Eclipse (continued)
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Lunar Eclipses
An eclipse of the Moon occurs when the Moon passes
through some part of the Earth's two shadows. The inner
shadow or umbra is that region where the Earth blocks all
direct rays from the Sun. The outer shadow or penumbra is a
zone where the Earth blocks part but not all of the Sun's rays.
The two shadows result in three types of lunar eclipses (Fig. 7):
Fig. 7 Cross section through the Earth's shadow cone (umbra) at the Moon's distance, showing the motion of the Moon at three types of lunar eclipses: penumbral eclipse, partial umbral
eclipse, and total umbral eclipse.
1. Penumbral eclipse. A part or all of the Moon passes
through the Earth's penumbral shadow.
2. Partial eclipse. A part but not all of the Moon passes
through the Earth's umbral shadow.
3. Total eclipse. The entire Moon passes through the Earth's
umbral shadow.
A lunar eclipse can occur only during the full moon phase.
Furthermore, the Moon must also be near one of the two nodes
of its orbit. The nodes are the points where the plane of the
Moon's orbit intersects the plane of the Earth's orbit. Since the
Moon's orbit is tipped 5° to the Earth's orbit (the ecliptic), the
Moon passes above or below the shadows during most full
moons. But several times each year, full moon occurs near one
of the nodes and some portion of the Moon passes through
one or both of the Earth's shadows.
All partial lunar eclipses begin and end with penumbral
phases. Similarly, all total eclipses begin with a penumbral
phase followed by a partial phase. Totality is then followed by a
partial and a penumbral phase in that order (Fig. 8).
