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Twist Issue 161 May 2024

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Tw st<br />

Moon<br />

craters<br />

identified<br />

I<br />

40<br />

‘VE picked out four different craters for us to track down,<br />

one in each of the four quadrants, and with the best<br />

viewing dates for them scattered throughout the month.<br />

All are visible with the naked eye, but anyone with binoculars or<br />

any size of telescope will be well set to spot even more detail.<br />

Our Lunar month for ‘<strong>May</strong>’ actually starts with the new moon on<br />

<strong>May</strong> 8th this year, and will continue right through to June 5th. Each<br />

quarter of a Lunar month takes approximately one week.<br />

The new Lunar month begins with the New Moon, when it is<br />

completely dark and obscured. Within the first couple of days, the<br />

bright crescent sliver will appear to the right in a 'reverse-C shape'.<br />

We'll introduce a term here called the 'Lunar Terminator',<br />

which is the technical name for the dividing line<br />

between day and night on the moon's surface. All<br />

objects orbiting a sun will have their own<br />

terminator - the Terrestrial (Earth's)<br />

Terminator passes over us every single<br />

dawn and dusk.<br />

After around one week, the Moon will<br />

be at First Quarter, with the Terminator<br />

exactly bisecting the disc, the left half in<br />

darkness and the right side well-lit.<br />

Second quarter will see the Moon<br />

become full, and the terminator<br />

disappear entirely off the left side of the<br />

disc. A new terminator will begin to<br />

cross, again from right-to-left. Third<br />

Quarter will see the terminator again<br />

exactly bisecting the face, but this time with<br />

the left half well-let, and the right side in<br />

darkness.<br />

The final days of the lunar month, in very-early<br />

June, will see a 'correct-C shape' crescent on the lefthand<br />

side, shrinking until it disappears for the next New<br />

Moon.<br />

It may be tempting to try and spot our lunar features<br />

during the Full Moon, and it's true that this is when the Moon is often<br />

brightest. However it's not necessarily the best option for two<br />

reasons. Firstly, the Full Moon can be so bright compared to the<br />

night, that it can saturate our night vision, ruining our ability to spot<br />

fine detail. Secondly, when the Moon is full, the angle of the sunlight<br />

illuminating the face is very direct, which tends to remove any<br />

shadows cast by the lunar geography, and remove a lot of contrast<br />

that would help us pick out the features, making everything blend<br />

into everything else.<br />

Instead, we'll come back to our terminator line. The best time for<br />

spotting detail in a lunar feature is the few days while it is only-just<br />

on the light-side of the terminator. Sometimes this will be a few days<br />

before the terminator crosses a feature, while in the other half of the<br />

D<br />

C<br />

A: Langrenus; B: Plato;<br />

C: Tycho; D: Grimaldi<br />

This month,<br />

Chesterfield<br />

astronomer Phil<br />

Unsworth takes <strong>Twist</strong><br />

readers crater<br />

spotting on the Moon<br />

Picture:<br />

Claire Davison<br />

lunar month, it will be the few days after the terminator has already<br />

passed. Best dates for spotting our four craters are listed at the end.<br />

We'll start with Langrenus Crater in the East, a light-coloured crater<br />

right on the Eastern edge of one of the darker 'Maria' (Seas).<br />

Langrenus is approximately round, but due to our viewpoint of it on<br />

the extreme Eastern edge, it will appear as an elongated oval. The<br />

darker Maria are actually lowlands that long ago filled with flowing<br />

lava. The lava cooled to form basalt plains. This basaltic lava is ironrich<br />

in comparison to the highlands, with the iron giving the resulting<br />

basalt a dark colour and lower reflectivity.<br />

Appearing around the same time of the month are Plato Crater in<br />

the North, and Tycho Crater in the South. In contrast to<br />

Langrenus, Plato is a dark-coloured crater against the<br />

lighter surroundings. At some point after its<br />

B<br />

A<br />

creation, Plato filled with the same basaltic lava<br />

as the Maria, to give the darker colouration.<br />

Tycho is one of the Moon's most<br />

prominent features. Have a look at Tycho<br />

on a few different nights, as the<br />

terminator moves. While the crater<br />

itself is small, you should see the very<br />

bright rays surrounding it. These are<br />

the lines of 'ejecta', that is smaller<br />

rocks and dust that were thrown out<br />

and away during the initial impact,<br />

similar to radiating cracks in glass, or a<br />

liquid splatter. Tycho crater itself is<br />

approximately 85km across, but the bright<br />

spokes of ejecta expanding out from there<br />

across the surface can be as long as<br />

1,500km.<br />

Our final crater in the West is Grimaldi. This is<br />

both a comparatively large crater, as well as being<br />

relatively old too. As such, the clean distinct edges that<br />

we've seen on the other craters have disappeared from<br />

Grimaldi over time, due to subsequent impacts and the<br />

crater walls crumbling away and smoothing out. Like Plato, Grimaldi<br />

is a darker basaltic crater against a lighter backdrop.<br />

Tycho Brahe, Michel Van Langren, and Francesco Grimaldi were all<br />

European astronomers of the Renaissance era and individually<br />

contributed to the Lunar observations and mapping of the period.<br />

Plato is named for the Ancient Greek philosopher of the same name.<br />

Lunar Month: <strong>May</strong> 8th-June 5th<br />

In the first half of the lunar month, look for this crater in the few<br />

days AFTER: Langrenus, <strong>May</strong> 10th; Plato, <strong>May</strong> 15th; Tycho, <strong>May</strong> 16th;<br />

Grimaldi, <strong>May</strong> 21st.<br />

In the second half of the lunar month, look for this crater in the few<br />

days BEFORE: Langrenus, <strong>May</strong> 25th; Plato, June 1st; Tycho, June<br />

5th; Grimaldi, June 5th.

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