SOLAR ECLIPSE. The phenomena of an eclipse of the sun resemble those of the moon in one respect only, namely, that the body of the luminary disappears. in all other respects there is so great a difference, both in the cause of the phenomenon and its attendant circumstances, that it is a pity one term, eclipse, should be used in senses so different. In the first place, the disappearance of the moon arises from the earth intercepting the light which she ought to receive, while that of the sun is the consequence of the moon coming between the sun and the earth. The body of the moon is never absolutely hidden, and is even slightly visible through a telescope during the darkest eclipse : but the body of the sun is really hidden by the inter vention of the opaque substance of the moon. Again, tho phenomena of an eclipse of the moon are the same for every point of the earth at which they are visible : the beginning, middle, and end of the phenomenon happen at the same instant of absolute time everywhere, and the same portion of the moon is hidden from all the earth at the same instant But in a solar eclipse, it entirely depends upon the position of the spectator whether there is any eclipse at all ; and of two persons at different parts of the earth, st the same instant, one may see the sun totally eclipsed, while the other may, by the bright ness of the ann's rays, not know that the moon is almost close to him. A screen held before a candle may be an eclipse of the candle for one person in the room, but not for another, on account of their difference of place ; this is an illustration of the solar eclipse : a ball thrown into a dark corner may be invisible to all the persona in a room at the same time ; this is the same illustration of a lunar eclipse.
If the earth had no motion of rotation, the inhabitants of any one place would see something exactly resembling a lunar eclipse; the sun being in place of the moon, and the moon in place of the earth's shadow. But different places would see different kinds of eclipses, some losing more of the sun's body, and others less. The rotation of the earth, without materially altering the character of the phenomenon, makes it much mote difficult to calculate: for it is as if each spot of the earth, instead of standing still to witness one phenomenon, or one simple eclipse, were constantly taking into view portions of different phenomena, a part of one followed by a subsequent part of another.
In an eclipse of the moon, whatever may be the phase for the time being, from the disappearance of the first to the reappearance of the last edge, the only question as to whether such phase will be visible or not at any place is the following :—Will the moon be above the horizon at that place when the phase occurs I Suppose, for instance, it were asked, what places on the earth will see the beginning of the eclipse, the disappearance of one edge of the at the instant when ' the moon rises / The answer is, calculate the absolute instant of the beginning of the eclipse, find out the spot to which the moon is vertical at that instant, and all places 90° distant from it will be exactly in the same predicament with respect to the eclipse. But hi an eclipse of the
sun, the beginning at two different places does not happen at the same instant; the inhabitants of any the same circle see very different phases, and a line drawn through all the places which see the same sort of phase under the same position of the luminaries with respect to their horizons, will be very different from a circle. Without attempting to give any account of the modes of ascertaining all these points, we subjoin, from the' Nautical Almanac,' a projection of the eclipse which took place on the 7th of July, 1342.
The southern line passes through all the places which see a simple contact of the luminaries and nothing more : the edges of the lumina ries unite for a moment and then separate. This line touches the two ends of a large figure of eight divided by another line passing through its loop ; and the portion of the earth which at any time sees a total eclipse is contained in the broad shaded band. On the line marked "beginning of eclipse at sunrise" live those to whom the lumina ries rise in contact : the other lines are similarly explained. The eclipse is first seen at the place marked "First contact" on the line just mentioned. One point is marked as having both beginning, middle, and end of the eclipse at sunrise : this means that the eclipse is there only a contact, so that its beginning, middle, and end take place at the same moment, and that moment is sunrise. At the loop of the figure of eight, the beginning, middle, and end are represented as each of them taking place both at sunrise and sunset : which must be a mystery to those who are not used to trace mathematical conceptions to their limits : are there two eclipses, one for sunrise and one for sunset I The explanation is this : there is at every moment of time a point in the arctic regions at which the sun is making its first appearance or its last appearance previously or subsequent to the long polar clay or night. As this moment approaches the days shorten, if the disappearance be coming on, and begin from nothing if the appearance be coming on : the long day or night being preceded by the ordinary days or nights of the rest of the earth. Now the point which is at the loop is that point of the earth at which the sun and moon are in contact (without any further eclipse) at the moment when the sun first grazes their horizon after their polar night : so that their day is but a moment, and at that moment the contact takes place.