The Earth casts a shadow that the Moon can pass through. When this happens we say that a LUNAR ECLIPSE occurs.

A Lunar Eclipse

  • Lunar eclipses (like solar eclipse) can be partial or total, depending on whether the light of the sun is partially or completely blocked from reaching the moon. The figure shows a total lunar eclipse with the moon lying in the UMBRA of the Earth's shadow.

    The UMBRA is the region of total shadow, while the PENUMBRA is the surrounding region of partial shadow. In the penumbra, sunlight is only dimmed and not blocked.

  • The umbra of the Earth's shadow is about 1.4 x 10^6 km long and points directly away from the sun.

  • If we place a screen at the distance of the moon, then the diameter of the dark umbral projection is 16,000 km, and the moon's diameter is only 3476 km.

    Total Eclipses

    A TOTAL LUNAR ECLIPSE occurs when no part of the moon is outside the umbra. We can see (fig 3-10) the stages as the eclipse proceeds from partial to total. It can only occur when the moon is in the FULL phase.

    The moon travels its diameter in 1 hour, so it takes about 1 hour to become totally eclipsed when favorably aligned to enter the umbra. A total eclipse, including the penumbral stage takes about 6 hours.

  • It is important to note that during a total lunar eclipse, the moon takes on a dark red color because it is being lighted slightly by sunlight passing through the Earth's atmosphere whose ray paths are bent because the light scatters off of the gas particles in the Earth's atmosphere.

    It turns out that the physics of light scattering tells that, for the conditions of the Earths atmosphere, blue light (high frequency) is scattered more than red light (lower frequency). Blue light is scattered too much to be seen at the small angles of scattering needed to illuminate the moon in a total eclipse. Only the red light ray paths are scattering appropriately. This particular scattering is called "Rayleigh Scattering" and is the SAME REASON THAT THE SKY IS BLUE IN THE DAY, but the sun looks red at the horizon! The blue light is scattered more and so our more direct view of the sun means that we see the light rays which are not bent.

    If the the Earth's atmosphere is particularly cloudy or dusty (e.g. from Volcanos) then the moon can be darker as some of the light is further absorbed by the atmosphere.

  • By similar reasoning, if we travel to the moon during a total lunar eclipse and looked at Earth, we would not see the sun, but we would see a reddish glow around the edge of the Earth.

  • Aristotle (384-322 BC) noticed that the shadow cast by the Earth on the moon was circular, and thus concluded that the Earth was spherical.

    Partial Eclipses

  • Because moon's orbital plane is inclined a little more than 5 degrees with respect to the Earth's orbital plane, the moon might not pass through the center of the umbra.

  • PARTIAL LUNAR ECLIPSE occurs when part of the moon remains outside the umbra. Any fainter red glow, as in a total eclipse, is drowned out by the glare of the illuminated part of the moon for all but nearly total eclipses.

    If the moon passes far enough north or south of the umbra, and the moon only passes through the penumbra, then the lunar eclipse is called a PENUMBRA ECLIPSE and are uneventful.

  • The last total eclipse of the moon ended at Jan 9 at 3:52pm and the duration was 1 hour.