Page 88 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

LUNAR ECLIPSES

WHILE SOLAR ECLIPSES ZOOM ACROSS THE FACE
OF EARTH, SPECTACULAR RED LUNAR ECLIPSES ARE
MUCH SLOWER SO SIT BACK, RELAX AND ENJOY
THE TRANQUIL PROCEEDINGS

Look up at the Moon at certain times of the Moon are lined up in space. However, most
year and you’ll be able to witness the motion months the Moon moves above or below
of the Solar System on a truly grand scale. Earth’s shadow, which is why eclipses don’t
There’s nothing quite like watching a lunar happen every time the Moon is full. They
eclipse as the Sun, Earth and Moon become occur when the Moon travels into Earth’s
perfectly aligned. shadow during full Moon. Slowly, the Moon
There are three types of lunar eclipse, approaches the exact opposite side of the
the most exciting of which is a total lunar sky to where the Sun is and as it does so
eclipse. The other two types are partial Earth blocks out more and more sunlight
and penumbral lunar eclipses, which we’ll from reaching it. As the Moon travels into
come to later. Earth’s shadow, the left side of the Moon
Total lunar eclipses can only happen gradually darkens until the Moon is totally
at full Moon when the Sun, Earth and covered – totality.

MECHANICS OF A

LUNAR ECLIPSE

All the lunar eclipse effects that happen, such as darkening and colour changes, are due
to Earth blocking off sunlight to the Moon. Only sunlight that has been bent (refracted) by Because the Sun is much bigger than
Earth’s atmosphere reaches the Moon (shown by the bent line in the diagram). When the Earth, it splits our planet’s shadow into two
Sun’s rays intercept Earth they form two zones of shadow: the penumbra, where some of parts: the darkest, called the umbra, and a
the Sun is always seen; and the umbra, where the Sun is completely hidden by Earth. The lighter outer ring, called the penumbra (see
eclipse appears to begin in earnest when the Moon starts to move into this umbra region the diagram, left). When the Moon is in the
and its surface begins to darken, before turning an unusual orange-red colour.
umbra, no direct light from the Sun reaches
it, whereas in the penumbra it is partially lit
by direct sunlight.
Only light that has been bent inwards
Light is refracted by by Earth’s atmosphere can make it to the
Earth’s atmosphere Moon’s orbit
Moon when it is in the umbral shadow. The
ILLUSTRATION BY STEVE MARSH, NASA X 3, PETE LAWRENCE X 2 The Sun Atmosphere Earth Umbra the sky gets darker too. You may not have
atmosphere fi lters out blue light, but leaves
red light, which gives the Moon a strange
orange-brown colour during totality.
Penumbra
As the Moon goes into eclipse and dims,
realised how bright a full Moon can be.
It lights up the sky around it with a blue
haze, out of which only the brighter stars
are visible. During a total lunar eclipse, the
Moon
darker Moon means that the fainter stars
sight of a deep-red Moon surrounded by
twinkling stars.
88 skyatnightmagazine.com 2012 can come out and we end up with the eerie