Tag: Physics

The first total lunar eclipse in nearly three years will be visible this
Monday/Tuesday over North and South America beginning at midnight on the
21st of December, 2010.

From the Wikipedia
article

There are three types of lunar eclipses. Penumbral, partial, and
selenelion (selenehelion).

A penumbral eclipse occurs when the Moon passes through the Earth’s
penumbra. The penumbra causes a subtle darkening of the Moon’s surface. A
special type of penumbral eclipse is a total penumbral eclipse, during
which the Moon lies exclusively within the Earth’s penumbra. Total
penumbral eclipses are rare, and when these occur, that portion of the
Moon which is closest to the umbra can appear somewhat darker than the
rest of the Moon.
A partial lunar eclipse occurs when only a portion of the Moon
enters the umbra. When the Moon travels completely into the Earth’s umbra,
one observes a total lunar eclipse. The Moon’s speed through the shadow is
about one kilometer per second (2,300 mph), and totality may last up to
nearly 107 minutes. Nevertheless, the total time between the Moon’s first
and last contact with the shadow is much longer, and could last up to 3.8
hours.^{wiki: [1]} The relative distance of the Moon from the
Earth at the time of an eclipse can affect the eclipse’s duration. In
particular, when the Moon is near its apogee, the farthest point from the
Earth in its orbit, its orbital speed is the slowest. The diameter of the
umbra does not decrease much with distance. Thus, a totally eclipsed Moon
occurring near apogee will lengthen the duration of totality.
A selenelion or selenehelion occurs when both the Sun and
the eclipsed Moon can be observed at the same time. This can only happen
just before sunset or just after sunrise, and both bodies will appear just
above the horizon at nearly opposite points in the sky. This arrangement
has led to the phenomenon being referred to as a horizontal eclipse. It
happens during every lunar eclipse at all those places on the Earth where
it is sunrise or sunset at the time. Indeed, the reddened light that
reaches the Moon comes from all the simultaneous sunrises and sunsets on
the Earth. Although the Moon is in the Earth’s geometrical shadow, the Sun
and the eclipsed Moon can appear in the sky at the same time because the
refraction of light through the Earth’s atmosphere causes objects near the
horizon to appear higher in the sky than their true geometric position.^{wiki:
[2]Wikipedia}