On the Nature of Eclipses

on the nature of eclipses

The Sun is the only body in the solar system which shines with its own light. All of the others, including the Earth and Moon, reflect the Sun's light. They also cast shadows, and when, in the course of their orbits, the Moon's shadow falls on the Earth, or vice versa, it is known as an eclipse.

There are two parts to these shadows. The umbra is a central cone of darkness which tapers away from the Earth or Moon, whilst the penumbra is an outer cone of partial shadow which diverges instead of tapering.

solar eclipse

The Moon's shadow cast on the Earth appears as a solar eclipse. An observer inside the circle of the penumbra will see a partial solar eclipse: the Moon shows as a bite taken out of the Sun's disc. Within the smaller, inner circle of the umbra the Sun will be entirely blotted out by the Moon. Anyone outside the the area covered by the penumbra (and therefore also the umbra) will see no eclipse at all.

The shadow travels across the surface of the Earth, generally along a curved track. The zone covered by the umbra is called the path of totality, and although no more than 270km (170 miles) wide (when the Moon is directly overhead) it may be thousands of kilometers long.

There is a system of terms and notations for the key stages of a solar eclipse:

First Contact (P1) - The observer enters the penumbra, the start of the partial eclipse.
Second Contact (U1) - Temporary night arrives with the umbra, and the total eclipse begins.
Third Contact (U2) - The total eclipse ends, the observer exits the umbra and sunlight returns.
Fourth Contact (P2) - The observer exits the penumbra and the partial eclipse ends.

annular eclipse

Sometimes, when the Moon is near apogee, the umbra will not quite reach the Earth's surface. An observer at the centre of the penumbra will see the black silhouette of the Moon surrounded by a ring of sunlight (the visible portion of the Suns larger disc). This is known as an annular eclipse, from the latin word annulus meaning ring.

lunar eclipse

Although it is caused in exactly the same way, we see a lunar eclipse from a very different viewpoint. As the Moon enters the shadow cast by the Earth, the event is visible to everyone who can see the Moon - essentially everyone on Earth's nightside..

The Earth's shadow cast on the Moon is predictably much larger than that of the Moon on the Earth. As a result the umbra may cover the entire lunar disc at the same time. Similarly, the entire Moon can be inside the penumbra without entering the umbra.

Lunar eclipses are therefore categorised as:

total umbral (the entire Moon enters the inner shadow cone)
partial umbral (part of the Moon enters the umbra the remainder stays in the penumbra)
total penumbral (the whole Moon enters the penumbra, but not the umbra)
partial penumbral (only part on the Moon enters the penumbra).

Again, there is a system of terms and notations for the key stages of a lunar eclipse:

First Contact (P1) - The penumbral eclipse begins, as the outer shadow touches the lunar disc.
Second Contact (U1) - The Moon enters the darker inner shadow, and the umbral eclipse begins.
Third Contact (U2) - The total umbral eclipse starts as the inner shadow covers the entire disc.
Fourth Contact (U3) - The Moon begins to exit the inner shadow and the total umbral eclipse ends.
Fifth Contact (U4) - The umbral eclipse ends, leaving only outer, penumbral shadow on the Moon.
Sixth Contact (P2) - The last shadow leaves the Moon and the penumbral eclipse ends.

atmosphere

Another major difference between solar and lunar eclipses is that whereas the Moon's umbra is totally dark, the Earth's is not. The Earth's atmosphere refracts some light into the shadow and onto the lunar surface. The colour of the Moon seen from the Earth will depend on atmospheric conditions. It can be measured using the Danjon scale.

Penumbral eclipses are similarly influenced by the atmosphere, and vary from pale amber to an almost undetectable yellow.

another perspective

From the lunar surface, eclipses would look rather different. Because the Moon's rotation has been captured the Earth will remain in roughly the same position in the lunar sky (moving slightly because of libration). It will be much larger: nearly 2° across compared with the Moon's ½° as seen from Earth.

During a solar eclipse, in the middle of the lunar night, the Moon's shadow will appear as a circular grey disc on the Earth, darkening towards the black umbra at the centre.

A lunar eclipse would be much more spectacular. The Sun, ½° across, would slip behind the Earth giving it an orange, red or deep brown rim and bathing the surrounding lunar landscape in a corresponding light.