Sons of Cambridge, Mass. It is one of the
three largest refracting telescopes at present in use. The Newall
refractor at Gateshead, Eng., has an objective 25 inches in diameter,
and a focal length of 29 feet. The great refractor at Vienna has an
objective 27 inches in diameter. There are several large refractors now
in process of construction.
[Illustration: Fig. 23.]
19. _The Wire Micrometer._--Large arcs in the heavens are measured by
means of the graduated circles attached to the axes of the telescopes;
but small arcs within the field of view of the telescope are measured by
means of instruments called _micrometers_, mounted in the focus of the
telescope. One of the most convenient of these micrometers is that known
as the _wire micrometer_, and shown in Fig. 23.
The frame _AA_ covers two slides, _C_ and _D_. These slides are moved by
the screws _F_ and _G_. The wires _E_ and _B_ are stretched across the
ends of the slides so as to be parallel to each other. On turning the
screws _F_ and _G_ one way, these wires are carried apart; and on
turning them the other way they are brought together again. Sometimes
two parallel wires, _x_ and _y_, shown in the diagram at the right, are
stretched across the frame at right angles to the wires _E_, _B_. We may
call the wires _x_ and _y_ the _longitudinal_ wires of the micrometer,
and _E_ and _B_ the _transverse_ wires. Many instruments have only one
longitudinal wire, which is stretched across the middle of the focus.
The longitudinal wires are just in front of the transverse wires, but do
not touch them.
To find the distance between any two points in the field of view with a
micrometer, with a single longitudinal wire, turn the frame till the
longitudinal wire passes through the two points; then set the wires _E_
and _B_ one on each point, turn one of the screws, known as the
_micrometer screw_, till the two wires are brought together, and note
the number of times the screw is turned. Having previously ascertained
over what arc one turn of the screw will move the wire, the number of
turns will enable us to find the length of the arc between the two
points.
The threads of the micrometer screw are cut with great accuracy; and the
screw is provided with a large head, which is divided into a hundred or
more equal parts.
These divisions, by means of a fixed pointer, enable us to ascertain
what fraction of a turn the screw has made over and above its complete
revolutions.
20. _Reflecting Telescopes._--It is possible to construct mirrors of
much larger size than lenses: hence reflecting telescopes have an
advantage over refracting telescopes as regards size of aperture and of
light-gathering power. They are, however, inferior as regards
definition; and, in order to prevent flexure, it is necessary to give
the speculum, or mirror, a massiveness which makes the telescope
unwieldy. It is also necessary frequently to repolish the speculum; and
this is an operation of great delicacy, as the slightest change in the
form of the surface impairs the definition of the image. These defects
have been remedied, to a certain extent, by the introduction of
silver-on-glass mirrors; that is, glass mirrors covered in front with a
thin coating of silver. Glass is only one-third as heavy as
speculum-metal, and silver is much superior to that metal in reflecting
power; and when the silver becomes tarnished, it can be removed and
renewed without danger of changing the form of the glass.
_The Herschelian Reflector._--In this form of telescope the mirror is
slightly tipped, so that the image, instead of being formed in the
centre of the tube, is formed near one side of it, as in Fig. 24. The
observer can then view it without putting his head inside the tube, and
therefore without cutting off any material portion of the light. In
observation, he must stand at the upper or outer end of the tube, and
look into it, his back being turned towards the object. From his looking
directly into the mirror, it is also sometimes called the _front-view_
telescope. The great disadvantage of this arrangement is, that the rays
cannot be brought to an exact focus when they are thrown so far to one
side of the axis, and the injury to the definition is so great that the
front-view plan is now entirely abandoned.
[Illustration: Fig. 24.]
_The Newtonian Reflector._--The plan proposed by Sir Isaac Newton was to
place a small plane mirror just inside the focus, inclined to the
telescope at an angle of 45°, so as to throw the rays to the side of the
tube, where they come to a focus, and form the image. An opening is made
in the side of the tube, just below where the image is formed; and in
this opening the eye-piece is inserted. The small mirror cuts off some
of the light, but not enough to be a serious defect. An improvement
which lessens this defect has been made by Professor Henry Draper. The
inclined mirror is replaced by a small rectangular prism (Fig. 25), by
reflection from which the image is formed very near the prism. A pair of
lenses are then inserted in the course of the rays, by which a second
image is formed at the opening in the side of the tube; and this second
image is viewed by an ordinary eye-piece.
[Illustration: Fig. 25.]
_The Gregorian Reflector._--This is a form proposed by James Gregory,
who probably preceded Newton as an inventor of the reflecting telescope.
Behind the focus, _F_ (Fig. 26), a small concave mirror, _R_, is placed,
by which the light is reflected back again down the tube. The larger
mirror, _M_, has an opening through its centre; and the small mirror,
_R_, is so adjusted as to form a second image of the object in this
opening. This image is then viewed by an eye-piece which is screwed into
the opening.
[Illustration: Fig. 26.]
_The Cassegrainian Reflector._--In principle this is the same with the
Gregorian; but the small mirror, _R_, is convex, and is placed inside
the focus, _F_, so that the rays are reflected from it before reaching
the focus, and no image is formed until they reach the opening in the
large mirror. This form has an advantage over the Gregorian, in that the
telescope may be made shorter, and the small mirror can be more easily
shaped to the required figure. It has, therefore, entirely superseded
the original Gregorian form.
[Illustration: Fig. 27.]
Optically these forms of telescope are inferior to the Newtonian; but
the latter is subject to the inconvenience, that the observer must be
stationed at the upper end of the telescope, where he looks into an
eye-piece screwed into the side of the tube.
On the other hand, the Cassegrainian Telescope is pointed directly at
the object to be viewed, like a refractor; and the observer stands at
the lower end, and looks in at the opening through the large mirror.
This is, therefore, the most convenient form of all in management.
[Illustration: Fig. 28.]
The largest reflecting telescope yet constructed is that of Lord Rosse,
at Parsonstown, Ireland. Its speculum is 6 feet in diameter, and its
focal length 55 feet. It is commonly used as a Newtonian. This telescope
is shown in Fig. 27.
The great telescope of the Melbourne Observatory, Australia, is a
Cassegranian reflector. Its speculum is 4 feet in diameter, and its
focal length is 32 feet. It is shown in Fig. 28.
[Illustration: Fig. 29.]
The great reflector of the Paris Observatory is a Newtonian reflector.
Its mirror of silvered glass is 4 feet in diameter, and its focal length
is 23 feet. This telescope is shown in Fig. 29.
21. _The Sun's Motion among the Stars._--If we notice the stars at the
same hour night after night, we shall find that the constellations are
steadily advancing towards the west. New constellations are continually
appearing in the east, and old ones disappearing in the west. This
continual advancing of the heavens towards the west is due to the fact
that the sun's place among the stars is _continually moving towards the
east_. The sun completes the circuit of the heavens in a year, and is
therefore moving eastward at the rate of about a degree a day.
[Illustration: Fig. 30.]
This motion of the sun's place among the stars is due to the revolution
of the earth around the sun, and not to any real motion of the sun. In
Fig. 30 suppose the inner circle to represent the orbit of the earth
around the sun, and the outer circle to represent the celestial sphere.
When the earth is at _E_, the sun's place on the celestial sphere is at
_S'_. As the earth moves in the direction _EF_, the sun's place on the
celestial sphere must move in the direction _S'T_: hence the revolution
of the earth around the sun would cause the sun's place among the stars
to move around the heavens in the same direction that the earth is
moving around the sun.
22. _The Ecliptic._--The circle described by the sun in its apparent
motion around the heavens is called the _ecliptic_. The plane of this
circle passes through the centre of the earth, and therefore through the
centre of the celestial sphere; the earth being so small, compared with
the celestial sphere, that it practically makes no difference whether we
consider a point on its surface, or one at its centre, as the centre of
the celestial sphere. The ecliptic is, therefore, a great circle.
The earth's orbit lies in the plane of the ecliptic; but it extends only
an inappreciable distance from the sun towards the celestial sphere.
[Illustration: Fig. 31.]
23. _The Obliquity of the Ecliptic._--The ecliptic is inclined to the
celestial equator by an angle of about 23-1/2°. This inclination is
called the _obliquity of the ecliptic_. The obliquity of the ecliptic is
due to the deviation of the earth's axis from a perpendicular to the
plane of its orbit. The axis of a rotating body tends to maintain the
same direction; and, as the earth revolves around the sun, its axis
points all the time in nearly the same direction. The earth's axis
deviates about 23-1/2° from the perpendicular to its orbit; and, as the
earth's equator is at right angles to its axis, it will deviate about
23-1/2° from the plane of the ecliptic. The celestial equator has the
same direction as the terrestrial equator, since the axis of the heavens
has the same direction as the axis of the earth.
[Illustration: Fig. 32.]
Suppose the globe at the centre of the tub (Fig. 31) to represent the
sun, and the smaller globes to represent the earth in various positions
in its orbit. The surface of the water will then represent the plane of
the ecliptic, and the rod projecting from the top of the earth will
represent the earth's axis, which is seen to point all the time in the
same direction, or to lean the same way. The leaning of the axis from
the perpendicular to the surface of the water would cause the earth's
equator to be inclined the same amount to the surface of the water, half
of the equator being above, and half of it below, the surface. Were the
axis of the earth perpendicular to the surface of the water, the earth's
equator would coincide with the surface, as is evident from Fig. 32.
[Illustration: Fig. 33.]
24. _The Equinoxes and Solstices._--The ecliptic and celestial equator,
being great circles, bisect each other. Half of the ecliptic is north,
and half of it is south, of the equator. The points at which the two
circles cross are called the _equinoxes_. The one at which the sun
crosses the equator from south to north is called the _vernal_ equinox,
and the one at which it crosses from north to south the _autumnal_
equinox. The points on the ecliptic midway between the equinoxes are
called the _solstices_. The one north of the equator is called the
_summer_ solstice, and the one south of the equator the _winter_
solstice. In Fig. 33, _EQ_ is the celestial equator, _EcE'c'_ the
ecliptic, _V_ the vernal equinox, A the autumnal equinox, Ec the winter
solstice, and _E'c'_ the summer solstice.
[Illustration: Fig. 34.]
25. _The Inclination of the Ecliptic to the Horizon._--Since the
celestial equator is perpendicular to the axis of the heavens, it makes
the same angle with it on every side: hence, at any place, the equator
makes always the same angle with the horizon, whatever part of it is
above the horizon. But, as the ecliptic is oblique to the equator, it
makes different angles with the celestial axis on different sides; and
hence, at any place, the angle which the ecliptic makes with the horizon
varies according to the part which is above the horizon. The two extreme
angles for a place more than 23-1/2° north of the equator are shown in
Figs. 34 and 35.
The least angle is formed when the vernal equinox is on the eastern
horizon, the autumnal on the western horizon, and the winter solstice on
the meridian, as in Fig. 34. The angle which the ecliptic then makes
with the horizon is equal to the elevation of the equinoctial _minus_
23-1/2°. In the latitude of New York this angle = 49° - 23-1/2° =
25-1/2°.
[Illustration: Fig. 35.]
The greatest angle is formed when the autumnal equinox is on the eastern
horizon, the vernal on the western horizon, and the summer solstice is
on the meridian (Fig. 35). The angle between the ecliptic and the
horizon is then equal to the elevation of the equinoctial _plus_
23-1/2°. In the latitude of New York this angle = 49° + 23-1/2° =
72-1/2°.
Of course the equinoxes, the solstices, and all other points on the
ecliptic, describe diurnal circles, like every other point in the
heavens: hence, in our latitude, these points rise and set every day.
26. _Celestial Latitude and Longitude._--_Celestial latitude_ is
distance measured north or south from the ecliptic; and _celestial
longitude_ is distance measured on the ecliptic eastward from the vernal
equinox, or the first point of Aries. Great circles perpendicular to the
ecliptic are called _celestial meridians_. These circles all pass
through the poles of the ecliptic, which are some 23-1/2° from the poles
of the equinoctial. The latitude of a heavenly body is measured by the
arc of a celestial meridian included between the body and the ecliptic.
The longitude of a heavenly body is measured by the arc of the ecliptic
included between the first point of Aries and the meridian which passes
through the body. There are, of course, always two arcs included between
the first point of Aries and the meridian,--one on the east, and the
other on the west, of the first point of Aries. The one on the _east_ is
always taken as the measure of the longitude.
27. _The Precession of the Equinoxes._--The equinoctial points have a
slow westward motion along the ecliptic. This motion is at the rate of
about 50'' a year, and would cause the equinoxes to make a complete
circuit of the heavens in a period of about twenty-six thousand years.
It is called the _precession of the equinoxes_. This westward motion of
the equinoxes is due to the fact that the axis of the earth has a slow
gyratory motion, like the handle of a spinning-top which has begun to
wabble a little. This gyratory motion causes the axis of the heavens to
describe a cone in about twenty-six thousand years, and the pole of the
heavens to describe a circle about the pole of the ecliptic in the same
time. The radius of this circle is 23-1/2°.
[Illustration: Fig. 36.]
28. _Illustration of Precession._--The precession of the equinoxes
may be illustrated by means of the apparatus shown in Fig. 36. The
horizontal and stationary ring _EC_ represents the ecliptic; the
oblique ring _E'Q_ represents the equator; _V_ and _A_ represent the
equinoctial point, and _E_ and _C_ the solstitial points; _B_
represents the pole of the ecliptic, _P_ the pole of the equator,
and _PO_ the celestial axis. The ring _E'Q_ is supported on a pivot
at _O_; and the rod _BP_, which connects _B_ and _P_, is jointed at
each end so as to admit of the movement of _P_ and _B_.
On carrying _P_ around _B_, we shall see that _E'Q_ will always
preserve the same obliquity to _EC_, and that the points _V_ and _A_
will move around the circle _EC_. The same will also be true of the
points _E_ and _C_.
29. _Effects of Precession._--One effect of precession, as has already
been stated, is the revolution of the pole of the heavens around the
pole of the ecliptic in a period of about twenty-six thousand years. The
circle described by the pole of the heavens, and the position of the
pole at various dates, are shown in Fig. 37, where o indicates the
position of the pole at the birth of Christ. The numbers round the
circle to the left of o are dates A.D., and those to the right of o are
dates B.C. It will be seen that the star at the end of the Little Bear's
tail, which is now near the north pole, will be exactly at the pole
about the year 2000. It will then recede farther and farther from the
pole till the year 15000 A.D., when it will be about forty-seven degrees
away from the pole. It will be noticed that one of the stars of the
Dragon was the pole star about 2800 years B.C. There are reasons to
suppose that this was about the time of the building of the Great
Pyramid.
A second effect of precession is the shifting of the signs along the
zodiac. The _zodiac_ is a belt of the heavens along the ecliptic,
extending eight degrees from it on each side. This belt is occupied by
twelve constellations, known as the _zodiacal constellations_. They are
_Aries_, _Taurus_, _Gemini_, _Cancer_, _Leo_, _Virgo_, _Libra_,
_Scorpio_, _Sagittarius_, _Capricornus_, _Aquarius_, and _Pisces_. The
zodiac is also divided into twelve equal parts of thirty degrees each,
called _signs_. These signs have the same names as the twelve zodiacal
constellations, and when they were first named, each sign occupied the
same part of the zodiac as the corresponding constellation; that is to
say, the sign Aries was in the constellation Aries, and the sign Taurus
in the constellation Taurus, etc. Now the signs are always reckoned as
beginning at the vernal equinox, which is continually shifting along the
ecliptic; so that the signs are continually moving along the zodiac,
while the constellations remain stationary: hence it has come about that
the _first point of Aries_ (the _sign_) is no longer in the
_constellation_ Aries, but in Pisces.
[Illustration: Fig. 37.]
Fig. 38 shows the position of the vernal equinox 2170 B.C. It was then
in Taurus, just south of the Pleiades. It has since moved from Taurus,
through Aries, and into Pisces, as shown in Fig. 39.
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
Since celestial longitude and right ascension are both measured from the
first point of Aries, the longitude and right ascension of the stars are
slowly changing from year to year. It will be seen, from Figs. 38 and
39, that the declination is also slowly changing.
30. _Nutation._--The gyratory motion of the earth's axis is not
perfectly regular and uniform. The earth's axis has a slight tremulous
motion, oscillating to and fro through a short distance once in about
nineteen years. This tremulous motion of the axis causes the pole of the
heavens to describe an undulating curve, as shown in Fig. 40, and gives
a slight unevenness to the motion of the equinoxes along the ecliptic.
This nodding motion of the axis is called _nutation_.
[Illustration: Fig. 40.]
31. _Refraction._--When a ray of light from one of the heavenly bodies
enters the earth's atmosphere obliquely, it will be bent towards a
perpendicular to the surface of the atmosphere, since it will be
entering a denser medium. As the ray traverses the atmosphere, it will
be continually passing into denser and denser layers, and will therefore
be bent more and more towards the perpendicular. This bending of the ray
is shown in Fig. 41. A ray which started from _A_ would enter the eye at
_C_, as if it came from _I_: hence a star at _A_ would appear to be at
_I_.
[Illustration: Fig. 41.]