Lunar Shadow Chase

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 is 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.

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’ can easily 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 via this eerie light.

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 days old, it has adopted perhaps its most classical appearance, when the polar regions form extended talons (or horns) into the darkness. For naked-eye observers, Earthshine may still be easily seen, but so far as the sunlit Moon is concerned, the major feature to emerge since new is the Mare Crisium (Sea of Crises). As a dark eye-like patch of grey in the northern part of the crescent, it’s unmistakable.

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.

As the waxing Moon progresses, new and fascinating landscapes come into view with each passing day. On the fifth day after the new Moon, the ever-shifting terminator throws into sharp relief one of the most striking crater chains on the lunar surface – Theophilus, Cyrillus, and Catharina. These three craters, each approximately 100 kilometers across, dominate the mid-southern latitudes.

If observed at the perfect moment, the early lunar morning light will cause the crater ramparts to project into the Moon’s dark hemisphere, making the terminator appear jagged even to the naked eye. For binocular observers, this trio of deep impacts stands out crisply against the much darker volcanic plain of Mare Nectaris. Telescopically, the northernmost crater, Theophilus, offers one of the best examples of a crater’s major features, with its central peak rising 1,400 meters high and massive slumps and landslides visible along the crater walls.

Seven days after the new Moon, at the first quarter phase, a bold diagonal line of three contiguous maria – Serenitatis, Tranquillitatis, and Fecunditatis – becomes visible in the Moon’s western hemisphere, running from the mid-northern latitudes to the equatorial limb. In the northern hemisphere, the ragged curved mountain range known as Montes Apenninus, the lunar Apennines, appears as the southwestern ramparts of the massive Mare Imbrium basin. This chain of mountains, essentially the remnants of Imbrium’s crater wall, is an ideal binocular target, with the terminator throwing it into deep relief.

On the eighth day, the Moon’s phase exceeds 50 percent, and it appears gibbous, or “humped.” That’s when observers can discern the familiar “Lady in the Moon” – the maria of Serenitatis, Tranquillitatis, and Fecunditatis forming her piled hair, the dark patch of Mare Vaporum as her eye, and the cleft of Sinus Medii mimicking a mouth. If timed correctly, the brilliant crater Tycho may appear as a sparkling jewel adorning her slender neck.

As the terminator creeps northward, a telescope will reveal the remarkable Rupes Recta, or “Straight Wall,” a lunar escarpment approximately 110 kilometers long. Nearby, the trio of colossal craters Ptolemaeus, Alphonsus, and Arzachel, ranging from 98 to 154 kilometers in diameter, showcase classic crater formations. Arzachel, in particular, exhibits craggy terracing and a central peak within its walls.