In the Shadow of the Earth
By Seanie Morris, Tullamore Astronomical Society
If you are one that likes to observe the Moon during the night, then get ready for
a Total Lunar Eclipse on the night of Thursday February 21st. On this night, you
will witness the Moon become blood red as it passes into the shadow of the Earth.
The event will be observable without telescopes through most of western Europe and
the Americas. Even though a Total Lunar Eclipse occurs twice a year, did you ever
wonder how it occurs?
The Mechanics
In the better-known Total Solar Eclipse, a rare alignment of the Sun, Moon, and Earth
occurs, where the Moon will completely block out the Sun’s light for a very small
portion of the Earth’s surface. In a Total Lunar Eclipse, the opposite occurs, whereby
the Earth comes between the Sun and the Moon. During any one calendar year, only 2
total lunar and 2 total solar eclipses will take place. However, not all of Earth can be
witness to each event.
Things are not aligned up in a perfect X-Y linear development in our part of the Solar
System. Earth is tilted in its orbit towards the Sun, and then, in a similar way, the
Moon is tilted in its plane of orbit around Earth. Inevitably, though rarely, a perfect
alignment does occur. This allignment is made even more dramatic during a Solar
Eclipse when you consider the following piece of astronomical trivia: The Sun is
almost 400 times further from Earth than the Moon is, and the Sun is almost 400
times larger than the Moon. Because these ratios are approximately the same, the size
of the Sun and the Moon each as seen from Earth appear to be approximately the
same when seen in the sky. Coincidence? Or a product of Intelligent Design? Who
knows.
However, this quirk of physics does not entirely lead to as great a phenomenon during
a Lunar Eclipse. The Moon is quite close to Earth, and also very small. Earth’s
shadow is also very broad in this close distance that the Moon’s orbit occupies. Still,
despite these less glamorous conditions, a Total Lunar Eclipse offers a completely
different view of the Moon to be observed.
The Event Itself
(Timings given in this section should be taken with a plus/minus of 1.5 minutes
depending upon your location in Ireland) First Contact occurs after midnight at
00:34hrs when the Moon ‘touches’ the outer shadow of Earth, called the penumbra.
You will notice the lower left of the disc begin to darken, and gradually get larger
across its surface. It will have a darkened grey appearance. This part is called a
Penumbral Eclipse.
Then, it begins to enter the darkest part of Earth’s shadow, called the umbra, at
01:41hrs. Again, from roughly the same spot (lower left), the Moon will begin to take
on a blood red appearance. This is referred to as (the start of) a Partial Lunar Eclipse.
Totality itself (Second Contact) occurs when all of the body of the Moon is encased
inside Earth’s umbra, at 03:00hrs. From here, it will take the Moon approximately 52
minutes to pass through the umbra, with the darkest part seen at 03:26hrs.
It begins to come out of the umbra (Third Contact) at 03:52hrs. From there, it takes on
the dark grey appearance again as it passes into the second penumbra.
The event then continues in reverse, and finishes at 06:17hrs when the last part of the
Moon (Fourth Contact) leaves Earth’s shadow completely, and things get back to
normal.
Observations
If you have a telescope or binoculars, then this is a good opportunity to observe the
Moon in a new light. Craters and maria (‘seas’) appear different when they don’t have
the bright rays of the Sun to cast peculiar shadows on the lunar surface. The red
colour of the surface during Totality is actually caused by Earthlight passing through
our atmosphere, and reflecting off the Moon’s surface. If the night is clear, the sudden
darkness will also allow time to observe other heavenly treasures in the new-found
darkness. During the entire event itself, if using optical equipment, you might even
spot the Moon occult a background star, and for the time taken to observe the eclipse,
see it reappear again later.
If you do manage to observe this event, Tullamore Astronomical Society would love
to hear of your thoughts and observation notes, even your pictures. You can contact us
via tullamoreastronomy@yahoo.co.uk, or check out our website at
www.tullamoreastronomy.com.