Denison Olmsted.

A compendium of astronomy; containing the elements of the science, familiarity explained and illustrated, with the latest discoveries. Adapted to the use of schools and academies, and of the general reader online

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Online LibraryDenison OlmstedA compendium of astronomy; containing the elements of the science, familiarity explained and illustrated, with the latest discoveries. Adapted to the use of schools and academies, and of the general reader → online text (page 1 of 22)
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Telescopic view of the Moon.

Telescopic view of the Moon when five days old.















, 1852.




Entered according to Act of Congress, in the year 1839, by


in the Clerk's office, of the District Court of Connecticut.


THIS small volume is intended to afford to the General
Reader, and to the more advanced pupils of our Schools and
Academies, a comprehensive outline of Astronomy with its
latest discoveries. For its perusal, no further acquaintance
with mathematics is necessary, than a knowledge of common
arithmetic ; although some slight knowledge, at least, of ge-
ometry an4 trigonometry will prove very useful.

By omitting mathematical formulae, and employing much
familiar illustration, we have endeavored to bring the leading
facts and doctrines of this noble and interesting science, within
the comprehension of every attentive and intelligent reader.
In no science, more than in this, are greater advantages to be
derived from a lucid arrangement an order which brings out
every fact and doctrine of the science, just in the place where
the mind is ready to receive it. A certain maturity of mind,
and power of reflection, are, however, indispensable for under-
standing this science. Astronomy is no study for children.
Let them be employed on subjects more suited to the state of.
their capacities, until those faculties are more fully developed,
which will enable them to learn to conceive correctly of the
celestial motions. A work on Astronomy that is very easy,
must be very superficial, and will be found to enter little into
the arcana of the science. The riches of this mine lie deep ;
and no one can acquire them, who is either incompetent or
unwilling to penetrate beneath the surface.

Although -this treatise is based on the larger work of the
author, (" Introduction to Astronomy,") prepared for the stu-
dents of Yale College, yet it is not merely an abridgment of
that. It contains much original matter adapted to the pecu-
liar exigencies of the class of readers for whom it is intended.
The few passages taken verbatim from astronomical writers,

O f\ f\


are not, as in the larger work, always accredited to their re-
spective authors, as this was deemed unimportant in a work
of this description.

It is strongly recommended to all who study this science,
even in its most elementary form, early to commence learning
the names of the constellations, and of the largest of the in-
dividual stars, in the order in which they are described in the
last part of the work. A celestial globe will be found a most
useful auxiliary in this as in every other part of Astronomy.
If it cannot supersede, it may greatly aid reflection. The
reader also should, if in his power, take frequent opportunities
of viewing the heavenly bodies through the telescope. This
will add much to his intelligence, and increase his interest in
the study.


SINCE the stereotype edition of this work was first pub-
lished, several new and interesting discoveries have been added
to Astronomy, an account of which will be found in the Sup-
plement. They make no change in the great facts and doc-
trines of the science, but these remain unaltered and immu-
table ; while the new discoveries extend still further our
knowledge of the Universe. We have, therefore, no occasion
to alter the text, except perhaps very slightly in one or two
statements, but by giving whatever is new and important in
the form of a supplement, (to which we may add as every
successive discovery is made,) we shall endeavor to secure to
this treatise the freshness and accuracy of the most recent
compilations, as well as furnish to the schools what has been
thoroughly tested and approved by the most able teachers of
the Union.


Preliminary Observations, - - Page


Chapter I. Of the Figure and Dimensions of the Earth,

and the Doctrine of the Sphere, ... 5
Chapter II. Of the Diurnal Revolution Artificial

Globes, - i^l

Chapter III. Of Parallax, Refraction, and Twilight, 36
Chapter IV. Of Time, - - 45

Chapter V. Of Astronomical Instruments Figure

and Density of the Earth, - 51


Chapter I. Of the Sun Solar Spots Zodiacal Light, 70
Chapter II. Of the Apparent Annual Motion of the

Sun Seasons Figure of the Earth's Orbit, - 79
Chapter III. Of Universal Gravitation Kepler's

Laws, Motion in an Elliptical Orbit Precession

of the Equinoxes, - - 91

Chapter IV. Of the Moon Phases, Revolutions, - 110
Chapter V. Of Eclipses, - - 137

Chapter VI. Of Longitude Tides, - 150

Chapter VII. Of the Planets the Inferior Planets,

Mercury and Venus, - 1 67

Chapter VIII. Of the Superior Planets Mars, Jupiter,

Saturn and Uranus Ceres, Pallas, Juno and Vesta, 183



Chapter IX. Of the Motions of the Planetary System
Quantity of Matter in the Sun and Planets
Stability of the Solar System, - - 205

Chapter X. Of Comets, - - - 218


Chapter I. Of the Fixed Stars Constellations, - 235

Chapter II. Of Clusters of Stars Nebulae Variable

Stars Temporary Stars Double Stars, - - 247

Chapter III. Of the Motions of the Fixed Stars Dis-
tances Nature, - ... - 255

Chapter IV. Of the System of the World, - - 265



1. ASTRONOMY is that science which treats of the heav-
enly bodies.

More particularly, its object is to teach what is known
respecting the Sun, Moon, Planets, Comets, and Fixed
Stars ; and also to explain the methods by which this
knowledge is acquired.

Astronomy is sometimes divided into Descriptive,
Physical, and Practical. Descriptive Astronomy re-
spects facts ; Physical Astronomy, causes ; Practical As-
tronomy, the means of investigating the facts, whether
by instruments, or by calculation. It is the province of
Descriptive Astronomy to observe, classify, and record,
all the phenomena of the heavenly bodies, whether per-
taining to those bodies individually, or resulting from
their motions and mutual relations. It is the part of
Physical Astronomy to explain the causes of these phe-
nomena by investigating and applying the general laws
on which they depend ; especially by tracing out all the
consequences of the law of universal gravitation. Prac-
tical Astronomy lends its aid to both the other depart-

2. Astronomy is the most ancient of all the sciences.
At a period of very high antiquity, it was cultivated in
Egypt, in Chaldea, and in India. Such knowledge of
the heavenly bodies as could be acquired by close and
long continued observation, without the aid of instru-

1 - Define Astronomy. What does it teach ? Name the three
parta into which it is divided. What does Descriptive Astron-
omy respect ? What does Physical Astronomy ? What does
Practical Astronomy ? What is the peculiar province of each ?


ments, was diligently amassed ; and tables of the celes-
tial motions were constructed, which could be used in
predicting eclipses, and other astronomical phenomena.

About 500 years before the Christain era, Pythago-
ras, of Greece, taught astronomy at the celebrated school
at Crotona, (a Greek town on the southeastern coast of
Italy,) and exhibited more correct views of the nature
of the celestial motions, than were entertained by any
other astronomer of the ancient world. His views, how-
ever, were not generally adopted, but lay neglected for
nearly 2000 years, when they were revived and estab-
lished by Copernicus and Galileo. The most celebrated
astronomical school of antiquity, was at Alexandria in
Egypt, which was established and sustained by the Ptol-
emies, (Egyptian princes,) 300 years before the Chris-
tian era. The employment of instruments for measur-
ing angles, and bringing in trigonometrical calculations
to aid the naked powers of observation, gave to the Alex-
andrian astronomers great advantages over all their pre-

The most able astronomer of the Alexandrian school
was Hipparchus, who was distinguished above all the
ancients for the accuracy of his astronomical measure-
ments and determinations. The knowledge of astron-
omy possessed by the Alexandrian school, and recorded
in the Almagest, or great work of Ptolemy, constituted
the chief of what was known of our science during the
middle ages, until the fifteenth and sixteenth centuries,
when the labors of Copernicus of Prussia, Tycho Brake

2. Trace the history of Astronomy. Among what ancient
nations was it cultivated ? What kind of knowledge of the
heavenly bodies was amassed ? Who was Pythagoras? When
and where did he live ? Where was his school ? How correct
were his views ? Were they generally adopted ? Give an ac-
count of the Alexandrian school. When was it established and
by whom ? What gave it great advantages over all its prede-
cessors ? Give some account of Hipparchus of Ptolemy of
Copernicus of Tycho Brahe of Kepler of Galileo of
Newton of La Place. Specify the respective labors of each


of Denmark, Kepler of Germany, and Galileo of Italy,
laid the solid foundations of modern astronomy. Coper-
nicus expounded the true system of the world, or the
arrangement and motions of the heavenly bodies ; Ty-
cho Brahe carried the use of instruments, and the art of
astronomical observation, to a far higher degree of accu-
racy than had ever been done before ; Kepler discovered
the great laws which regulate the movements of the
planets ; and Galileo, having first enjoyed the aid of the
telescope, made innumerable discoveries in the solar
system. Near the beginning of the eighteenth century,
Sir Isaac Newton discovered, in the law of universal
gravitation, tho great principle mat explains the causes
of all celestial phenomena ; and recently, La Place has
more fully completed what Newton begun, having fol-
lowed out all the consequences of the law of universal
gravitation, in his great work, the Mecanique Celeste.

3. Among the ancients, astronomy was studied chiefly
as subsidiary to astrology. Astrology was the art of di-
vining future events by the stars. It was of two kinds,
natural and judicial. Natural Astrology, aimed at pre-
dicting remarkable occurrences in the natural world, as
eathquakes, volcanoes, tempests, and pestilential dis-
eases. Judicial Astrology, aimed at foretelling the fates
of individuals, or of empires.

4. Astronomers of every age, have been distinguished
for their persevering industry, and their great love of ac-
curacy. They have uniformly aspired to an exactness
in their inquiries, far beyond what is aimed at in most
geographical investigations, satisfied with nothing short
of numerical accuracy wherever this is attainable ; and
years of toilsome observation, or laborious calculation,
have been spent with the hope of attaining a few se-

3. Define Astrology. What was Natural and what Judicial
Astrology ?

4. What is said of the industry and accuracy of astrono-
mers ? Can this science be taught by artificial aids alone ?


conds nearer to the truth. Moreover, a severe but de
lightful labor is imposed on all, who would arrive at a
clear and satisfactory knowledge of the subject of astron-
omy. Diagrams, artificial globes, orreries, and familiar
comparisons and illustrations, proposed by the author or
the instructor, may afford essential aid to the learner,
but nothing can convey to him a perfect comprehension
of the celestial motions, without much diligent study
and reflection.

5. In this treatise, we shall for the present assume the
Copernican system as the true system of the world,
postponing the discussion of the evidence on which it
rests to a late period, when the learner has been made ex-
tensively acquainted with astronomical facts. This sys-
tem maintains (1,) That, the apparent diurnal revolution
of the heavenly bodies, from east to west, is owing to
the real revolution of the earth dn its own axis from
west to east, in the same time ; and (2,) That the sun
is the center around which the earth and planets all re-
volve from west to east, contrary to the opinion that the
earth is the center of motion of the sun and planets.

5. What system is assumed as the true system of the world ?
Specify the two leading points in the Copernican system. ,




6. The figure of the earth is nearly globular. This
fact is known, first, by the circular form of its shadow
cast upon the moon in a lunar eclipse ; secondly, from
analogy, each of the other planets being seen to be
spherical ; thirdly, by our seeing the tops of distant ob-
jects while the other parts are invisible, as the topmast
of a ship, while either leaving or approaching the shore,
or the lantern of a light-house, which when first descried
at a distance at sea, appears to glimmer upon the very
surface of the water ; fourthly, by the testimony of nav-
igators who have sailed around it ; and, finally, by ac-
tual observations and measurements, made for the ex-
press purpose of ascertaining the figure of the earth, by
means of which astronomers are enabled to compute the
distances from the center of the earth of various places
on its surface, which distances are found to be nearly

The effect of the rotundity of the earth upon the ap-
pearance of a ship, when either leaving or approaching
the spectator, is illustrated by Fig. 1.

As light proceeds in straight lines, it is evident that,
if the earth is round, the top of the ship ought to come
into view before the lower parts, when the ship is ap-
proaching the spectator at A, and to remain longest iii
view when the ship is leaving^him. But, were the eartL

6. What is the figure of the earth"? Enumerate the various
proofs of its rotundity.

a continued plane, then the spectator would see all parts
of the ship at the same time, as is represented in the an-
nexed figure.

Fig. 2.

7. The foregoing considerations show that the form
of the earth is spherical ; but more exact determinations
prove, that the earth, though nearly globular, is not ex-
actly so ; its diameter from the north to the south pole
is about 26 miles less than through the equator, giving
to the earth the form of an oblate spheroid, or a flattened
sphere resembling an orange. We shall reserve the ex-


planations of the methods by which this fact is estab-
lished, until the learner is better prepared than at present
to understand them.

The mean or average diameter of the earth, is 7912.4
miles, a measure which the learner should fix in his
memory as a standard of comparison in astronomy, and
of which he shoulc 1 endeavor to form the most adequate
conception in his power. The circumference of the
earth is about 25,000 miles. Although the surface of
the earth is uneven, sometimes rising in high mountains,
and sometimes descending in deep valleys, yet these ele-
vations and depressions are so small in comparison with
the immense volume of the globe, as hardly to occasion
any sensible deviation from a surface uniformly curvi-
linear. The irregularities of the earth's surface, in this
view, are no greater than the rough points on the rind
of an orange, which do not perceptibly interrupt its con-
tinuity ; for the highest mountain on the globe is only
about five miles above the general level ; and the deep-
est mine hitherto opened is only about half a mile.*

~ T~W or a ^ out one sixteen hundredth part

of the whole diameter, an inequality which, in an arti-
ficial globe of eighteen inches diameter, amounts to only
the eighty eighth part of an inch.

8. The greatest difficulty in the way of acquiring
correct views in astronomy, arises from the erroneous
notions that pre-occuoy the mind. To divest himself

7. What is the exact figure of the earth ? How much greater
is its diameter through the equator than through the poles ?,
What is the mean average diameter of the earth ? What is its
circumference 1 Do the inequalities on the earth's surface af-
fect its rotundity ? To what may these be compared ? How
high is the highest mountain above the general level ? How
deep is the deepest mine ? To how much would this amount
on an artificial globe eighteen inches in diameter ?

* Sir John Herschel.


of these, the learner should conceive of the earth as a
huge globe occupying a small portion of space, and en-
circled on all sides with the starry sphere. He should
free his mind from its habitual proneness to consider one
part of space as naturally up and another down, and
view himself as subject to a force which binds him to
the earth as truly as though he were fastened to it by
some invisible cords or wires, as the needle attaches it-
self to all sides of a spherical loadstone. He should

Fig. 3.

dwell on this point until it appears to him as truly up in
the direction of BB, CC, DD, (Fig. 3,) when he is at
B, C, and D, respectively, as in the direction *AA, when
he is at A.


9. The definitions of the different lines, points, and
circles, which are used in astronomy, and the proposi-
tions founded upon them, compose the Doctrine of the

8. Whence arises the greatest difficulty in acquiring correct
views in astronomy ? How should the learner conceive of
the earth ? Illustrate by figure 3.

9. Doctrine of the sphere define it.


10. A section of a sphere by a plane cutting it in any
manner, is a circle. Great circles are those which pass
through the center of the sphere, and divide it into two
equal hemispheres : Small circles, are such as do not
pass through the center, but divide the sphere into two
unequal parts. Every circle, whether great or small, is
divided into 360 equal parts called degrees. A degree,
therefore, is not any fixed or definite quantity, but only
a certain aliquot part of any circle.*

The axis of a circle, is a straight line passing through
its center at right angles to its plane.

Fig. 4.

* As this work may be read by some who are unacquainted with
even the rudiments of geometry, we annex a few particulars respecting
angular measurements.

A line drawn from the center to the circumference of a circle is
called a radius, as CD, fig. 4.

Any part of the circumference of a circle is called an arc, as AB,
or BD.

An angle 'is measured by the
arc included between two radii.
Thus, in the annexed figure, the
angle contained between the two
radii CA and CB, that is, the an-
gle ACB, is measured by the arc
AB. But this arc is the same part
of the smaller circle that EF is of
the greater. The arc AB there-
fore contains the same number of
degrees as the arc EF, and either
may be taken for the measure of-
the angle ACB. As the whole
circle contains 360, it is evident
that the quarter of a circle, or'quad-

rant ABD, contains 90, and the
semicircle ABDG contains 180.
r The complement of an arc or an-
gle,^ what it wants of 90. Thus BD is the complement of AB, and
AB is the complement of BD. If AB denotes a certain number of de-
grees of latitude, BD will be the complement of the latitude or the co-
latitude, as it is commonly written.

The supplement of an arc or angle, is what it wants of 180.
Thus BA is the supplement of GDB, and GDB, is the supplement
of BA. If BA were 20 of longitude, GDB its supplement would
be 160.

An angle is said to be subtended by the side which is opposite to it.
Thus in the triangle ACK, the angle at C is subtended by the side AK,
the angle at A by CK, and the angle at K by CA. In like manner a
side is said to be subtended by an angle, as AK by the angle at C.



The pole of a great circle, is the point on the sphere
where its axis cuts through the sphere. Every great
circle has two poles, each of which is every where 90
from the great circle.

All great circles of the sphere cut each other in two
points diametrically opposite, and consequently, their
points of section are 180 apart.

A great circle which passes through the pole of an-
other great circle, cuts the latter at right angles.

The great circle which passes through the pole of an-
other great circle and is at right angles to it, is called a
secondary to that circle.

The angle made by two great circles on the surface
of the sphere, is measured by the arc of another great
circle, of which the angular point is the pole, being the
arc of that great circle intercepted between those two

11. In order to fix the position of any plane, either on
the surface of the earth or in the heavens, both the earth
and the heavens are conceived to be divided into sepa-
rate portions by circles, which are imagined to cut
through them in various ways. The earth thus inter-
sected is called the terrestrial, and the heavens the ce-
lestial sphere. The learner will remark, that these cir-
cles have no existence in nature, but are mere land-
marks, artificially contrived for convenience of refer-

10. What figure is produced by the section of a sphere?
Define great circles. Define small circles. Into how many
degrees is every circle divided ? Is a degree any fixed or defi-
nite quantity ? What is the axis of a circle ? What is the pole
of a circle? How do all great circles cut each other? How
is a great circle cut by another great circle passing through its
pole ? What is the secondary of a circle ? How is the angle
made by two great circles on the surface of the sphere measured?

11. How are the earth and the heavens conceived to be di-
vided ? What constitutes the terrestrial sphere ? What the
celestial ? Have these circles any existence in nature ? In
what do the heavenly bodies appear to be fixed ?


ence. On account of the immense distance of the heav-
enly bodies, they appear to us, wherever we are placed,
to be fixed in the same concave surface, or celestial
vault. The great circles of the globe, extended every
way to meet the concave surface of the heavens, become
circles of the celestial sphere.

12. The Horizon is the great circle which divides
the earth into upper and lower hemispheres, and sepa-
rates the visible heavens from the invisible. This is
the rational horizon. The sensible horizon, is a circle
touching the earth at the place of the spectator, and is
bounded by the line in which the earth and skies seem
to meet. The sensible horizon is parallel to the ra-
tional, but is distant from it by the semi-diameter of the
earth, or nearly 4,000 miles. Still, so vast is the dis-
tance of the starry sphere, that both these planes appear
to cut that sphere in the same line ; so that we see the
same hemisphere of stars that we should see if the up-
per half of the earth were removed, and we stood on the
rational horizon.

13. The poles of the horizon are the zenith and na-
dir. The Zenith is the point directly over bur head,
and the Nadir that directly under our feet. The plumb
line is in the axis of the horizon, and consequently di-
rected towards its poles.

Every place on the surface of the earth has its own
horizon ; and the traveller has a new horizon at every
step, always extending 90 degrees from him in all di-

12. Define the horizon. Distinguish between the rational
and the sensible horizon. What is the distance between the
sensible and rational horizons ? How do both appear to cut
the starry heavens ?

13. What are the poles of the horizon ? Define the zenith.
Define the nadir. How is the plumb line situated with respect
*o the horizon? How manv horizons are there on the earth 1


14. Vertical circles are those which pass through the
ooles of the horizon, perpendicular to it.

The Meridian is that vertical circle which passes
through the north and south points.

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Online LibraryDenison OlmstedA compendium of astronomy; containing the elements of the science, familiarity explained and illustrated, with the latest discoveries. Adapted to the use of schools and academies, and of the general reader → online text (page 1 of 22)