Doorstep Astronomy: Tour the 7-day-old Moon

For most beginning astronomers, the moon represents themost accessible target in the sky, whether you're using binoculars, a smalltelescope, or just your eyes. It is the only "world" which we can seein breathtaking detail: mountains, plains, volcanoes, craters, and othertopographic features.

The moon is half a degree in diameter as seen with thenaked eye (your fist on an outstretched arm covers about 10 degrees of sky).When Mars is next closest to the Earth in January next year, it will take atelescope magnifying 130 times just to show Mars the same size as you can seethe Moon without any telescope at all.

With a small binocular, 7x50 or 10x50, you can see far moredetail on the moon than any backyard telescope will ever show on Mars. Even thesmallest amateur telescope will reveal the riches of the lunar surface inenormous detail, providing hours of exploration and enjoyment.

When andwhere to look?

The moon is a sphere lit by the sun from various angles asit moves in orbit around the Earth.

The best time to observe the moon is when the sun is justrising or setting on the lunar surface: the shadows will be long and surfacerelief will be exaggerated. This means observing along what is called theterminator, the boundary on the moon between sunlight and shadow. Theterminator sweeps across the lunar surface night after night, exposing more ofthe lunar surface, and then covering it up again two weeks later as the moongoes through its phases.

Tonight (Wednesday) the moon will be seven days old, andthe terminator will be close to a straight line. (Tomorrow night will besimilar circumstances.)

The moon is high in the southern sky at sunset and will bewell placed all evening. The area on the moon to concentrate on is close to thesunlit side of the terminator.


If you're observing with your naked eyes or withbinoculars, the directions on the moon are easy. If you live in Earth'sNorthern Hemisphere, north is up, south is down, east is to the right, and westis to the left.

Through a telescope, these directions will probably bereversed, either east and west (refractors, Cassegrains) or both east and west,and north and south (reflectors). It's probably best to orient yourself by themoon's topography: the moon's Northern Hemisphere has many open plains, knownas "maria" (singular "mare," pronounced"mahr-ey," meaning "sea" in Latin) and relatively fewmountains and craters. The moon's Southern Hemisphere is much more mountainous,and has many more craters.

The terminator runs in an arc from the moon's north pole toits south pole.

The most obvious features on the first quarter moon are thethree linked maria from north to south: the Mare Serenitatis (Sea of Serenity), Mare Tranquillitatis (Sea of Tranquility), and the Mare Fecunditatis (Sea of Fecundity).

A tour ofthe terminator

The most striking feature at first quarter is the chain ofthree giant craters, 60 to 90 miles in diameter (100 to 150 km), just to thesouth of the center of the terminator. The largest of these, Ptolemaeus, has abroad flat floor, like a miniature mare. The middle crater, Alphonsus, has amuch more complicated floor, with a central peak and many ridges and rillesscarring its surface. The southernmost crater, Arzachel, is a classic terracedcrater with a high central peak.

About half way between these three craters and the northpole is a fascinating area where the Mare Serenitatis meets the Mare Imbrium,where tonight the sun is just rising. Twin mountain chains on the south ?shores?of both these ?seas? form a wedge, and right at the tip of this wedge is thespot which was chosen by NASA to land the fourth expedition to the moon, Apollo15, on July 30, 1971.

The most prominent feature in this area is the mountainMons Hadley. Immediately to the west of Mons Hadley, visible in smalltelescopes, is the small flat area where Apollo 15 landed. A large amateurtelescope will reveal the sinuous rille (a collapsed lava tube), visible inmany of the photographs taken by the astronauts.

To the south of the three giant craters is just about themost rugged and mountainous terrain to be found anywhere on the moon. With agood map of the moon you can navigate all the way to the south pole.

Should you wish to explore further, Starry Night willidentify hundreds of lunar features for you.

Thisarticle was provided to by Starry Night Education, theleader in space science curriculum solutions.

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Geoff Gaherty
Starry Night Sky Columnist

Geoff Gaherty was's Night Sky columnist and in partnership with Starry Night software and a dedicated amateur astronomer who sought to share the wonders of the night sky with the world. Based in Canada, Geoff studied mathematics and physics at McGill University and earned a Ph.D. in anthropology from the University of Toronto, all while pursuing a passion for the night sky and serving as an astronomy communicator. He credited a partial solar eclipse observed in 1946 (at age 5) and his 1957 sighting of the Comet Arend-Roland as a teenager for sparking his interest in amateur astronomy. In 2008, Geoff won the Chant Medal from the Royal Astronomical Society of Canada, an award given to a Canadian amateur astronomer in recognition of their lifetime achievements. Sadly, Geoff passed away July 7, 2016 due to complications from a kidney transplant, but his legacy continues at Starry Night.