ILLUSTRATED ASTRONOMY.

ON XXXVI.

CLIPSES.

e?

erception of the sun's rays by is opposite to the sun.

, with respect to the body eclipsed? ar and lunar.

?

he sun.

?

eclipse of the sun?

e moon's passing between the ing its shadow upon the earth.

f the sun take place?

at new moon.

he moon. (Fig. 3.)

of the moon?

he moon's passing through the -)

of the moon take place?

at full moon. (Fig. 3 and 4.) d, with respect to the amount eclipsed? ial.

the whole of the sun or moon.

only a part of the sun or moon.

ose?

the central part of the sun, when e earth, that the edge of the sun tring around it. (Fig. 9, Note.) of the sun at every new moon?

ECLIPSES.

ALL opake bodies cast a shadow when the rays from any luminous body fall upon them. Every primary and secondary planet in the solar system casts a shadow towards that point of the heavens which If the sun were smaller than the earth, the earth's shadow would increase in diameter as the distance increases from the earth, (See Fig. 1;) but if the sun and earth were of the same size, the shadow would be of the same size, no matter how great the distance from the earth, (See Fig. 2.) But as the sun is immensely larger than the earth, the earth's shadow terminates in a point at about 850,000 miles from the earth; the length of the earth's shadow is, however, subject to considerable variation. When the earth is nearest to the sun, which takes place about January 1st, the shadow is much shorter than when the earth is at its greatest distance, which is about the 1st of July. The moon revolves around the earth in about 29 days, from one new moon to another. If the moon passed at every new moon exactly between the centres of the sun and earth, we should have a great eclipse of the sun at every new moon, and a total eclipse of the moon at every full moon, (See Fig. 3;) but the moon's orbit or path makes an angle with the plane of the ecliptic, (the plane of the ecliptic is described by a line drawn from the centre of the sun, passing through the centre of the earth and extended, to the heavens,) of about 54 degrees, consequently one half of the moon's orbit is above the ecliptic, and the other half is below it.

The two opposite points where the moon's orbit cuts the plane of the ecliptic, are called the moon's nodes; the nodes do not keep in the same position with respect to the earth and sun, but have a retrograde motion of about 19 degrees in a year. This causes the moon at new moon to be too high or too low, so that the moon's shadow passes above the north pole or below the south pole, hence there is no eclipse, and at full moon, the moon passes either above or below the earth's shadow. A total eclipse of the moon occurs when the whole of the moon is immersed in the earth's shadow, (See Fig. 3;) but we occasionally have a partial eclipse of the moon which is caused by the moon's being so high or so low as only to be partially immersed in the earth's shadow, (See Fig. 4.) The diameter of the sun and moon's discs is divided into twelve equal parts, called digits (See Fig. 6;) but by inspecting the diagram, it will be seen that when the sun is said to have six digits eclipsed, that only about one third of the disc of the sun is covered by the moon, although one half of the diameter of the sun is hidden from view. The sun and moon appear to be about the same size, but the apparent size of both is subject to some variation: when the earth is in that point of its orbit nearest the sun (January 1st,) the sun appears larger than at any other time during the year, and, when the moon is at the greatest distance from the earth, she appears the smallest. If an eclipse of the sun should take place exactly at this time, the shadow of the moon would terminate in a point before it reached the earth, and the moon would not appear large enough to cover the whole disc of the sun, but would produce what is called an annular eclipse, or the sun would appear like a luminous ring around the moon, (See Fig. 9;) but if the earth was at its greatest distance from the sun (July 1st) and the moon the nearest to the earth, then the moon would appear larger than the sun, and the shadow of the moon would touch the earth before it terminated in a point; this would produce a total and as great an eclipse of the sun as can take place, (See Fig. 3.) A total eclipse of the sun is visible only to a small portion of the earth at one and the same time, the shadow of the moon where it touches the earth would be only about 150 miles in diameter, consequently there would be only & space across the earth from west to east about 150 miles wide, in which it would appear total, but, a partial eclipse would be seen from a space more than 2,000 miles wide on each side of t of the apparent diameter of the umbra, or dark shadow. Those who lived north of the dark (Fig. 6.)

eclipse of the sun at every new moon? 1oon, the moon is generally too shadow to fall upon the earth.

f the moon at every full moon?
ull moon the moon generally
he earth's shadow.
earth's shadow ?

s. (Note. This is the mean or

moon's shadow?

es. (Note. This is the mean or

aber of eclipses that can take place in un and two of the moon.

of eclipses that can take place in a st be of the sun.

shadow would see the southern portion of the sun eclipsed, and those who lived south of it, would see the northern limb of the sun eclipsed.

Eclipses of the sun are more frequent than of the moon, because the sun's ecliptic limits are greater than the moon's, yet we have more visible eclipses of the moon than of the sun, because eclipses of the moon are visible from all parts of the earth where the moon is above the horizon, and are equally great to each of those parts; but eclipses of the sun are visible only to those places upon which the moon's ||

CLIPSES.-Continued.

ILLUSTRATED ASTRONOMY.

Q. What are the effects of a total eclipse of the Sun ?

A. The heavens are shrouded in darkness, so that the stars and planets become visible; the animal tribes become agitated; and a general gloom overspreads the landscape.

Q. How long can a total eclipse of the Sun continue at any one place on the earth?

A. A total eclipse of the Sun cannot continue at any one place over four minutes.

Q. Why is the dark portion of the New Moon visible when seen in the west soon after the Sun is set?

A. Because the rays of the Sun falling upon the
Earth are reflected back upon the dark portion of the
Moon and render it faintly visible, see Fig. 4. [This
appearance of the Moon at new moon is sometimes
called." the Old Moon in the New Moon's arms."]

Q. Are the Stars or planets ever hidden from our view by the Moon?
A. They frequently are, which is called the Occul-

hadow surrounding the Umbra, Fig. tation of a star or planet.

nt of the Moon's Umbra upon the Earth when

exceed 175 miles in diameter, and ch less, see Fig. 3.

s of the Sun always begin on the western side

ECLIPSES.

ECLIPSES are among the most interesting phenomena presented to us by the heavenly bodies. In all ages, when an eclipse has taken place, it has excited the profound attention of the learned, and the fears and superstitions of the ignorant. The causes of eclipses before the seventeenth century were known only to a few, and they generally took advantage of this knowledge to impose upon the credulity of the ignorant by pretending that they were inspired by the Gods. Among the ancient nations, the Chaldeans were the foremost in their observations of the phenomena of the heavens; per

Moon passes from west to east be-haps this was owing in some measure to their occupation; They being shepherds were nd Sun.

the Moon begin on the eastern side of the Moon? Moon passes from west to east 's shadow, see Fig. 1.

does the Moon's shadow pass over the Earth

er the earth from west to east.

obliged to watch their flocks by night to protect them from the wild beasts which were
at that time numerous. Men under such circumstances would naturally be led to watch
closely the movements of the heavenly bodies, and more especially so; for in the
earlier periods of the world they had no correct mode of reckoning time in order to
determine the seasons or the proper seed time and harvest.

Eclipses attracted the particular attention of the Chaldeans, and by a series of ob-
servations extended through several centuries, they discovered a very important fact
relating to eclipses although they did not understand the cause.

By comparing the records which had been made for a great length of time, they found that a certain period of time elapsed between eclipses of the same kind and magnitude; that is, if 18 years, 11 days, 7 hours and 43 minutes, were added to the

tion of the Earth's surface may be covered by time of the happening of any eclipse, it would show the time of the return of the same

ut 4,393 miles in diameter. eclipses of the Sun total?.

Moon being so far from the Earth, ates in a point before it reaches the

f its orbit must the Moon be to cause a total

t or near its perigee, when the Um-
would reach the Earth, see Fig. 3.
an eclipse of the Moon continue?
tinue four hours; in this case the
directly through the centre of the
ee Fig. 1.

visible when it is totally eclipsed?

eclipse; the only differences would be that it would not happen at the same time in
the day and it would be a little greater or less than the previous eclipse-thus they
were able to predict eclises with sufficient accuracy to answer their designs upon the
ignorant without understanding the laws by which these periodical returns were
produced.

To explain this briefly, it must be remembered that the moon's orbit makes an angle
with the plane of the earth's orbit of 50; these two points where the moon's orbit

cuts the plane of the earth's orbit, are called nodes, (see diagram, greatest number of
eclipses in one year, page 45). Now we will suppose that on any day at noon it is new
moon, and the moon is just 16° degrees from her descending node, the shadow of the
moon would just touch the earth at the south pole; in 223 lunations or 18 years, 11 days,
7 hours, 43 minutes thereafter, the moon would come nearly to the same position as it
was at the beginning, consequently there would be another small eclipse of the Sun,
and at the expiration of every 223 lunations it would return, and at each return the
moon's shadow would pass across the earth a little more to the north until the eclipse
had appeared about 77 times, when it would pass off at the north pole, occupying a

period of 1388 years: The same period would not commence again until the expiration

of 12492 years. Each eclipse which takes place during any year, belongs to a separate

and similar period. Those periods of eclipses which come in at the Moon's ascending

node, first come on to the earth at the north pole and at each return the moon's shadow
passes across the earth more to the south and after appearing about 77 times they finally
leave the earth at the south pole.

In those periods of eclipses of the moon which come in at the moon's descending
pears
of a reddish color like tarnish-node, the moon first touches the upper portion of the earth's shadow and at each return

the moon passes through the earth's shadow more to the south and at the thirty-second
return the moon would pass directly through the centre of the earth's shadow, produc-
ing a great eclipse of the moon; also in those eclipses of the moon which come in at
the moon's ascending node, the moon first comes in contact with the lower portion of
the earth's shadow and at each return the moon passes through the shadow more to the
north and goes through a similar course as in the former case; it must be remembered
that there are a number of e
each year, the greatest number is seven the least

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EARTH'S

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EARTH'S PENUN