The Moon is our nearest neighbor, except for certain asteroids and man-made satellites. This bleak, airless sphere is about 2,160 miles in diameter, and revolves around Earth at an average distance of some 238,857 miles, completing one revolution in about 27 days. The lunar orbit is an ellipse, not a true circle; so the distance of the Moon from Earth changes.
Since the Moon rotates on its axis in the same time it takes to revolve around Earth, the lunar hemisphere visible to us remains about the same. Librations (apparent tilting due to the Moon’s motions with respect to Earth) make a total of about 59 per cent of the lunar surface visible each month.
Earth’s nearest neighbor: This composite of the Moon, showing details with exquisite clarity, was made by combining photographs taken at first and last quarter. The shadows on the opposite hemispheres differ in direction. The Moon at full phase would look flat, show little detail, because of lack of shadows. (Lick Obs.)
THE LIGHT OF THE MOON Sunlight falls on the Moon as it does on Earth. But the Moon has no atmosphere to filter the fierce rays. The lunar surface in daylight, therefore, gets intensely hot—something like 250°F., or hotter than boiling water. (The dark side probably gets to 243°F. below zero!) What we call “moonlight” is simply sunlight which the Moon is reflecting toward Earth. Except during lunar eclipses, a full half of the Moon is always lighted by the Sun. But we see this full half only when Earth is between Sun and Moon—the phase called full Moon. When the Moon is not in line with Earth and Sun, we see only part of the lighted half.
Just after new Moon, a thin bright crescent is seen. The rest of the disk is faintly lighted. This faint light, called “the old Moon in the new Moon’s arms,” is light reflected from Earth to the Moon’s dark side, and is known as earthshine.
The Moon rises about 50 minutes later each night on the average, but the actual time from month to month varies considerably. For some evenings around full Moon near the autumnal equinox (about September 23) moonrise will be only about 20 minutes later each night, because the angle between ecliptic and horizon is then near the minimum. Thus we have moonlight in early evening longer than usual. This phase is called Harvest Moon. The next full phase after Harvest Moon is known as Hunter’s Moon.
Lunar halo: Refraction of moonlight by ice crystals in high atmosphere produces this spectacle. Usually the halo is of 22° radius, sometimes 46°.
The lunar pathway stays near the ecliptic, or path of the Sun. However, while the Sun rides high in summer and low in winter, the Moon rides low in summer and high in winter. At the full phase, the lunar disk may take odd shapes as it rises or sets, particularly when seen through a dense or smoky atmosphere. Sometimes refraction makes it look oval.
The Moon is one of the most satisfying objects for the amateur. The best times to observe are between last and first quarter; then there is less glare. The shadows of the mountains and in the craters are longest and set off the rugged landscape in sharp relief. Often the Moon can be well observed during daylight hours.
When beyond the crescent stage, the Moon reflects considerable light. For eye safety during prolonged telescopic observing, a filter should be used over the eyepiece, or a filter cap placed over the objective, or the area of the objective reduced by placing over it a cardboard cap with a hole of the desired size. (See here and here.) Another way to cut down glare is to use an eyepiece of high enough power so that only part of the Moon is in the field of view. Then less light reaches the eye.
After a period of lunar observing, the sensitivity of the eye to fainter objects is much reduced. Plan observing sessions accordingly.
Some mountain ranges, seas, and craters can be seen even with binoculars. Much more can be seen with a small telescope. With a 3- or 4-inch instrument, use a lower power—30 to 100×. Don’t use power beyond what atmospheric conditions allow.
For serious lunar work, a 6- or 8-inch telescope is needed, with a power up to 300x or 400x. This will clearly show details only a half mile across.
Atmospheric effects: The Moon may appear reddish and flattened when near the horizon, because then the light coming from it to us has a longer path through the atmosphere. Red rays penetrate the atmosphere more easily than others. Refraction causes the flattening.
Based on a more detailed map published by Sky and Telescope, © copyright 1956 by Sky Publishing Corp., Cambridge, Mali.
Mountains, Valley, and Scarps
| A | Alpina Valley |
| B | Alps |
| C | Altai |
| D | Apennine Mts. |
| E | Carpathian |
| F | Caucasus |
| H | Harbinger |
| I | Hyginus Cleft |
| J | Jura Mts. |
| K | Pico |
| L | Pyrenees |
| M | Riphaeus |
| N | Straight Range |
| O | Straight Wall |
| P | Teneriffe |
Craters
| 1 | Agrippa |
| 2 | Albategnius |
| 3 | Alphonsus |
| 4 | Apianus |
| 5 | Apollonius |
| 6 | Archimedes |
| 7 | Aristarchus |
| 8 | Aristillus |
| 9 | Aristoteles |
| 10 | Arzachel |
| 11 | Atlas |
| 13 | Autolycus |
| 13 | Bayer |
| 14 | Bullialdus |
| 15 | Bürg |
| 16 | Cassini |
| 17 | Catharina |
| 18 | Clavius |
| 19 | Cleomedes |
| 20 | Colornbo |
| 21 | Copernicus |
| 22 | Dawes |
| 23 | Encke |
| 24 | Eratosthenes |
| 25 | Eudoxus |
| 26 | Fabricius |
| 27 | Flamsteed |
| 28 | Fracastorius |
| 29 | Franklin |
| 30 | Gassendi |
| 31 | Gauricus |
| 32 | Geber |
| 33 | Gemma Frisius |
| 34 | Godin |
| 35 | Goodacre |
| 36 | Grimoldi |
| 37 | Hell |
| 38 | Heraclitus |
| 39 | Hercules |
| 40 | Herschel |
| 41 | Herschel, J. |
| 42 | Hevelius |
| 43 | Hipparchus |
| 44 | Isidorus |
| 45 | Julius Caesar |
| 46 | Kepler |
| 47 | Lambert |
| 48 | Licetus |
| 49 | Linné |
| 50 | Longomontanus |
| 51 | Macrobius |
| 52 | Maginus |
| 53 | Manilius |
| 54 | Mercator |
| 55 | Messier |
| 56 | Moretus |
| 57 | Newton |
| 58 | Orontius |
| 59 | Pallas |
| 60 | Petavius |
| 61 | Picard |
| 63 | Pickering, W. H. |
| 63 | Plato |
| 64 | Plinius |
| 65 | Posidonius |
| 66 | Ptolomaeus |
| 67 | Purbach |
| 68 | Pythagoras |
| 69 | Rabbi Levi |
| 70 | Regiomontanus |
| 71 | Reinhold |
| 72 | Riccioli |
| 73 | Sacrobosco |
| 74 | Schiller |
| 75 | Snellius |
| 76 | Stevinus |
| 77 | Stöfler |
| 78 | Theaetetus |
| 79 | Theophilus |
| 80 | Tycho |
| 81 | Vitruvius |
| 82 | Vlacq |
| 83 | Walter |
Copernicus: Mountain-ringed plain (right) has eight peaks; one is 2,400 feet high. Note rays around Copernicus; also “drowned” craters (high center). (Mt. Wilson and Palomar Obs.)
THE LUNAR LANDSCAPE On the dry lunar surface, mountains rise to heights of over two miles, and among the numerous craters are some as wide as 60 and 70 miles. Vast flat areas—the maria, or “seas”—are visible. Great mountain ranges such as the Apennines, Alps, and Caucasus, are always of interest. Numerous craters deserve special attention—Plato and Archimedes, Copernicus and Tycho and Kepler. There are the long, straight cliffs; deep, narrow, or crooked valleys; wide cracks sometimes extending hundreds of miles; light-colored streaks or rays extending from some of the craters, the most striking of which are those from Tycho.
Phases of the Moon: At all times, a full hemisphere of the Moon is lighted by the Sun. How much of the lighted area we can see at any time depends upon the position of the Moon in its orbit at that time. In the northern hemisphere, the Moon appears to grow from right to left, as in this diagram (bottom). In the southern hemisphere it grows from left to right. The full cycle covers 29 days 12 hours 44 minutes.
Straight Wall: This steep escarpment (near center), about 80 miles long, probably results from a fault in Moon’s crust. The “drowned” craters (lower right) were perhaps filled by lava flows. (Lick Obs.)
NAMES ON THE MOON In 1650 the astronomer Riccioli produced the first map of the Moon. He is said to have originated the system of naming that we use today. The great plains or flat areas are called “seas” (Mare Nubium, Sea of Clouds; Mare Imbrium, Sea of Rains; and so forth). Most of the mountain ranges bear some resemblance to those on Earth, such as the Alps and Apennines, and are named after them. Conspicuous craters bear the names of ancient philosophers and of astronomers—Plato, Archimedes, Kepler. Some features are named for counterparts on Earth—bays and gulfs, capes and lakes.
RECENT DISCOVERIES The Moon’s far side-the 41 per cent never seen by observers from Earth—was an unknown landscape until photographed by a Russian spacecraft in 1959. Photographs showed a cratered landscape lacking the large “seas” observed on the lunar surface that faces Earth. Since 1959, photographs and direct observations by U.S. astronauts, along with instrument readings, have provided much detailed information.
Lunar Apennines (left): Here is part of a splendid range, up to 14,000 feet high, which runs 450 miles around the west side of Mare Imbrium. Big crater is Archimedes, 70 miles wide. (Lick Obs.)
Four lunar features (right): The crater Plato, 60 miles wide, dominates the scene. Extending to the left and above Plato are the Alps. To Plato’s right, near the edge of the photo, is the Straight Range, a chain of a dozen peaks stretching 45 miles. Above and slightly to the right of Plato, the mountain Pico towers 8,000 ft. above Mare Imbrium. (Lick Obs.)
Tycho (left): This is a mountain-ringed plain like Copernicus, with width of 54 miles. The rays extend hundreds of miles. (Yerkes Obs.)
Pickering (right): A meteoroid approaching from the left struck the lunar surface at a sharp angle, making a crater and strewing impact debris, visible as “rays” extending toward the right.
The Moon has a rigid rock crust some 620 miles thick, covering a relatively soft interior. The rock is mainly basalt and anorthosite, formed by cooling of molten materials. Samples dated by radioactivity are as old as 4.4 billion years (the solar system is about 4.6 billion years old). The maria are solidified lava flows, some resulting from collisions of the Moon with big meteoroids long ago. Many craters are volcanic; others result from meteoroid impacts. Impacts probably are responsible also for the “rays,” which consist of fragmented rock extending out from craters like spray. Numerous scarps and “rills” (valley-like depressions) result from faulting.
Recent discoveries make the Moon more, not less, interesting for amateur observers. Meteoroid impacts are watched for. Alphonsus and other craters are inspected regularly for signs of reddening or haze that would indicate volcanic activity. Observers check their observations against maps and photographs to detect recent changes on the lunar surface. Occultations and eclipses are viewed. Perhaps most of all, the amateur can still enjoy the face of the Moon, with the play of sunlight on its stark features, as one of the world’s grandest spectacles.
ECLIPSES Once in a while, at full phase, the Moon passes through Earth’s shadow, and we have one of nature’s most glorious phenomena: an eclipse. In any one year there may be two or even three lunar eclipses, or none. A total lunar eclipse lasts as much as 1 hour and 40 minutes—much longer than a total solar eclipse. There is plenty of time to see it and observe the ever-changing colors. During an eclipse, familiar lunar features take on a new appearance.
TOTAL LUNAR ECLIPSES, 1985-2000
1985 May 4
1985 Oct. 28
1986 Apr. 24
1986 Oct. 17
1987 Oct. 7
1988 Aug. 27
1989 Feb. 20
1989 Aug.16
1990 Feb. 9
1990 Aug. 6
1991 Dec. 21
1992 Jun. 15
1992 Dec. 9
1993 Jun. 4
1993 Nov. 29
1994 May 25
1995 Apr. 15
1996 Apr. 4
1996 Sep. 2
1997 Mar. 24
1997 Sep. 16
1999 Jul. 28
2000 Jan. 21
2000 Jul. 16
Lunar eclipse: Here, the Sun’s light comes from the left. The Moon is eclipsed as it enters Earth’s shadow. This is not completely dark, because some sunlight is refracted into it by Earth’s atmosphere. Since red rays penetrate the atmosphere most easily, the Moon’s disk looks reddish.
In a total solar eclipse, the disk of the Sun is completely hidden by the Moon. But in a lunar eclipse, the disk of the Moon can often be seen, even in Earth’s shadow. Some of the sunlight passing through Earth’s atmosphere is refracted so that it falls on the Moon, giving it a coppery hue.
Occultation: Jupiter and satellites appear after being occulted by the Moon. The sharpness of Jupiter’s image when the planet is just at the edge of the Moon indicates that the Moon lacks an appreciable atmosphere.
OCCULTATIONS Now and then the Moon passes in front of a star or planet, hiding it briefly. This event, called an occultation, is of great interest to mapmakers. If the instants of disappearance and reappearance are accurately timed by observers at different locations on Earth, the data can be used to determine the exact distances between those locations. Exact positions of many geographical points have been checked in this way.
Many experienced observers do occultation work. A small telescope, a short-wave radio, and a good watch or stopwatch are the essentials. Accurate time signals can be obtained from Radio Station WWV. Dates when occultations will occur can be found in Sky and Telescope magazine.
THINGS TO DO (1) Observe occultations. (2) Make detailed drawings of Moon’s surface. (3) Observe eclipses. (4) Watch sunrise over the lunar mountains. (5) Watch the ever-changing appearance of several particular lunar features, night after night. (6) Take photographs (see here). (7) Time the Moon’s rising and setting times, over a lunar month. (8) Study the rays. (9) Watch for impacts. (10) Watch Alphonsus and the area near Aristarchus for signs of volcanic activity.
