Observing the First Quarter Moon
September 2012 :
The Moon is an easy target for event the cheapest of small telescopes. Even binoculars can show a casual stargazer the more prominent features that populate the lunar landscape.
With those parameters in mind, I'm going to highlight some of the major features that are easily discernible if you know where to look and what to look for. During the monthly lunar cycle an observer can watch the sunlit portion of the Moon wax (get bigger) and wane (get smaller). I don't want to overwhelm you if you're just a budding amateur astronomer, so I've restricted this observing session to the first quarter Moon, technically a waxing phase that can be observed during the early evening hours after sunset.
The accompanying first quarter Moon map has my selection of features labeled which I will now highlight.
The map is oriented north/south, top/bottom, the way the Moon appears to the naked eye. Your telescope may invert the image, or may shift it right to left. If this is the case, you'll have to mentally allow for these ifferences when referencing the map. When binoculars are used, the Moon's image will be as the eye sees it in the sky. Though manned and unmanned missions to the Moon have unveiled its secrets, one can relax in the backyard and explore the Moon. For the beginning observer, as well as the seasoned amateur astronomer, the Moon is quite an exciting world to discover with binoculars or telescope.
Though the Moon reaches first quarter on September 22, you may begin observing it anytime after new moon on the 15th. If you get a chance to observe each night, you'll see new features emerge from lunar night into sunlight. This sunrise point, called the terminator, is a wonderful area to train your telescope or binoculars. Much detail can be seen here because the sun is low on the lunar horizon, thereby casting exceedingly long shadows. As the sun rises higher into the lunar sky, you'll see the same features "change" their appearance. You'll see what I mean when you observe the phenomena firsthand.
We will begin our exploration of the first quarter Moon with the six seas that can be seen best during this lunar phase. Lunar seas are not seas as we know them on the Earth, though early astronomers thought they were bodies of water. Rather, they are seas of solidified lava.
When huge meteors struck the lunar surface billions of years ago, lava welled up through the resultant craters and flowed out onto the low plains. These "maria," or seas, are the smooth plains we can easily view with the naked eye. Since lunar seas formed after the heavy bombardment of cosmic debris, the sea floors are marred only by a small number of craters. This will be quite evident in binoculars and telescopes.
Six major seas can be seen at first quarter. Start your observations at the northern (top) portion of the Moon. You will see (A) Mare Frigoris, Sea of Cold; (B) Mare Serentatis, Sea of Serenity; (C) Mare Tranquillitatis, Sea of Tranquillity; (D) Mare Nectaris, Sea of Nectar; (E) Mare Crisium, Sea of Crises; and (F) MareFoecunditatis, the Sea of Fertility. The names applied to these features indicate that early astronomers had quite a different landscape in mind for our desolate neighbor.
Next we'll examine some prominent craters that can be seen at first quarter. Just south of the Sea of Cold (A) is the crater Aristoteles (G). It is 55-miles in diameter and about 12,000-feet deep. On the crater floor are two central peaks that should be easy to spot with a small telescope. Notice the massive crater walls -- they remind me of the terraced farmlands of China. To the east (right) of Aristoteles is a close pair of interesting craters, Hercules (H) and Atlas (I). Hercules is 45-miles in diameter and 12,500-feet deep, whereas Atlas is 54-miles in diameter and 10,000-feet deep. Both have very steeply terraced walls and are well preserved ---- they have not been obliterated by later impacts. Hercules has a small crater on its floor just south of its center. Atlas also has many interesting features on its floor, including a crisscrossing of six rilles or clefts, which were caused by the collapsing of material below. (It looks similar to earthquake faults or sinkholes.) So much detail is visible in this region that you could easily spend an entire evening observing Hercules and Atlas. However, this is just an introduction to lunar observing. Additional craters merit more than a casual glance as well.
South of Hercules and Atlas and on the northeast edge of the Sea of Serenity (B) is found one of the Moon's finest craters, Posidonius (J). Posidonius is 60-miles in diameter and has very low and narrow walls. Considered to be a walled plain, it is the remnant of an ancient crater that was partially melted and filled with lava. An inner ring spirals out from north of Posidonius' center toward its south wall. There are also several central peaks visible on the floor, as well as an impact crater. Several other craters can be found on and just outside of Posidonius' northeast crater rim.
At the juncture of the Sea of Serenity (B) and the Sea of Tranquillity (C) lies a 27-mile wide crater called Pliny (K), standing like a sentinel between the two seas. Pliny has a very bright floor 10,500 feet below the crater rim. It is also well preserved, having been of fairly recent origin. The small central peak, however, looks like it has been pulverized by some past action.
A good distance to the southwest lies ancient crater Hipparchus (L). Not only has this crater been lava flooded, but also just inside the north wall is another impact crater. Its south wall has been breached by an additional impact as well. Though Hipparchus is one of the largest craters, its heyday as a lunar showpiece was perhaps billions of years ago, before it was dramatically altered by impacts and lava flows.
In stark comparison to the crater Hipparchus is Albategnius (M), a neighbor to the south. It too is an old feature, but unlike Hipparchus, it has not been obliterated. Albategnius is 81-miles in diameter and 14,400-feet deep. Solidified lava has also partially filled it. A large impact crater is visible on its southwestern wall. This 30-mile scar almost reaches the small central peak on Albategnius' floor. This too has been filled with lava, though a central peak remains visible.
To the east we find Theophilus (N) on the shores of the Sea of Nectar (D). This extremely well-preserved crater is 65-miles in diameter and 23,300-feet deep. Its walls are very steep and full of detail. The southwest wall encounters another crater of almost identical size. (I call them the "Siamese Craters.") Theophilus also has a multiple central peak region that catches the early sunlight while the deep floor remains in darkness. This peak stands out like a beacon when the terminator approaches the eastern wall of the crater.
The last feature we'll explore today resides on the edge of the Sea of Fertility (F). It is an 85-mile wide crater called Langrenus (O). This crater stands in great contrast to the lava plain. The crater walls rise about 6500-feet from the floor of the plain, then steeply descend 16,200-feet to the crater floor. A small telescope will show a small central peak at the bottom.
In conclusion, please do not observe only the features I have outlined. There are many more craters, as well as majestic mountain ranges, to behold. Hundreds of rilles and valleys also await your gaze. If you've enjoyed the challenge of locating the features detailed in this column, please continue your lunar exploration by acquiring a detailed Moon map. Remember, we have looked at the Moon only during first quarter. There's yet another half of the lunar surface which faces the Earth that we haven't even examined!
Keep your eyes to the skies!!
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