How Long Is The Period Of Totality

All the total eclipses in Figure 2-2 lasted for about seven minutes. What factors control that time span? The duration of totality depends upon the relative angular sizes of Sun and Moon. The greatest interval of obscuration is when a solar eclipse occurs (1) when the Moon is at perigee, so that the lunar diameter is maximized; and (2) when the Earth is at aphelion, so that the solar diameter is minimized (this is why those long eclipses straddled July, aphelion occurring early in that month).

The changing speeds of these bodies also affect the duration of totality: the apparent angular speed of the Sun is lowest when we are at aphelion, as above, and this enhances the duration of totality. On the other hand, when the Moon is at perigee its angular speed is the maximum it ever attains, and that has a contrary effect. Basically, seven-minute-plus eclipses result from the greatest feasible difference in lunar versus solar apparent diameter, about one-fortieth of a degree: Moon 0.548 degrees, Sun 0.524 degrees. The converse can also be true, the Moon appearing smaller than the Sun, making the duration of totality zero; that is, an annular eclipse occurs.

Apart from the stage of totality, we saw in Chapter 2 that there is an extended period—some hours—of partial eclipse that precedes and follows the main event. This is the time it takes for the Moon gradually to cover the Sun, and then to uncover it again later. More often, there is no totality (or even an annular eclipse) because the Moon does not pass centrally across the solar disk, and so only a partial eclipse takes place. Lunar eclipses may similarly be subdivided, and we should consider the different circumstances that can occur for those.

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