When, in their eternal movement through the vast wastelands of space, one celestial body comes between two other such bodies, completely or partially obscuring the direct view that would normally exist between the two, we say that there is an eclipse; the obscured body having been eclipsed from the view of the other by the obscuring one. Given this definition, a solar eclipse, from an earthbound viewpoint, is one in which Sol, our local star, the sun, is eclipsed by another celestial body. In real terms, it is the obscuration of the sun by the moon when the moon intervenes between the earth and the sun.
From an earthbound viewpoint, such an event only happens when the sun and the moon are in certain specific positions in relation to earth. Eclipses, viewed from earth occur only when the moon is in its new moon phase, that is, at a time when both bodies are in the same longitudinal aspect as seen from earth. The moon, a cold rocky orb, emits no light of its own. Moonlight, as seen on earth, is merely the reflection of sunlight that hits the lunar surface. The new moon itself is not visible on earth, because at the time of the new moon, the illuminated surface of the moon is faced away from earth.
A solar eclipse can appear as one of four types. The first, and most spectacular, is the total eclipse, when, as the name suggests, the sun is completely obscured by the moon. At the other end of the scale is the partial eclipse, when the sun and the moon are not exactly in line and the resulting obscuration is not complete. An annular eclipse is one when, although the sun and the moon are perfectly aligned, the apparent size of the moon is less than the apparent size of the sun so that, although the moon is perfectly situated against the sun, it does not cover the entire sun. The final type of solar eclipse, one which is comparatively rare, is the hybrid eclipse which is seen as a total eclipse at some points on earth’s surface and as an annular eclipse at other points.
A total eclipse seen here on earth occurs because of a simple mathematical correlation. The distance of the sun from earth, approximately 148.8 million kilometres, is 400 times the distance of the moon from the earth, 372,000 kilometres. Now, because the sun’s diameter is approximately 400 times that of the moon, when both bodies are in proper conjunction, a total obscuration will take place. Given this factor, it might be thought that every eclipse would be a total one and that every new moon would provide us with an eclipse. This is not the case and there are a number of factors which do not allow such.
The first is that in its orbit around the earth, the moon is tilted somewhat over five degrees to the plane of the ecliptic, i.e. earth’s own orbit around the sun. The result of this tilt is that at the time of the new moon, the moon will often be above or below the sun, so that no conjunction is possible. An eclipse will occur only at those times that the new moon intersects the ecliptic. Those points at which the new moon does intercept are known as nodes.
Secondly, because the moon’s orbit around the earth is oval in shape rather than circular, just as is the earth’s own orbit around the sun, the apparent sizes of the sun and the moon vary depending on what particular point of their orbits are the earth and the moon. When the moon is near its closest point to earth in its orbit (perigee), total eclipses are likely to occur because the moon will appear to be large enough to cover the entire photosphere of the sun, i.e. the sun’s bright disk, its magnitude being greater than one. On the other hand, when the moon is near to its farthest point in its orbit (apogee), eclipses will be annular because the apparent size of the moon is not large enough to cover the entire sun, its magnitude being less than one. Hybrids occur when the magnitude moves from being greater than one to being less than one or vice versa so that the eclipse will be seen as a total one at some points on the earth’s surface and an annular one at other points. The movement of the earth in its orbit around the sun can also affect what kind of eclipse we see, although not to so great a degree as the position of the moon in its own orbit around the earth. In January, when the earth is at perihelion, i.e. its closest point to the sun, annular eclipses are indicated; in July, when the sun is at aphelion, i.e. its farthest point from the sun, total eclipses are indicated.
It would seem that the impressive total eclipses that we see here on earth are likely to be a rarity in the universe as a whole, the phenomenon being a lucky union of size and distance. Millions of years past, when the moon was much closer to the earth, total obscuration of the sun’s disk would have been impossible. And, half a billion years or so in the future, the moon, whose orbital distance increases by almost four centimetres per annum, will be to far away to allow for the complete obscuration of the sun.
There is a final point. Earth is not the only one of the sun’s daughters on which a solar eclipse can be seen. A viewer standing on the surface of Jupiter or Pluto can also witness this spectacle. Jupiter has five satellites, Amalthea, Io, Europa, Ganymede and Callisto which are able to completely obscure the sun as seen from the Jovian surface; and the phenomenon can be followed from earth with the aid of telescopes. Indeed, it was the tracking of this phenomenon that first permitted an accurate calculation of the speed of light sometime in the 17th century. All of the other Jovian satellites are either too small or too far away from their primary to permit obscuration of the sun.
Solar eclipses can also be viewed from the surface of Pluto, when any of its three satellites, Charon, Nix and Hydra, pass in front of the sun and obscures the sunlight as seen from the surface of the planet.
