Observing the Moon

by Duncan Lunan

Recently I was asked to provide a general article about the Moon for Amara’s Seashore Journals – Wave 1, a new journal edited by a mermaid.  On checking my articles on the Moon for ‘Beginner’s Astronomy’ in Orkney News, back in 2021, I had plunged straight in with theories about the origin of the Moon and its features, and had little on its movements or appearance, except for a shorter piece on lunar eclipses the following week.  I wasn’t illustrating the column then either, except for a basic Moon map.  The Amara’s Seashore Journals – Wave 1 piece ends at Full Moon, due to restrictions of space, so with Amara the Mermaid’s permission I’m extending it here.

If you’re lucky enough to live by the seaside, you may be able to see a great deal more of the sky than people who live in towns and cities can;  and if you’re still luckier and can get to somewhere where artificial lights around you aren’t too much of a problem, you should be able to see a great many more of the stars.  But even if you live in the heart of a city, the one thing we can all see in the night sky is the Moon.

diagram showing the lunar orbits in relation to the Earth and The Sun
Fig. 1. Lunar orbit inclination

The Moon and Earth travel together around the Sun, and because they’re both in motion, it takes the Moon 28-29 days to go round the sky as we see it from here.  The Moon’s path is inclined to the Earth’s by 5 degrees  (Fig. 1), so the Moon doesn’t always pass directly between us and the Sun.  When it does we get a solar eclipse, which is a rare event affecting only part of the world, so generally you have to travel in order to see one  (Fig. 2).  The Earth is larger than the Moon, so it casts a wider cone of shadow through space, and when the Moon passes through it, a lunar eclipse can be seen from the half of the Earth which is facing it  (Fig. 3).

In an ordinary month, however, the Moon is invisible, with its dark side facing us, as it passes the Sun.  [IMPORTANT WARNING:  never look for the Moon when it’s close to the Sun, and especially not with any instrument like binoculars or a telescope.  If you accidentally catch sight of the Sun that way, you could easily lose your eyesight forever.]  Note that the Moon doesn’t have a permanent ‘dark side’:  one side is always lit by the Sun, but it moves around as the Moon moves round the Earth, while always keeping the same face turned towards us.  

Venus and the moon in the night sky
Fig. 4. Venus and Moon

The Moon first becomes visible in twilight, as a thin crescent above where the Sun went down, and itself sets soon afterwards  (Fig. 4).  The following night, it will be higher in the sky and its crescent will be fatter  (waxing).  At this time of the lunar month, you can usually see the rest of the Moon glowing in reflected light from the Earth.  The old name for this effect is ‘the old Moon in the new Moon’s arms’, and it was Leonardo da Vinci who first identified it as Earthlight  (Fig. 5).

the moon in the night sky
Fig. 5. Crescent Moon and earthshine, spring 2013

If you can see the stars as well, you’ll notice that from night to night, the Moon is travelling towards the eastern horizon.  In the northern hemisphere, it’s moving from right to left across the sky  (Fig. 6), passing through each constellation of the Zodiac in turn.  If it begins the lunar month in Taurus, near the Pleiades  (‘the Seven Sisters’), it will travel from there into Gemini, then Cancer, then Leo, then Virgo, and all the way round as the ‘moonth’ goes on.  After the first week has gone, the Moon will be half-full – confusingly, that’s called ‘First Quarter’ (Fig. 7) – and by that time it will be easy to find in daylight.  Dark markings will be obvious on its face, and in the evening, with a telescope, craters will be clearly visible near the dividing line between night and day on the Moon, which is called the ‘terminator’  (Fig. 8).  To the left of it, with a telescope, you will probably see mountain peaks and crater rims catching the first rays of the Sun;  and as the night goes on, if you can stay up for it, you’ll see the terminator advancing towards them, until they merge into full lunar daylight.

In a few more days, the Moon will be three-quarters full, and is then said to be ‘gibbous’  (Fig. 9).  By then you can recognise the familiar markings making up the face of ‘the Man in the Moon’.  These dark plains are called ‘maria’  (‘seas’, in Latin), because they were once thought to be liquid water, like the Earth’s oceans.  Another idea was that the Moon was a perfect, polished ball, and the maria actually were the Earth’s oceans, reflected in it.  But in fact they are huge craters, formed by impacts in the early history of the Moon, and later filled with dark lava from below the surface.  For some reason the similar craters on the Farside of the Moon didn’t fill in the same way, and scientists are still arguing about why that should be the case. 

Fig. 9. Gibbous Moon 25.02.18, Alan Cayless, Bridge of Allan

In Fig. 9, taken on 25th February 2018 by Dr. Alan Cayless in Bridge of Allan, the dark crater near the top is Plato;  the bright one near the equator is Copernicus;  the two vertically placed ones at the bottom are Tycho  (uppermost)  and Clavius, near the Moon’s south pole.  All lunar craters are named after great scientists, mathematicians or philosophers.

When the Moon is Full, we can’t see so much detail, because it’s so bright, but there are no shadows to highlight the features.  What we can see are the bright splashes of broken rock around the more recent large craters.  The only one visible to the naked eye is the crater Tycho  (Fig. 10), near the bottom of the disc as we see it from here.

Elsewhere on Earth, the view is different.  We see the Man in the Moon from the northern hemisphere, but in China, nearer the equator, they see a Rabbit  (Fig. 11).  And from Australia and New Zealand, down in the southern hemisphere where the whole sky is the other way up from our view, instead of the Man in the Moon they see a Crab  (Fig. 12).  And that’s why, in the first film of The Lord of the Rings, when we see the Moon, it’s the Crab that’s visible – because they filmed it in New Zealand!

Fig. 12. The Crab, seen in the southern hemisphere

In theory, the Full Moon rises at sunset.  But as we’re not standing at the centre of the Earth, and the Earth’s atmosphere makes things at the horizon look higher in the sky than they truly are, from different viewpoints on Earth it can be seen to rise at significantly different times.  The Moon’s appearance changes very little from the night before Full Moon to the night after, and African hunters often refer to ‘the three nights of the Full Moon’.  But once you’re aware of the Moon’s motion, it’s amazing how often in detective programmes and films the Moon can remain Full  (or a crescent, or gibbous)  for the whole length of the episode.  Less common, but not unknown, is showing the waning Moon instead of the waxing one.  Greeting card artists don’t necessarily have it wrong, but if it’s not wrong, it’s amazing how many scenes on greeting cards take place in the small hours of the morning.  When my sister commissioned a scene of sunset over Arran for my 50th birthday, from the artist Janet Foley  (Fig. 13), they went to great lengths to make sure the crescent would be facing the right way – but it’s very easy if you just remember that the horns of the crescent always point away from the Sun.

The Moon’s path around the Earth is not circular, and though it doesn’t diverge far from an ellipse, it’s quite significant when imposed on a circle with an average radius of 239,000 miles  (Fig. 14).  At its nearest to Earth  (perigee)  it’s 226,000 miles away, and at its furthest  (apogee)  it’s 252,000 miles away – enough to make a difference of three to five days on a low-energy spacecraft trajectory like Apollo’s.  It’s enough to make a noticeable difference in the apparent size of the Full Moon  (Fig. 15), apparently exaggerated when it’s on the horizon.  (Actually, the Moon is nearer when overhead – Fig. 16).  In recent years the media have made a big deal of these ‘supermoons’, and after missing out on a BBC interview by saying they weren’t unusual or important, I’ve made a point of including them in ‘The Sky Above You’ since.

After Full Moon the terminator reappears, on the right-hand side of the disc  (seen from the northern hemisphere), but this time marking sunset on the Moon instead of sunrise.  The first prominent feature to disappear is Mare Crisium, the Sea of Crises  (Fig. 17 – cp. Fig. 10), the site of the extraterrestrial transmitter in Arthur C. Clarke’s story ‘The Sentinel’, which inspired 2001, A Space Odyssey

Fig. 17. Waning Moon June 23rd 2016, Dublin

In the 1950s H,P. Wilkins, Patrick Moore’s collaborator in mapping the Moon pre-Apollo, believed he had seen the setting Sun streaming through a bridge on the western rim of Mare Crisium.  As there’s no surface water on the Moon to form such a structure, it seemed unlikely, and was eventually identified as an optical illusion caused by sunlight passing between two ridges.  Nevertheless, Mare Crisium was the target of the Soviet sample-return missions of the late 60s and 70s  (Fig. 18), and the Russians have announced their intention to go back there, though international cooperation has gone by the board for now.

The advancing terminator crosses Taurus-Littrow, Mare Tranquillitatis, Descartes and Hadley Rille, scenes of the Apollo 17, 11, 16 and 15 landing sites respectively  (Fig. 19).  All have been photographed several times over in recent years by Lunar Reconnaissance Orbiter, and the trails left by astronauts and vehicles are plainly visible, along with the Scientific Experiment Packages which they left.  (‘Yes, We Really Did Land on the Moon – Part 3, the later missions’, ON November 6th 2022).  All the flags are still standing except Apollo 11’s, which Buzz Aldrin saw fall over as they took off.

Fig. 20. Waning Third Quarter Moon, 9th June 2015, 4 am,

As the terminator crosses Hadley Rille, on the border between Mare Imbrium in sunlight and Mare Serenitatis in darkness  (Fig. 20), the Moon is half-full, at Third Quarter.  The features highlighted by shadows in Fig. 8 are all highlighted again, but lit from the opposite direction.  Notice the time of Fig. 20, at 4 a.m. on the 9th of June.  This is my favourite telescopic view of the Moon, because it was my first one.  I had the school telescope on loan for the summer vacation;  I had got up early in hopes to see the latest Soviet Sputnik before sunrise, but having failed in that, I took the telescope through to the front garden and there was the Moon, at Third Quarter, lit in colours like Fig. 20 in the predawn June sky.

In my first serious attempt at nonfiction in my teens, I wrote a guide to the Solar System as I imagined it would be in 2061.  Using the first edition of Patrick Moore’s Guide to the Moon as a handbook, in the lunar chapter I portrayed a journey by rover, monorail and rocket from top to bottom of Fig. 20, starting at top left in Sinus Iridum, the Bay of Rainbows, the lava-flooded crater which was the site of the Moon landing in the BBC’s Journey into Space in 1953  (Fig. 21), and of the Moonbase in the second and third serials.  Odd things happened to that mission, and there are actual oddities about the Bay which were highlighted by the late Jean Sendy in his book L’Ere du Verseau  (Laffont, 1970).  There may be no connection, but it was one of the sites photographed in detail by the Chinese probe Chang’e-2 in 2010, and their Chang’e-3 lander set down near it in 2013  (Fig. 22).  From the Bay my travellers drove along the lunar Alps to the dark-floored crater Plato, visiting the double fault of the Alpine Valley (Fig. 23), before taking the monorail south, passing the standalone Mt. Pico and the double-ringed crater Cassini, once suspected of having undergone major changes due to volcanic action

Plato is notorious for observers’ reports of mists, the ‘Transient Lunar Phenomena’ (TLP)  for recognition of which Patrick Moore campaigned assiduously.  R.A. Smith painted one dramatically for The Exploration of the Moon by Arthur C. Clarke  (Muller, 1954, Fig. 24), and another featured in ‘The Light’ by the late Poul Anderson, first published in Galaxy, March 1957, and reprinted in one of the first anthologies of adult science fiction which I read, 13 Great Stories of Science-Fiction, edited by Groff Conklin  (Gold Medal Books, Fawcett, 1957). 

Fig. 24. R.A. Smith, lunar mist, from The Exploration of the Moon

My travellers went south across Mare Imbrium by monorail, and then by rocket to the crater Alphonsus, the middle one of three giants running north-south from the lunar equator  (Fig. 20).  Alphonsus was the site of the only professionally observed TLP, by the Soviet spectroscopist Kozyrev in 1959.  Kozyrev’s authority was unquestioned  (he also discovered the atmosphere above the rings of Saturn, verified by the Pioneer 11 mission in 1979), but there were no other witnesses to the event and it remained controversial.  In 1964 the first successful US moonprobe, Ranger 9, made a headlong plunge to impact in Alphonsus  (Fig. 25).  Watching the images come in live, the Project Mercury astronaut Wally Schirra shouted “Bail out, you fool!  Bail out!”  The pictures showed dark-rimmed craters on the floor of Alphonsus, apparently volcanic, and there would definitely have been a mission to the central peak if Project Apollo hadn’t been cancelled.

From Alphonsus, my travellers visited the Straight Wall, a geological fault in Mare Nubium.  700-800 feet high, the cliff faces west and is easier to see from here when it’s sunlit at Third Quarter  (Fig. 26).  Chesley Bonestell painted a dramatic view of it for Willy Ley’s The Conquest of Space, but it turns out that the slope is only 7 degrees, and there would be little or nothing to see at ground level. 

From Alphonsus my party travelled on to Tycho, one of the most recent large craters on the Moon – then thought to be 60 million years old, now put at 100 million.  Rays of debris from Tycho spread as far as Alphonsus, and there are many unexplained features.  More recent coverage of the central peak, from Japan’s Kayuga-Selene and from Lunar Reconnaissance Orbiter, appears to show uplifted strata, lava flows and a collapse caldera, like the Cleopatra one on Venus’s Maxwell Montes.  During the Apollo programme, it was said that ‘If you had only two missions left to fly, you’d send one of them to Tycho’;  but in the event, both the last two were cancelled.

I’ve written extensively about the 2001 views of the Earth from Tycho, and from the moonbase in Clavius, further south  (‘Howlers in Space, Part 2’, ON 24th December 2022).  It’s only from the rim of the Moon, as seen from here, that the Earth can be seen near the horizon as so often depicted.  Chesley Bonestell painted it strikingly for The Conquest of Space, and the actual view was first captured by Kayuga-Selene in November 2007.  Mount Malapert  (Fig. 27)  aroused particular interest as a possible site for a lunar radio observatory, because continuous solar power would be available on the summit and the instruments lower down would be shielded from Earth’s radio noise.  But the Clementine mission to the Moon in 1994  (Fig. 28)  aroused fresh interest in the lunar poles.

On the last three Apollo missions the Service Module included a scientific instrument bay for detailed study of the terrain below.  One of the missions which NASA never got to fly would have gone to polar orbit, without a landing, and extended that survey to the whole Moon.  Repeated applications for lunar studies got nowhere, and Clementine, the first US lunar mission in 22 years was a USAF test of sensors for the SDI ‘Star Wars’ programme.  Ironically, the big thing to come out of it was verification of a Soviet hypothesis, that there could be deposits of ice at the lunar poles  (V.S. Safronov, Ye. I. Rushkol, History of the Lunar Atmosphere and the Possibility of Ice and Organic Compounds Existing on the Moon, NASA Technical Translation F-232, US Government Printing Office, 1964).  Clementine’s initial detection was followed up by Lunar Prospector, the probe NASA had wanted for so many years, then Kayuga-Selene and India’s Chandrayaan-1, which established that there are large deposits of ice in the permanently shadowed basins of the polar craters  (Fig. 29).  Some of it may be formed by interaction of the solar wind with moonrock  (Fig. 30);  some of it may come from hydrated layers in the lunar interior  (‘The Moon’, ON 18th April 2021);  and some may come from cometary impacts, like the possible ones of 1178 which Sydney Jordan illustrated for Children from the Sky and Incoming Asteroid  (Fig. 31).  Determining its precise composition and origin may tell us a great deal about the history of the Solar System, the Earth and the Moon.

But it’s not the scientific importance of the deposits which has sparked ‘the new Moon race’.  If there’s enough of it, it can provide life-support for lunar bases and even be used to make rocket fuel.  It’s no coincidence that Chang’e-2 photographed the south pole, though its main mission was to the Bay of Rainbows, as above.  Several of the cubesats carried by NASA’s Artemis-1 were intended to study the shadowed crater floors;  they failed due to the delay in the launch, but Lunar Flashlight, which wasn’t ready in time, was released by Hakuto-R on its way to the Moon and is now at work, having lit up the dark floor of the crater Shackleton  (Fig. 27).  Hakuto-R was targeted for Atlas crater, in the north polar area, Israel’s Beresheet lander targeted Mare Serenitatis, but India’s Chandrayaan-2 went for the south pole region.  All three failed, and their remains have been photographed by Lunar Reconnaissance Orbiter  (another knock on the head for conspiracy theorists who say its Apollo landing site photos must be faked).  On the current schedule, the next one up is NASA’s Polar Resources Ice Mining Experiment  (PRIME-1), built by Intuitive Machines, which is going for a really difficult target on the rim of Shackleton crater – where NASA’s Artemis 3 mission is scheduled to land astronauts, for the first time in 53 years, in 2025.  Or maybe 2026;  we shall see.

Duncan Lunan’s Children from the Sky was published by Mutus Liber, Edinburgh, in 2012, and Incoming Asteroid!  What could we do about it? by Springer, New York, 2013.  Both are available from Amazon or through booksellers.   

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