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must appear dark, and make so much of the sun's disc appear so, as is hid from us by her interposition.
What is called an eclipse of the sun, is, therefore, in reality, an eclipse of the earth, which is deprived of the sun's light, by the moon's coming between, and casting a shadow upon it. The earth being a globe, only that half of it, which at any time is turned towards the sun, can be enlightened by him at that time; it is upon some part of this enlightened half of the earth, that the moon's shadow, or penumbra, falls in a solar eclipse.
The sun is always in the plane of the ecliptic; but the moon being inclined to this plane, and only coinciding with it at the nodes, it will not cover either the whole or a part of the sun ; or, in other words, the sun will not be eclipsed, unless the moon at that time is in or near one of her nodes.
The moon, however, cannot be directly between the sun and us, unless they are both in the same part of the heavens ; that is, unless they are in conjunction. Therefore, the sun can never be eclipsed but at the new-moon, nor even then, unless the moon at that time is in or near one of her nodes.
From hence it is easy to shew, that the darkness of our Saviour's crucifixion, was not owing to an eclipse of the sun. For the crucifixion happened at the time of the Jewish passover, and the passover, by the appointment of the law, was to be celebrated at the full-moon; the sun could not, therefore, be eclipsed at the time of the passover. An intelligent tutor will find many opportunities of observing to his
pupil, that nature, and philosophy which explains the phenomena of nature, do always agree with divine revelation,
The moon being much smaller than the earth, and having a conical shadow, because she is less than the sun, can only cover a sinall part of the earth by her shadow; though, as we have observed before, the whole body of the moon inay be involved in that of the earth. Hence an eclipse of the sun is visible but to a few inhabitants of the earth ; whereas an eclipse of the moon may be seen by all those that are on that hemisphere which is turned towards it. In other words, as the moon can never totally eclipse the earth, there will be many parts of the globe that will suffer no eclipse, though the sun be above their horizon.
An eclipse of the sun always begins on the western, and ends on the eastern side; because the moon moving in her orbit from west to east, necessarily first arrives at and touches the sun's western limb, and goes off at the eastern.
It is not necessary, in order to constitute a central eclipse of the sun, that the moon should be exactly in the line of the nodes, at the time of its conjunction ; for it is sufficient to denominate an eclipse of the sun central, that the centre of the moon be directly between the centre of the sun, and the eye of the spectator ; for to him, the sun is then centrally eclipsed. But as the shadow of the moon can cover but a small portion of the earth, it is obvious this may happen when the moon is not in one of her
nodes. Further, the sun may be eclipsed centrally, totally, partially, and not at all, at the same time.
A total eclipse of the sun is a very curious spectacle : Clavius says that, in that which he observed in Portugal, in 1650, the obscurity was greater, or more sensible than that of the night: the largest stars made their appearance for about a minute or two, and the birds were so terrified, that they fell to the ground.
Thus, in plate 10, fig. 3 ; let ABC be the sun; MN the moon ; hlg part of the cone of the moon's shadow ; fd the penumbra of the moon : from this figure it is easy to perceive,
1. That those parts of the earth that are within the circle represented by gh, are covered by the shadow of the moon, so that no rays can come from any part of the sun into that circle, on account of the interposition of the moon.
2. In those parts of the earth where the penumbra falls, only part of the sun is visible; thus, between d and
8 the parts of the sụn near C cannot be seen, the rays coming from thence towards d'or g being intercepted by the moon ; whereas, at the same time, the parts between f and h are illuminated by rays coming from C, but are deprived by the moon of such as come from A.
3. The nearer any part of the earth, within the penumbra, is to the shadow of the moon, as in places near g, h, or h, the less portion of the sun is visible to its inhabitants ; the nearer it is to the outside of the
penumbra, as towards d, e, or f, the greater portion of the sun may
be seen. 4. Out of the penumbra, the entire disc of the
sun is visible,
OF THE LIMITS OF SOLAR. AND LUNAR
The distance of the moon, in degrees and minutes, above or below the ecliptic line, is called her latitude. If she be above the ecliptic, she is said to have north; if below it, south latitude.
If the latitude at any time exceed the sum of the semidiameter of the moon, equal to 16 min. and the earth's shadow equal to 454 minutes, the moon at that time cannot be eclipsed ; but will either pass under or over the shadow, according as she happens to be above or below the ecliptic line.
The distance from the node, either before or after it, corresponding to the above extent, is about 12 degrees, which is consequently the limit of lunar eclipses : for when a full-moon happens within 12 degrees of the nodes, she will be eclipsed; and the nearer to the nodes, the greater will the eclipse be.
If at the new-moon, the latitude exceeds the sum of the semidiameters of the sun 164 minutes, and of the moon 163 minutes, we should see no eclipse of the sun from the centre of the earth. But as we view the luminaries from the surface, which is much
higher, we are obliged to take in the semidiameter of the earth as seen from the moon. Then, if the latitude of the moon be greater than the sum of these three numbers, 94 minutes, the sun will not be eclipsed; for the moon will pass either over or under his dise, according as she is above or below the ecliptic line. The distance from the node on either side agreeing to the above-mentioned extent, is the 18 degrees, which is the utmost limit of solar eclipses ; whence it follows, that if the sun and moon, at the time of new-moon, happen to be within 18 degrees of the node, the sun will be eclipsed.
OF THE PERIOD OF ECLIPSES.
If the places of the moon's nodes were fixed, eclipses would always happen nearly at the same time of the year; but as they have a motion of about 3 min. 11 sec, every day backwards, or contrary to the order of the signs, the succeeding eclipse must recede likewise; and in one revolution of the nodes, which is completed in 18 years, 224 days, 3 hours, they will revolve in a retrograde manner through the year, and return to the same places again.
But there is a more correct period, called the Chaldean Saros, which is 18 years, 11 days, 7 hours, 43 min.; for, in that time, the sun and moon advance just as far beyond a complete direct revolution in the ecliptic, as the nodes want of com