Moon tour

Why the day af­ter first quar­ter is one of the best nights for ob­serv­ing the lu­nar sur­face

All About Space - - Contents -

The dif­fer­ent phases of the Moon of­fer dif­fer­ent vis­ual treats and de­lights.

The cres­cent phase, whether it’s a very young ‘new’ Moon hang­ing in the western sky af­ter sun­set, or an old wan­ing Moon glow­ing above the eastern hori­zon be­fore sun­rise, is a beau­ti­ful sight. It can look par­tic­u­larly strik­ing if it hap­pens to be shin­ing close to a bright planet. If the bright, sun­lit cres­cent is quite thin you can of­ten see the rest of the Earth-fac­ing side of the Moon glow­ing with the sub­tle laven­der light of Earthshine, too.

De­spite what many ob­servers will tell you, the full Moon is not the worst lu­nar phase to ob­serve. True, with the Sun beat­ing down mer­ci­lessly from high above there is no sur­face re­lief to see, no shad­ows are cast be­hind the Moon’s jagged moun­tains or into the bowls of its deep craters, but the full Moon is when it is eas­i­est to see the con­trast be­tween the dark lu­nar seas and its rugged high­lands, and to iden­tify its ma­jor fea­tures too. Full Moon is also the best time to see the bright ‘rays‘ streak­ing across the Moon’s face – trails of dusty de­bris sprayed out across the Moon by the im­pacts which blasted the youngest craters out of the sur­face. Also, few sights in as­tron­omy can com­pare with see­ing a bloated full Moon ris­ing up from be­hind the trees like an enor­mous sil­very hot air bal­loon.

How­ever, I have al­ways thought that one spe­cial day of the lu­nar month of­fers the best of both worlds, and pro­vides stun­ning views through binoc­u­lars and small tele­scopes. When the Moon is just slightly gib­bous, a day past first quar­ter – what many peo­ple call a ‘half Moon‘ – it of­fers the ob­server fan­tas­tic views of ev­ery type of lu­nar fea­ture. With the ter­mi­na­tor – the line be­tween lu­nar night and day – run­ning al­most straight down the mid­dle of the Moon’s face the light is just per­fect for see­ing its craters, moun­tain ranges, sprawl­ing seas and long de­bris rays, too.

Binoc­u­lar views of the Moon the day af­ter first quar­ter are fas­ci­nat­ing, with the seas on the eastern side of the Moon’s face clearly vis­i­ble as dark, blue­grey splodges, and the largest craters along the ter­mi­na­tor look­ing like pock marks. Through a small tele­scope with a low-power eye­piece, with the Moon al­most fill­ing the eye­piece, you can eas­ily imag­ine you’re a space tourist, fly­ing to­wards the

Moon in a space­ship.

In­crease the mag­ni­fi­ca­tion so you’re look­ing straight down into the craters along the ter­mi­na­tor and you’ll feel like you’re stand­ing be­hind the as­tro­nauts of the fu­ture as they de­scend to­wards the sur­face, look­ing for a safe land­ing site, just as Arm­strong and Aldrin did in 1969 when they guided the Eagle lu­nar mod­ule to­wards its his­toric land­ing on the Sea of Tran­quil­ity.

When the Moon has just passed first quar­ter you will be able to see the sweep­ing curve of the jagged Ap­pe­nine moun­tain range, right on the ter­mi­na­tor to­wards the north. Just above those moun­tains the crater Archimedes will stand out from the sur­face in stark re­lief, look­ing as fresh as if it had been made the day be­fore. To the east, next to the curv­ing limb, the oval Mare Cri­sium will look like a dark thumbprint on the Moon, and be­tween it and the ter­mi­na­tor other dark seas will form the shape of a crab’s claw. In the cen­tre, just to the right of the ter­mi­na­tor, a chain of three craters, Ptole­maeus, Alphon­sus and Arzachel will look very im­pres­sive. At the top of the disc, on the ter­mi­na­tor, the dark-floored crater

Plato will stand out clearly, while back to­wards the bottom of the ter­mi­na­tor, the young crater Ty­cho will be start­ing to emerge from the shad­ows. As you stare down into it, Ty­cho might re­mind you of a bul­let hole in a wall, or a pit left on the sur­face of a frozen lake af­ter the im­pact of a heavy stone.

This month the Moon is at first quar­ter on the evening of 22 April, and if you ob­serve it on that night you will still have fan­tas­tic views, but the fol­low­ing night those views will be just a lit­tle bet­ter. If you don’t be­lieve us, take a look your­self!

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