Lunar
eclipses
A
lunar eclipse occurs when the Earth passes between the Moon and Sun,
aligning the three celestial bodies and casting its shadow over the
surface of the Moon. Thus, lunar eclipses always occur at full Moon
- but not all full moons. Because the orbit of the Moon around the Earth
is slightly out of alignment with that of the orbit of the Earth around
the Sun, not every full Moon creates a lunar eclipse. It is only when
the two orbits coincide that a lunar eclipse occurs. The correct alignment
of the three celestial bodies occurs roughly every six months, thus
lunar eclipses can be expected, on average, twice a year.
Unlike solar eclipses, which can usually only be viewed on a very limited
part of the Earth's surface, lunar eclipses can be seen by all people
on the night side of the Earth when the eclipse occurs.
Totality
Although less spectacular than solar eclipses, lunar
eclipses last longer, with a gradual darkening of the Moon and totality
taking a few hours to occur. The first part of a lunar eclipse occurs
when the Moon starts to pass through the Earth's partial shadow, the
penumbra. This causes a slight darkening of the Moon which is hardly
noticeable to the eye.
An
eclipse properly starts when the Moon enters the umbra, or full shadow,
and the Earth's shadow can be seen crossing the Moon's surface. The
transition to totality takes about an hour, at which time the whole
disc of the Moon is within the umbra. However, even then the Moon is
not completely darkened, as some light passes through the Earth's atmosphere
onto the Moon. This light has a reddish-orange colour, because more red
light than blue light passes through the Earth's atmosphere. A totally
eclipsed Moon therefore appears as a reddish-orange disc in our night
sky.
Totality can last up to one and three-quarter hours if the Moon passes
through the centre of the umbra. If the Moon only passes through the
edge of the umbra, a partial lunar eclipse occurs.
Viewing a lunar eclipse
Viewing
a lunar eclipse is quite safe, as you are looking only at the reflected
light from the Moon and this is no brighter than a normal full moon.
Remember, however, the direct viewing a solar eclipse is not safe,
as looking directly at the Sun, whether by naked eye, through a telescope,
binoculars, sunglasses or tinted glass can cause permanent damage to
the eye.
The old adage - "Never look directly at the Sun "
is true at all times!
Lunar eclipses can be safely observed by naked eye, binoculars or telescopes.
Using a telescope, you can even watch as the Earth's shadow crosses
the mountains and craters of the Moon.
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