MOON WATCH NIGHT: Using just your eyes
For years prior to the Elizabethan astronomer, Thomas Harriot,
(A Brit, and the first to draw the Moon through a telescope, even beating Galileo) people had to
make do with their own eyes when looking at the Moon.
This is what you will see with your eyes….
Your eyes are limited by “resolution”, or the smallest angular
part of the sky they can adequately see. You should be able to resolve objects
about 1/25th the diameter of the Moon, but no smaller. This of
course assumes you have perfect eyesight. This will let you see surface
features down to about 140 km across in size.
At night the iris to your eyes is as wide open as possible, and
this will give you much better resolution than in day light when the iris gets
smaller!
You should be able to make out dark areas called “Mare” and bright
areas called “Highlands”.
Hundreds of years ago people used to think that Mare areas were
lunar seas, but we now know that liquid water cannot exist freely on the Moon
as it would vaporize immediately as there is no atmosphere. In fact the Mare
areas were where molten lava collected in big deep basins and cooled down
thousands of millions of years ago. The Mare is really quite dark and if you
went to the Moon would be as black as coal. It is not the colour
that makes it look dark, it purely because it is was once filled with tiny
volcanic gas bubbles. The gas has now pretty much all gone but we are left with
lots of tiny holes or perforations. The holes from the bubbles contain shadow
for much of the time (except at Full Moon) and that is why the Mare looks dark
to us!
The highland by contrast is more reflective and higher than the
Mare. It is also often older and made of tougher rock. It has been pounded over
thousands of millions of years by meteorites and sometimes mini-asteroids and
so has many more craters.
Can you imagine a man or women’s face or shape
in the Moon. Many people did before the telescope was
invented! Some people see somebody reading a book,
others see an old woman’s face. In China people see a Rabbit in the Moon. If
you ever visit the southern hemisphere, like Australia, New Zealand, China,
South America etc, take the time and trouble to look up at the Moon with your eyes
and you will find that it is upside down compared to what it looks like back in
the British Isles!
Now on Moon watch night remember what the Moon looks like – take a
look again a few days later, or even next month. Do you notice that apart from
the changing shape of the crescent or phase, that the Moon’s Mare and highland
shapes always look the same? This tells us that the Moon always has the same
side facing the Earth. We never really see much of the far-side. The Moon’s day
is therefore the same length as it takes to orbit the earth, or approximately
just under a month. This is known as “synchronized” or “captured rotation”.
Webmaster: Dr Anthony Cook
Web version 2013/10/11
(Email: atc @ aber.ac.uk)