Event driven astronomy
The Earth’s umbral shadow is larger than that of the Moon and at
the distance of the Moon is still quite large, presenting a larger target
for the Moon to pass through. Unlike a solar eclipse, a lunar eclipse
can be seen from anywhere on Earth as long as the Moon is above
the horizon.
The two main types of lunar eclipse are partial and total. A third type is
known as a penumbral eclipse but this is a tricky thing to see visually unless
the Moon’s passage through the penumbral shadow takes it close to the
umbral shadow. When this occurs, one edge of the Moon may appear to be
slightly more shaded than normal.
You might expect the Moon to go totally dark during a total lunar eclipse
but this is not the case. The reason for this is the Earth’s atmosphere. As
light passes through the atmosphere it gets bent, or refracted, so that it
in-fills the umbral shadow. As the bluer components of this light get
scattered by the atmosphere, the colours left are towards the red end of
the spectrum. The Earth’s umbral shadow is quite reddish in colour as a
consequence of this.
As the Moon enters the umbral shadow it appears to change colour until
at the time of totality it will look quite reddish in colour. The darkness and
depth of the colour depend on the state of the Earth’s atmosphere. Volcanic
eruptions and heavy cloud cover can all have an effect. Some lunar eclipses
take on a light gold-coppery hue, while others can go a deep blood red, the
Moon virtually disappearing from view.your location is such that you have a good flat view around you, it’s possible
to sense the approach and despatch of the Moon’s shadow just before and
after complete totality.
We are very fortunate to be able to witness this amazing event. The
Moon’s distance from the Earth is slowly increasing over time. Hundreds
of millions of years in the past, the Moon would have appeared too big for
the precise fit that’s required for a proper total eclipse of the Sun. Similarly,
it’s estimated that in 600 million years time, the Moon will appear too small
for a total eclipse to be seen at all. During the course of a year, there will be
between two and five solar eclipses, though of course seen from different
locations on Earth. Not more than two will ever be total eclipses, however.
Lunar Eclipses
The counterpart to a solar eclipse is the lunar eclipse, an event which occurs
when the Moon lines up with the Sun on the opposite side of the Earth. When
this occurs, the Moon will pass through the shadow of the Earth and what
was a bright full Moon will appear dim by comparison.
The Earth casts a shadow in space which is similar in make-up to the one
described for the Moon above. There’s a dark umbral shadow and a lighter
surrounding penumbra. If you are in the umbral shadow of the Earth looking
back at the Sun, the Sun’s light would be totally blocked. Inside the Earth’s
penumbral shadow looking back at the Sun, part of the Sun’s disc would be
blocked out.
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