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degrees from either node at the time of oppor sition, she will pass through a greater or less portion of the earth's shadow, according to her situation And as the sun commonly passes by the nodes but twice in a year; and the moon's orbit contains 360 degrees, (of which 17, the limit of solar eclipses on either side of the nodes, and 12 the limit of lunar eclipses, are but small portions) it is obvious that there must be many new and full moons without any eclipse
Having already had occasion to mention the nodes of the moon, it may be advisable in this place to explain the term to our young readers. The moon's orbit intersects the ecliptic in two points diametrically opposite to each other, and these intersections are called the nodes. The ascending node, called the dragon's head, is the northern intersection of the moon's orbit with the ecliptic, and is marked thus by astronomers, the descending node, or dragon's tail, is the southern intersection, and is marked thus, &.
Supposing the line of the nodes to be always carried parallel to itself round the sun, there would be exactly half a year between the conjunctions of the sun and nodes. But as the nodes shift toward the west, or contrary to the annual motion of the globe, about nineteen degrees and one third in a year, the same node will come round to the sun about nineteen days sooner every year than on the preceding one: hence, from the time of the
ascending node passing by the sun, as seen from the earth, there will be only 173 days before the descending node passes that luminary: at whatever time of the year, therefore, we have eclipses about either of the nodes, we may expect to have eclipses about the other node in a hundred and seventy-three days afterwards.
The nodes shifting through all the signs and degrees of the ecliptic in 18 years, 225 days, there would always be a regular period of eclipses, if any complete number of lunations were finished without a fraction-This, however, never occurs; but in 223 mean lunations, or about 18 years and 10 days after the sun, moon, and nodes, have been once in a line of conjunction, they return so nearly to the same state again that the same node which was in conjunction with the sun and moon at the commencement of the first of these lunations, will be within less than half a degree of a line of conjunction with the sun and moon again, when the last of these lunations is completed. In that time, therefore there will be a regular period of eclipses, or returns of the same eclipses, for many centuries. The falling back, however, of the line of conjunction of the sun and moon, with regard to the line of the nodes, in every period, will eventually exhaust it and after that it will not return again in less than twelve thousand four hundred and ninety-two years.
Those eclipses of the sun which happen about the ascending node, and begin to come in at the north pole of the earth, will go a little southerly at each return, till they go quite off the earth at the opposite pole; and those which happen about the descending node, and begin to come in at the south pole of the earth, will go a little northerly at each return, till at last they quite leave the earth at the north pole.
Every variety of eclipse that can possibly occur, either with respect to the sun or moon, is owing to the elliptical figure of their orbits, and the position they are in when the eclipse happens.
When the moon changes at her least distance from the earth, and is within the proper limits of the node, she will appear large enough to cover the whole disk of the sun: and those inhabitants of our globe where her shadow falls will have the sun entirely hid from their view for some minutes. But when the moon changes at a greater distance from the earth, and is sufficiently near to the node, her diameter will subtend a less angle than that of the sun; consequently, her dark shadow must terminate in a point before it reaches the earth; and at the place over which it hangs, the sun's edge will appear like a luminous ring encircling the body of the moon.
An eclipse of the former kind is called a total, and of the latter an annular oneAnd as the apparent diameter of the moon,
when largest, exceeds that of the sun when least, by only one minute and a half a degree in the greatest solar eclipse that can possibly happen, the total darkness will continue no longer than while the moon passes through a minute and a half of her orbit from the sur; which she describes in about three minutes of time. But when the change happens within seventeen degrees of the node and the moon is at her mean distance, the point of her shadow will just reach the earth, and on the small spot where it falls the darkness can be only momentary.
A total eclipse of the sun may be justly considered as one of the most curious and most solemn spectacles in nature: for at this time the darkness is intense, the stars appear, and the brute creation, as well as superstitious or ignorant observers, seem struck with terror; but its duration, in general, is but for an instant, and, at the utmost, continues little more than three minutes of time.-It may, also, be remarked, that such eclipses occur but seldom at any particular place, and annular ones are still more uncommon.
In total eclipses of the moon, that planet is seldom invisible, but generally appears of a dusky hue, like the colour of tarnished copper.
This appearance has been thought to result from the moon's native light; but it should rather be referred to the scattered beams of the sun, which are so bent into the earth's shadow, in passing through the atmosphere, as to afford
us a sufficient portion of light to render the moon visible. There have, indeed, been some eclipses in which the moon, (being in that part of her orbit nearest the earth,) has totally disappeared; but such instances are rarely heard Hevelius mentions one eclipse of this kind, which happened on the 25th of April, 1642, when it was impossible to discover the place of the moon, even with the assistance of a telescope, although the sky was sufficiently clear to discover stars of the fifth magnitude.
Our readers will easily perceive from what has been said, that a total eclipse of the moon may occur, although she be not exactly in either of the nodes at the time of such eclipse; for as the diameter of the earth's shadow is considerably larger than that of the moon, she may, of course, be wholly involved in the dark cone, without passing directly through its axis. The moon, also, may be at such a distance from the node, that only a portion of her body can enter the earth's shadow, and in that case, there may be a partial eclipse of the moon, which will be greater or less according to her situation. But when the full moon happens exactly in one of the nodes, then the axis of the earth's shadow will pass through the centre of the moon, and the eclipse will consequently be total and central.
As the diameters of the sun and moon are divided by astronomers into twelve equal parts, an eclipse is said to be of so many