Types Of Eclipse

A scientific understanding of any phenomenon starts by sorting the available observations into appropriate groupings based upon some fundamental characteristic. We sort small six-legged beasts into the category "insects," while eight-legged ones are called "arachnids" (spiders, scorpions, mites, ticks, etc.). Naturally other considerations also apply: an octopus is not an arachnid.

Similarly the basic eclipse phenomenon is subdivided into different types. The first distinction, as we have already seen in Chapter 1, is between lunar and solar eclipses. Up to this juncture we have been concerned mainly with solar eclipses, produced by the Moon passing between us and the Sun. Similarly, the Earth may circulate between the Sun and the Moon, putting the latter into its shadow. This is a lunar eclipse.

A second distinction is between total and partial eclipses. In a partial eclipse the alignment of Earth, Moon, and Sun is not exact, so only part of the disk of the Sun (in a solar eclipse) or Moon (in a lunar eclipse) is obscured.

Considering now only solar events, one can get a perfect alignment, but nevertheless not a total eclipse. This is due to the mutual separations of the three bodies varying. As the Moon intercedes between the Earth and the Sun, it may or may not be large enough to block out the whole of the solar disk. This is because its orbit is not circular, so it is sometimes closer to the Earth (at perigee) and sometimes further away (at apogee). When near perigee its disk appears comparatively large (refer to Figure 1-2) and so can cover the Sun completely—a total eclipse. When it is at apogee, its disk appears smaller and so it is unable to obscure the Sun completely. A bright "annulus" then appears around the circumference of the Moon, and so this is called an annular eclipse. These three situations are depicted in Figure 2-1.

There is a fourth type known as a grazing eclipse. It occurs when the limb of the Moon just touches the apparent edge of the Sun in the sky, but does not overlap it. Solar eclipses may also be of

FIGURE 2-1. Three basic forms of solar eclipse can occur. If the Moon does not pass centrally over the solar disk, then the eclipse is only partial. If the passage is close to central, then the eclipse may be either total or annular, depending on the distance from us of the Moon and, to a lesser extent, the Sun. A total eclipse occurs if the rays from top and bottom of the Sun touching top and bottom of the Moon do not cross before reaching the Earth. If those rays do cross above the Earth's surface, then the eclipse will be annular, with a ring of the solar surface being visible around the Moon's periphery. In this diagram the sizes of the three bodies are highly exaggerated.

Sun Moon Earth

FIGURE 2-1. Three basic forms of solar eclipse can occur. If the Moon does not pass centrally over the solar disk, then the eclipse is only partial. If the passage is close to central, then the eclipse may be either total or annular, depending on the distance from us of the Moon and, to a lesser extent, the Sun. A total eclipse occurs if the rays from top and bottom of the Sun touching top and bottom of the Moon do not cross before reaching the Earth. If those rays do cross above the Earth's surface, then the eclipse will be annular, with a ring of the solar surface being visible around the Moon's periphery. In this diagram the sizes of the three bodies are highly exaggerated.

hybrid nature, total in some locations and annular in others. All these situations are explained in more detail in the Appendix, including the additional influence of the Earth-Sun distance varying during the year.

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