The last total solar eclipse of the century

What is an eclipse?

A solar eclipse occurs when the moon comes between the Sun and the observer. This happens when the shadow cone of the Moon intersects the surface of the Earth, and is observable by anyone within this shadow zone (see diagram).

Two conditions have to be met for a solar eclipse to occur. The first concerns the relationship between the orbits of the Earth and the Moon, which are not in the same plane, but are inclined at around 5 degrees (5 degrees 8'43") to each other. The moon crosses the plane of the Earth's orbit twice in each complete orbit. For an eclipse to occur the moon must be near one of these intersection points (or nodes).

The second condition is that the Sun, the Earth and the moon must also be lined up, corresponding to the phase of the New Moon.

Solar eclipses: annular, partial, total

The Moon's shadow consists of two cone-shaped areas (see diagram), known as the umbra (externally tangent to the Sun and Moon) and the penumbra (internally tangent to the Sun and Moon). For an observer standing between the Moon and the umbra cone summit the eclipse is total. If the observer is beyond the cone summit, the eclipse is annular (ring-like): the apparent diameter of the Moon is too small to mask the whole solar disk. For an observer standing in the penumbra, only a part of the Sun is masked: the eclipse is partial.

The most favourable conditions for a total eclipse are when the Moon is at its perigee, the Earth is farthest from the Sun (around July) and when the Sun is observed near zenith. When these conditions are at best, one can have a totality duration of more than 7 mn. These conditions will be partly met on 11 August 1999, leading to a totality duration of up to 2mn 23s .

What is a lunar eclipse?

A lunar eclipse occurs when the Earth comes between the Sun and the Moon. This phenomenon can be seen by any observer on Earth for whom the Moon is above the horizon, so is much more frequent. Lunar eclipses occur at the time of a full moon, and when the Moon is near one of the nodes of intersection between its orbit and the ecliptic plane.

The Earth umbra is larger than the whole Moon. So, one will observe either a total eclipse by the umbra (which can well be observed), a partial eclipse by the umbra and penumbra, or a total or partial eclipse by the penumbra only. The duration of a lunar eclipse is much longer than a solar eclipse, and can take as much as six hours.

In practice, the lunar eclipse conditions are modified due to the refraction of the Sun's rays by the Earth's atmosphere. This refraction (of 35 minutes of arc) allows some light to penetrate the cone of geometric umbra. So even during total lunar eclipse, the lunar disk is not completely dark. This grazing light is more absorbed by the Earth's atmosphere in the blue and yellow (as light at sunset) portions of the spectrum, giving a particular reddish light during total lunar eclipse.