What to Expect With a Lunar Eclipse

The Mechanics of a Lunar Eclipse

• During a lunar eclipse, Earth stands between the moon and the sun, casting its shadow on the moon's surface. That shadow has two parts, shaped like two concentric cones. The outer cone is known as the "penumbral shadow," and only some of the sun's light is blocked when the moon moves through this area of Earth's shadow. When the inner cone, which is called the "umbral shadow," falls over the moon, none of the sun's light reaches the moon's surface.

Penumbral Eclipses

• During a penumbral eclipse, the moon passes through Earth's penumbral shadow. This type of eclipse has an appearance that is much fainter than that of a partial or total eclipse. This is because the penumbral shadow only blocks a portion of the sun's rays. When the moon passes through the penumbral shadow zone, enough light can still reach the moon's surface that observers on Earth may not be able to detect any difference in the moon's appearance, or will only notice a subtle dimming of the moonlight.

Partial Eclipses

• Partial eclipses occur when part of the moon's surface is obscured by Earth's umbral shadow. Because Earth is a sphere, and therefore casts a circular shadow, the umbral shadow forms a darkened semi-circle on the moon's surface during a partial lunar eclipse. Unlike penumbral eclipses, observers can easily see partial eclipses with the naked eye. The contrast between the dark and light portions of the moon's surface make the shadowed area appear much darker than the lighter area, although it is also partially darkened by the penumbral shadow.

Total Eclipses

• Total eclipses go through penumbral and partial eclipse phases, but they differ from the other eclipse types in that the moon eventually moves entirely into Earth's umbral shadow. When the surface of the moon is fully obscured by Earth's penumbral shadow, the moon can shine with bright red, orange or dark brown light. Although Earth's shadow blocks direct sunlight from reaching the moon during this phase of the eclipse, light is still filtered and refracted through Earth's atmosphere into the shadow zone, giving the eclipse its dramatic coloring. This phase of a total eclipse may last longer than an hour before the moon begins to move out of the umbral shadow.