HOLD -- Doorstep Astronomy: Tour the 7-day-old Moon

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

The moon is half a degree in diameter as seen with the naked 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 a telescope magnifying 130 times just to show Mars the same size as you can see the Moon without any telescope at all.

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

When and where to look?

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

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

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

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

Getting oriented

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

Through a telescope, these directions will probably be reversed, either east and west (refractors, Cassegrains) or both east and west, and north and south (reflectors). It's probably best to orient yourself by the moon's topography: the moon's Northern Hemisphere has many open plains, known as "maria" (singular "mare," pronounced "mahr-ey," meaning "sea" in Latin) and relatively few mountains 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 to its south pole.

The most obvious features on the first quarter moon are the three linked maria from north to south: the Mare Serenitatis (Sea of Serenity), Mare Tranquillitatis (Sea of Tranquility), and the Mare Fecunditatis (Sea of Fecundity). Set apart from these and between them and the edge of the moon is the smaller oval Mare Crisium (Sea of Crises). This last is really a gigantic circular basin, but the curve of the moon's surface makes it appear oval. Pay attention to its shape and position from one month to the next, as it goes through a number of changes as the moon rocks back and forth relative to the earth.

A tour of the terminator

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

About half way between these three craters and the north pole is a fascinating area where the Mare Serenitatis meets the Mare Imbrium, where 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 the spot which was chosen by NASA to land the fourth expedition to the moon, Apollo 15, on July 30, 1971.

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

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

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

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.

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