ASTRONOMICAL NOTES FOR OCTOBER, 1883. 357 
great the distance from the earth. But as the Sun is immensely larger than the 
earth, the earth's shadow terminates in a point at about 850,000 miles distant; 
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 ist, the 
shadow is much shorter than when the earth is at its greatest distance, which is 
about the ist of July. The Moon revolves around the earth in about twenty-nine 
and one-half days, from one new moon to another. If the Moon passed at every 
new moon exactly between the centres of the Sun and the earth, we would have 
a great eclipse of the Sun at every new moon, and a total eclipse of the Moon at 
every full moon, 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 5J^°, 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° 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 shadow of the earth, but we oc- 
casionally have a partial eclipse of the Moon {as the one above), which is caused 
by the Moon's being so high or low as to be only partially immersed in the shadow 
of the earth. The Sun and Moon appear to be about the same size, but the appar- 
ent size of both is subject to some variation. Eclipses of the Sun are more fre- 
quent than of the Moon, because the ecliptic Hmits of the Sun are greater than 
the Moon's, yet we have more visible eclipses of the Moon than of the Sun, because 
ecHpses 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 these parts ; but eclipses of 
the Sun are visible only at those places upon which the Moon's shadow falls. 
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 ; perhaps this was owing in some measure to their 
occupation. They being shepherds, were obliged to watch their flocks by night to 
protect them from the wild beasts which were at that time very numerous. 
The Chaldeans, by a series of observations extending through several cent- 
uries, discovered a very important fact in relation to eclipses, although they did 
not understand the cause. By comparing the records for many years past they 
VII— 23 
