Chasing lunar shadows

An eight-day-old gibbous Moon imaged on 16 August 2021. Image: Fernando Oliveira de Menezes.

In its 29.5-day voyage around Earth, the Moon makes its first observable appearance as a scimitar of light to the west of the Sun perhaps a day or so past new. Still in the Sun’s proximity, the thickening crescent remains accessible not long after sunset. However, as the Moon steadily puts more sky between itself and the Sun, it sets ever later into the twilight.

Increasingly taking charge of the evening sky, it becomes more obvious against a darkening backdrop as its phase advances towards first quarter. Afterwards circling further behind our planet, it makes its appearance ever later into the night as sunlight steadily consumes the Earth-facing surface until the maximum illumination of full Moon.

In the 14 or so days between new and full, the shadowed line between light and dark that we call the terminator passes through some of the Moon’smost fascinating features – some well-trammelled, others less so. I would like to take you on that journey, sampling just a few of those treats, looking through three visual windows: with the eye alone, through binoculars (by which I mean the ‘classical’ 7× to 10 × 50mm class) and with the aid of a small telescope of maybe 70- 80mm (~three-inch) aperture.

There’s a chance to see a 24-hour-old or so Moon just after sunset on 9 April. This is the scene from London at 7.50pm BST, with the Moon, an extremely slender crescent, sitting some 11° high. There’s a similar scene from Scotland, though the Sun sets around 20 minutes later.

Catch a young Moon

Tracking down a young Moon early in its lunation is a popular observing adventure. On 9 April there’s a chance to see the Moon when it’s just over 24 hours old (1.02 days with a 1.37 per cent phase) following the new Moon of 8 April at 18:21 UT. At sunset from London and Edinburgh, at 7.50pm and 8.08pm BST, the Moon lies 11° high. 

Sweeping with a pair of binoculars or a small telescope AFTER sunset across an unobstructed western should snare the Moon. Let’s hope local atmospheric conditions are particularly fine, free from obscuring thin cloud and haze.

Earthshine captured on 13 June 2021 by the author using a 355mm (14-inch) Meade LX200 telescope and a Canon 5DSr camera for a ten-second shot at ISO 100.

A day or so after new Moon: Earthshine

The Sun-illuminated phase presented to the Earth is the thinnest crescent – a mere glancing blow of sunlight. However, an observer on the Moon looking towards us would see the exact contrary. In the lunar sky, an almost full Earth would be a brilliant source of light. This light illuminates the rest of the Moon’s surface, making it dimly visible to terrestrial observers.

This ‘Earthshine’, or ’Earthlight’ caneasily make major features like the lunar mare visible to the unaided eye. Against a dark sky the effect is a wonder.Through binoculars, even some of the Moon’s lesser features are easily picked out viathis eerie light.

The Moon buzzes the Pleiades (M45) on the early evening of 11 April, with Jupiter located below. This is the scene from the south of England to the west at about 9pm BST. AN graphics by Greg Smye-Rumsby.

Three-day-old Moon: Buzzing the Pleiades, Polar horns and Mare Crisium

On the evening of 11 April the Moon lies around 3° above and to the left of the magnificent Pleiades open cluster (M45). At about 9pm BST the sky will have darkened sufficiently to make the scene very picturesque, especially with brilliant Jupiter lying below, though you’ll need a good western horizon to see it all.

By the time that the Moon is three daysold, it has adopted perhaps its mostclassical appearance, when the polarregions form extended talons (or horns)into the darkness. Illustrators will have owls, fairies and other fantastical creatures ‘sitting’ nonchalantly on the south polar ledge. Others will naively have the curve of this evening phase pointing the wrong way (not as it should, towards the unseenSun beneath the horizon) and might even have ‘foreground’ stars embedded against the Moon’s un-illuminated surface.

For naked-eye observers Earthshinemay still be easily seen, but so far asthesunlit Moonis concernedthe major feature to emerge since new is the MareCrisium (Sea of Crises). As a dark eye-likepatch of grey in the northern part of thecrescent it is unmistakable. Its oval shape is merely an effect of foreshortening since in reality it is an approximately circular impact basin 600 kilometres in diameter.

In addition to being a fascinating limb feature, Mare Crisium also serves as a useful indicator of lunar libration. Libration is an effect of the Moon’s elliptical orbit, by which we sometimes see‘around the corner’ of the face of the Moon turned towards us. We consequently sometimes see Mare Crisium appear well away from the visual edge of the Moon and other times hard up against it. Libration is avery useful phenomenon, by which the Moon offers us a view of 5percent of its surface, rather than just the 50 per cent one might expect from a ‘locked’ orbit. After new Moon, it is always fascinating to discover how far this time theMoon’s disc has ‘nodded’, one way or the other.

The rugged southern uplands

For binocular and telescopic observers another treat is available, for which we must descend to the very southern tip of the crescent.

The south pole of the lunar surface offers some of the highest and roughest terrain on the Moon. At the sharp end of the Moon’s southern horn the local sunrise is now grazing the very tips of this chaotic region. The result is a steadily diminishing chain of illuminated mountain tops divided by chasms of inky blackness. The effect is dramatic in binoculars, but a telescopic view will reveal an almost star-like procession of incandescent points trailing away into nothingness. It is often great sport to see how far into the razor’sedge you can resolve those sunlit rocky crests.

Lunar day five: Craters Theophilus, Cyrillus and Catharina

With the dawning of lunar day five, the ever-moving terminator throws into deep relief one of the best crater chains that the Moon has to offer – that of Theophilus, Cyrillus and Catharina. Located in the mid- southern latitudes, these three features, all approximately 100 kilometres across, command the area at this time. If the observation opportunity is just right, the early lunar morning crater ramparts will sufficiently project into the Moon’s dark hemisphere to make the terminator look jagged to the naked eye alone.

For binoculars, this trio of deep impacts is very rewarding, standing out crisplya gainst the much darker volcanic plain of Mare Nectaris (Sea of Nectar) to the right.Telescopically, the northernmost of this group, Theophilus, provides one of the best examples of a crater’s major features; its entral peak is a multiple summit1, 400 metres high, with the crater walls showing massive slumps and landslides.

Montes Apenninus (the lunar Apennines) majestically guard the south-eastern shore of Mare Imbrium (Sea of Rains). This mountain range is the largest and most striking on the Moon. They begins to appear at the lunar terminator at first quarter phase. Also pictured is 60 kilometre-diameter crater Eratosthenes. Image: NASA/LRO.

Seven days in: First Quarter phase reveals trio of Maria

Seven days after new Moon, we reach the first cardinal point of the Moon’s orbit, when (at 90° to the Sun) a precisely vertical terminator line cleaves the Moon exactly in two. By this time, the unaided observer will notice that a bold diagonal line of three contiguous Mare have joined Crisium in the Moon’s western hemisphere, running roughly from the terminator’s mid-northern latitude to the equatorial limb.

Beginning with the topmost, these are the Mare of Serenitatis (Serenity), Tranquillitatis (Tranquillity) and Fecunditatis (Fertility). Tranquillity was of course the site of anything but, when humans made their first lunar landing there 55 years ago.

In the northern hemisphere, a ragged curved feature now appears a massive chain of mountains called Montes Apenninus, the lunar Appenines. They form the south-western ramparts of the Mare Imbrium (Sea of Showers) – a massive flooded crater plain 1,250 kilometres across only partially revealed at this phase. These mountains are essentially what remains of Imbrium’s crater wall. The seven-to-eight-day-old Moon will throw this ideal binocular feature into deep relief.

The ‘Lady in the Moon’ can be imaged by naked-eye observers from lunar day eight. Maria Serenitatis, Tranquillitatis and Fecunditatis form her piled hair, while the dark patch of Mare Vaporum beneath Serenitatis forms the eye and beneath that a cleft created by Sinus Medii mimics a mouth. AN graphic by Greg Smye-Rumsby.

Day eight dawns: a lunar lady unfolds

By day eight the Moon is really getting into its stride. Its phase, now greater than 50 per cent, is termed gibbous (from a Latin term indicating ‘humped’).

At this time, the naked-eye observer gets an anthropomorphic treat, for a young lady appears. For myself, I perceive her as a late Victorian woman, with the three contiguous Mare of the north- western hemisphere forming her piled hair. The dark patch of Mare Vaporum (Sea of Vapours) beneath Serenity forms the eye and beneath that a cleft created by Sinus Medii (Middle Bay) mimics a mouth. She appears to be gazing wistfully towards the Moon’s top left quadrant.

If you catch the lunar lady rather late on day eight, the creeping terminator may have moved sufficiently eastwards to adorn her slender neck with a sparkling jewel – this being the intensely bright crater Tycho. It may qualify as the easiest crater to be visible to the naked eye.

The Straight Wall (Rupes Recta), a lunar escarpment, is one of the more remarkable features on the Moon. It’s quite a transitory feature that’s visible around lunar day eight. Image: Damian Peach.

Tycho, a wall and colossal craters

This is a great time to look at Tycho with binoculars. It is a deep, crisp excavation 86 kilometres in diameter with an impressive central peak. Relative to the rest of the Moon it is young, perhaps only 100 million years old – though it looks like the impact occurred only yesterday. Streaking away from it, curving upwards towards the north-western limb, faint brushstrokes of light can be seen. But more on these later.

Scanning slowly northwards up the terminator, just halfway towards the lunar equator, a telescope will show something entirely incongruous amongst the dominant circularity of crater formations – a straight line. This is no unearthly signature of an ancient alien civilisation, but rather it is a slump in the lunar crust – a linear fault 110 kilometres long known as Rupus Recta (Straight Wall).

This huge escarpment looks like a cliff face, but is in reality a slope sufficiently shallow to be scaled by walking! Its appearance is actually quite transitory and within another day or so its local Sun climbs above the slope and the shadow vanishes. It can thereafter only be seen with difficulty as a faint line.

Close by the Straight Wall, a little farther up and to the right, we chance upon a second prominent trio of colossal craters: Ptolemaeus, Alphonsus and Arzachel (with diameters of 154km, 118km and 98km, respectively). They are easily visible in binoculars and like their predecessors a couple of days prior, these impacts are classics of crater formation, although appearing rather more worn with floors heavily smoothed by upwelled lava.

A small telescope will show that of the three Arzachel does present evidence of craggy terracing within its walls and a rather tame central peak.

The full 1,000 kilometre extent of the lunar Appenines can now be seen in binoculars. If telescopic observers follow this range’s southern tip into the terminator, they will find a deeply

punctured 60 kilometre-diameter crater called Eratosthenes. Its internal walls are heavily terraced and the prominent central peak stands very tall at the centre.

Should the local Sun still be low, this central peak may be visible as a star-like spark against the unlit crater floor. If conditions are very good and your ‘scope will allow higher magnification, the crater floor will be seen as not smooth, as is the case with so many, but strewn with rubble.

The mighty crater Copernicus is probably the most observed on the Moon. You can’t miss it as lunar day ten dawns. Image: Damian Peach.

Lunar days 10: Mighty Copernicus rules

Day 10 sees much of the Moon’s eastern hemisphere coming into view. The majority of the Imbrium basin is now in sunlight. Together with Mare Insularum (Sea of Islands), Cognitum (the Known Sea) and Nubium (Sea of Clouds) beneath, it now forms a contiguous dark laval region running almost the full length of the terminator. Like a long shadow, this is easily noticed without optical aid.

Slap bang in the middle of this otherwise featureless band of Mare is the vivid splat of Copernicus, a monstrous impact crater nearly 100 kilometres across. Binoculars will reveal the massive spray of ejecta surrounding it, the bright residue starkly seen against the heavy grey of the surrounding lava plain. Drawing closer with a telescope will reward the observer with deeply terraced crater walls and a jumble of central mountains.

Aristarchus is one of the brightest craters on the Moon. North of it lies Vallis Schröteri (Schröter’s Valley), a snaking lava channel. Image: Damian Peach.

Lunar day 12: the Aristarchus Plateau

The dawning of lunar day 12 sees the bulk of the last major ‘sea’ to emerge blinking into the sunlight, Oceanus Procellarum (Ocean of Storms). It inaugurates the appearance of two of my favourite areas of the telescopic Moon, the Aristarchus Plateau and the Marius Hills.

Like Copernicus, Aristarchus is another very bright crater (albeit a little smaller at 40 kilometres in diameter) struck lonely into the gloomy barren backdrop of this stormy ocean. Indeed, some say it has the brightest reflectance of any lunar feature. Slightly higher and nearer the terminator than Copernicus, with Tycho these three bright craters comprise those most likely to be spotted unaided by those with acute vision.

A comparatively fresh impact, a telescope may reveal Aristarchus’ best known characteristic, the enigmatic vertical banding that at this time is visible against its deep multi-level internal terracing. The contrast of these mysterious scree slopes changes gradually with the Sun’s elevation. There are other craters that show a similar phenomenon, but it is in Aristarchus that it is most pronounced.

Observers using binoculars will enjoy a view of Vallis Schröteri (Schröter’s Valley), a snaking lava channel cut deep into the surrounding surface that emerges between Aristarchus and its neighbour, Herodotus. 

Below Aristarchus, about the same distance from the terminator, we alight upon dark-floored crater Marius. The area immediately to the west, between the crater and the terminator, is a raised plateau containing one of the Moon’s highest concentrations of volcanic activity, the Marius Hills. If timed correctly, long shadows will be cast by a series of domes, volcanic features averaging ½km in height and ranging in size between house-sized cones to mounds a few kilometres across. I will admit that this may be a challenging observation for a small telescope (possibly requiring at least an 80mm [three-inch] APO refractor), but even with lower resolution the area will look mottled.

The ‘Man in the Moon’ is a familiar feature for visual observers. The ‘face’ arise from Imbrium being the left eye, Serenity being the right, Vaporum the shadow beneath his nose with Humorum and Nubium joining forces as the mouth. AN illustration by Greg Smye-Rumsby.

Full Moon after a fortnight: The Man in the Moon

During day 14, the terminator completes its 10 m.p.h. (equatorial) crawl across the surface and disappears (around the corner of the eastern limb). The silvery gaze of the full Moon has arrived. As with the early crescent, poetic license is upon us once again, for the Lady of the Moon is now joined by her gender counterpart.

Familiar to most naked-eye observers, the Man in the Moon’s features arise from Imbrium being the left eye, Serenity being the right, Vaporum the shadow beneath his nose with Humorum and Nubium joining forces as the mouth. I have always thought his turned down grimace and the streak of tearful sadness (formed by Tranquillitatis and Fecunditatis) flowing away from his right eye make him look a little morose.

But we may be cheered by the full Moon’s other inhabitant, the rabbit. This takes a bit more effort to see – and very much depends on the Moon’s orientation with respect to the horizon, but here goes. Serenity is the rabbit’s snout, with Tranquillity, Fecundity and Nectar forming the back of its head and floppy ears. Imbrium constitutes its chest, with the darker areas below, the hind legs and feet.

Don’t ignore the full Moon as you’ll miss some of the extensive rays systems centred on craters such as Tycho, seen to the south, and Copernicus, just left of centre. Image: Miguel Claro.

Glorious lunar rays

In binoculars the full Moon provides one of astronomy’s most undervalued glories, the lunar rays. In those distant days when the lunar surface was being pummelled by incoming debris, each crater impact sprayed out vast clouds of tiny molten globules which froze before falling back to the surface as billions of perfectly spherical glassy beads. This misty ejecta was unevenly funnelled through the ragged terrain of the impact, so once settled, it created surface tracks of material that can be traced back to its origin.

At full Moon, with the Sun behind the observer’s shoulder (and the Earth!) these glass beads are serving as perfect reflectors, bouncing fierce solar light straight back towards the Earthly observer. This effect is so marked that the intensity of the rays can change quite markedly in the hours either side of full Moon, reaching their peak brightness as the phase angle reaches 100-per cent. So do not dismiss the rays as they portray the violence of the Moon’s history just as vividly as the craters they spawned from.

Get yer ‘scope out at full Moon

Don’t give up on the telescope either. Using a neutral density filter (and experimenting, as always, with others), a higher-power view of these ejecta blankets can be fascinating – and I think, brings home the violence of the original impacts like nothing else. With adequate filtration and favourable observing conditions, some of the lunar rays (particularly those originating at Tycho) can be traced all the way across the Moon’s face.

As soon as the terminator disappears from the Moon’s eastern flank, it makes its appearance in the west – but instead, begins to steadily extinguish the Moon’s surface rather than illuminating it.

I have spotlighted just a little of the Moon’s bounty. I hope it has whetted your appetite for continued exploration. Never forget that with ever-changing angles of solar illumination, the chances of an observer seeing features on the Moon in exactly the same light twice are vanishingly remote. Good hunting.