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CHAPTER VIII.

Eclipses.

An eclipse is a partial or total obscuration of a heavenly body.

So far as astronomical observation has extended, the Sun is the only heavenly luminary in the solar system, that shines by its own light. The planets are in themselves opaque, and shine only by reflecting the solar rays. Hence on the side of these not illuminated by the Sun, dark shadows are cast. These shadows are in the form of vast cones, extending into the heavens. They are but privations of light in the space hid from the Sun. That they are not coextensive with the Sun's light, but terminate at a distance far more limited, is evident, because the primary planets never eclipse each other. Mars, though often in opposition to the Sun, is never eclipsed by the Earth's shadow. This must therefore terminate before it reaches that planet. (Plate vi. Fig. 8.). Let S be the Sun, A E the Earth, A B E the Earth's dark shadow, terminating at B. From this figure it is evident, that, when a luminous body is larger than a dark body, intercepting its rays, and causing a shadow, such shadow must end at the point where the rays from the extremes of the luminous body cross each other beyond the dark body; and that, as the Sun is far larger than the planets of our system, their shadows must terminate at points beyond the planets opposite to the Sun, at the intersection of the solar rays. The primary planets eclipse their secondaries, and the secondaries their primaries. The Earth's shadow eclipses the Moon; the Moon's shadow the Earth. But, when the Earth is eclipsed by the Moon, the Sun is darkened to

some of the inhabitants of the Earth. Hence eclipses of the Earth are usually denominated eclipses of the Sun.

The shadow of the Earth, when longest, is about 219 of its semi-diameters. Different computations make a trifling difference in the mean extent of this shadow. If the diameters of the Earth and Sun be taken as before stated, and the shadow be computed from these, it will be found to be about( 217 semi-diameters of the Earth ; equal to 864,094 miles

If the Moon revolved in the plane of the ecliptic, an eclipse would happen at every conjunction and opposition, or at every change and full. (But, her orbit being inclined to that circle in an angle of 50 94 3“, varying a little at different times, eclipses cannot happen except when she is in or about her nodes. In every other

part of her orbit, she is either too far north or south to eclipse the Sun, or to fall into the Earth's shadow and be herself eclipsed. Plate vi. Fig. 11. represents the number of digits eclipsed up to 12 on the right hand, where the eclipse, being at the node, is total. The limit is different in different species of eclipses. For if the Moon be within about 170 of either of her nodes at the change, there will be a solar eclipse. But lunar eclipses can happen but when she is within about 11° of her nodes. The greatest limit in solar eclipses, according to the tables in the author's larger work, is 18° 11', the least, 16° 28'; the greatest in lunar, 11° 51', the least, 100 11'.

In lunar eclipses, when a part only of the Moon's disk is covered, the eclipse is denominated partial ; when the whole disk is covered, total when the centre of the disk passes through the centre of the shadow, central. (Plate vi. Fig. 10.)

The Moon is visible, when totally immersed in the Earth's shadow, appearing of a dusky red color, like burnished copper. It is probable, that the refracted rays of the Sun cause this phenomenon. These, trav

ersing the atmosphere of the Earth, are by it turned inward, so as to fall on the Moon, and render her distinctly to be seen.

In a lunar eclipse, all to whom the Moon is visible, see her in the same instant of absolute time.

Solar eclipses are much more frequent than lunar; but most of the former are invisible at any particular part of the Earth.

The dark shadow of the Moon sometimes reaches to the Earth, eclipsing a small portion of its surface ; sometimes that dark shadow is terminated before it arrives at the Earth. In the latter case, the Sun, at the centre of an eclipse, appears like a luminous ring. The eclipse is then called annular. (Plate v. Fig. 5.) This beautiful phenomenon was seen in some parts of New England on the morning of April 3, 1791; at Washington, September 17, 1811 ; and in the eastern parts of the United States, February 12th of the year 1831. The dark shadow of the Moon is longest, when she is in perigee and the Earth in aphelion ; shortest, when she is in apogee, and the Earth in perihelion. The inhabitants of our republic have had the satisfaction of viewing two annular eclipses, since the commencement of the present century; one, September 17, 1811, the other, February 12, 1831. According to computation, they will have the pleasure of seeing another, September 18, 1838; the annular eclipses being three for the century.

Two total solar eclipses are computed for the United States during the century ; one, June 16, 1806, the other, August 7, 1869. It will appear from this, and from inspection of the tables of the semi-diameters of the Sun and Moon, that annular eclipses of the Sun are more frequent than total eclipses of the same luminary.

The Moon's partial shadow is called her penumbra. All the inhabitants over whom this shadow extends, see the Sun partially eclipsed. In Plate vi. Fig. 10.

a b c d represent the Moon's penumbra ; the arch b d, its extent on the Earth. The darkness of the

penumbra decreases, as it diverges from the dark shadow of the Moon. The motion of the dark shadow and penumbra over the Earth is nearly from west to east; except at the polar regions, when they sometimes pass in an opposite direction.

The whole number of eclipses in any one year is never less than two, nor more than seven : when two, both are of the Sun ; when seven, four are of the Sun, three of the Moon.*

The line of the Moon's nodes has a constant motion from east to west, or backwards in the ecliptic; making a complete revolution in 18 y. 223 d. 20 h. 13 m. 32 s. In a year of 365 days, its motion is 19° 19' 43'', completing a revolution in 18 y. 224 d. 4 h. 53 m. when leap year is four times taken ; in 18 y. 223 d. 4 h. 53 m. when leap year is five times included. By the retrograde motion of the nodes, either of them is brought round to the Sun, or passes from the Sun to the Sun again, in 346 d. 14 h. 52 m. 14 s. on a mean. Half of this time only intervenes between one node and the other passing the Sun. When eclipses happen at the ascending node, other eclipses may be expected at the descending node in about 173 d.; and, after a lapse of the same time, at the ascending node, thus continuing in rotation.

When the Sun and Moon have been in conjunction with the Moon's ascending or descending node, they will be in conjunction again within 28' 12" of the same node, after 223 mean lunations. Thus is formed a regular period of eclipses. It is completed in 18 y. 11 d. 7 h. 43m. 19 s. when 'leap year is four times included; 18 y. 10 d. 7 h. 43 m. 19 s. when leap year is

* In some books it is stated, that, when there are seven eclipses in a year, five are of the Sun. Such an event seems barely possible. Should it ever happen, two of them must be very slight, the penumbra just touching the pole.

five times included. There is a regular series of returns to each eclipse.' Eclipses at the ascending node first strike the Earth at the north, and pass off at the south pole, moving a little southward at each return. Eclipses at the descending node commence at the south, and retire at the north pole. After an eclipse has completed a series, and left the Earth, it will not again return and commence a new series at the same node, till after an absence of more than 12,000 years. The eclipses commencing at one pole are equal in number to those commencing at the other. The irregular motion of the Earth and Moon may accelerate or retard the commencement of a series about one hundred years. In one series an eclipse may visit the Earth but seventy times; it will not surpass seventy-seven times. When an eclipse returns but seventy times, it will occupy about 1,262 years; when it returns seventy-seven times, it will require 1,388 years. The memorable eclipse of June 16, 1806, was total to a large part of New England. It happened at the Moon's descending node. Having traversed the mighty void from the creation, it first met the south pole on the morning of the 16th of March, O. S. 1049, at 10 h. 11 m. 39 s. Each visit has shown it a little farther north. The last return was June 24, 1824. It happened in the evening, the Sun going down a little eclipsed at Washington. It will again visit the Earth, July 8th, 1842 But, being at 2 h. 2 m. 2 s. in the morning, at Washington, it will be invisible in the United States; but will be large and total over a wide extent of the eastern continent. This eclipse will leave the Earth at, the north pole on the 11th of May, in the year 2347, N. S. of the Christian era..

The dark shadow of the Moon, when longest, and falling directly on the Earth, extends about 107 miles. In most cases, however, it falls obliquely; in some, very obliquely, when it may cover an extent of more than 900 miles.

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