Twist Issue 161 May 2024

Tw st
Moon
craters
identified
I
40
‘VE picked out four different craters for us to track down,
one in each of the four quadrants, and with the best
viewing dates for them scattered throughout the month.
All are visible with the naked eye, but anyone with binoculars or
any size of telescope will be well set to spot even more detail.
Our Lunar month for ‘May’ actually starts with the new moon on
May 8th this year, and will continue right through to June 5th. Each
quarter of a Lunar month takes approximately one week.
The new Lunar month begins with the New Moon, when it is
completely dark and obscured. Within the first couple of days, the
bright crescent sliver will appear to the right in a 'reverse-C shape'.
We'll introduce a term here called the 'Lunar Terminator',
which is the technical name for the dividing line
between day and night on the moon's surface. All
objects orbiting a sun will have their own
terminator - the Terrestrial (Earth's)
Terminator passes over us every single
dawn and dusk.
After around one week, the Moon will
be at First Quarter, with the Terminator
exactly bisecting the disc, the left half in
darkness and the right side well-lit.
Second quarter will see the Moon
become full, and the terminator
disappear entirely off the left side of the
disc. A new terminator will begin to
cross, again from right-to-left. Third
Quarter will see the terminator again
exactly bisecting the face, but this time with
the left half well-let, and the right side in
darkness.
The final days of the lunar month, in very-early
June, will see a 'correct-C shape' crescent on the lefthand
side, shrinking until it disappears for the next New
Moon.
It may be tempting to try and spot our lunar features
during the Full Moon, and it's true that this is when the Moon is often
brightest. However it's not necessarily the best option for two
reasons. Firstly, the Full Moon can be so bright compared to the
night, that it can saturate our night vision, ruining our ability to spot
fine detail. Secondly, when the Moon is full, the angle of the sunlight
illuminating the face is very direct, which tends to remove any
shadows cast by the lunar geography, and remove a lot of contrast
that would help us pick out the features, making everything blend
into everything else.
Instead, we'll come back to our terminator line. The best time for
spotting detail in a lunar feature is the few days while it is only-just
on the light-side of the terminator. Sometimes this will be a few days
before the terminator crosses a feature, while in the other half of the
D
C
A: Langrenus; B: Plato;
C: Tycho; D: Grimaldi
This month,
Chesterfield
astronomer Phil
Unsworth takes Twist
readers crater
spotting on the Moon
Picture:
Claire Davison
lunar month, it will be the few days after the terminator has already
passed. Best dates for spotting our four craters are listed at the end.
We'll start with Langrenus Crater in the East, a light-coloured crater
right on the Eastern edge of one of the darker 'Maria' (Seas).
Langrenus is approximately round, but due to our viewpoint of it on
the extreme Eastern edge, it will appear as an elongated oval. The
darker Maria are actually lowlands that long ago filled with flowing
lava. The lava cooled to form basalt plains. This basaltic lava is ironrich
in comparison to the highlands, with the iron giving the resulting
basalt a dark colour and lower reflectivity.
Appearing around the same time of the month are Plato Crater in
the North, and Tycho Crater in the South. In contrast to
Langrenus, Plato is a dark-coloured crater against the
lighter surroundings. At some point after its
B
A
creation, Plato filled with the same basaltic lava
as the Maria, to give the darker colouration.
Tycho is one of the Moon's most
prominent features. Have a look at Tycho
on a few different nights, as the
terminator moves. While the crater
itself is small, you should see the very
bright rays surrounding it. These are
the lines of 'ejecta', that is smaller
rocks and dust that were thrown out
and away during the initial impact,
similar to radiating cracks in glass, or a
liquid splatter. Tycho crater itself is
approximately 85km across, but the bright
spokes of ejecta expanding out from there
across the surface can be as long as
1,500km.
Our final crater in the West is Grimaldi. This is
both a comparatively large crater, as well as being
relatively old too. As such, the clean distinct edges that
we've seen on the other craters have disappeared from
Grimaldi over time, due to subsequent impacts and the
crater walls crumbling away and smoothing out. Like Plato, Grimaldi
is a darker basaltic crater against a lighter backdrop.
Tycho Brahe, Michel Van Langren, and Francesco Grimaldi were all
European astronomers of the Renaissance era and individually
contributed to the Lunar observations and mapping of the period.
Plato is named for the Ancient Greek philosopher of the same name.
Lunar Month: May 8th-June 5th
In the first half of the lunar month, look for this crater in the few
days AFTER: Langrenus, May 10th; Plato, May 15th; Tycho, May 16th;
Grimaldi, May 21st.
In the second half of the lunar month, look for this crater in the few
days BEFORE: Langrenus, May 25th; Plato, June 1st; Tycho, June
5th; Grimaldi, June 5th.