Touring the Universe through Binoculars: A Complete Astronomer's Guidebook

1

Why Binoculars?

Why bother with binoculars? Today’s amateur astronomers have a vast array of telescopic equipment and accessories from which to choose. Huge telescopes, advanced optical designs, and special accessories that were once considered to be only in the realm of the professional are now readily available to the hobbyist. With all this, why would anyone want to use plain old binoculars?

The answer is that, for all the diversity of instrumentation, one of the most useful, yet often neglected instruments to tour the universe with is a pair of binoculars. Their low power and wide field of view make binoculars ideal for either a casual scan or some pretty sophisticated observing.

Research has shown that when it comes to viewing the heavens, two eyes are definitely better than one. Our power of resolution and ability to detect faint objects are dramatically improved by using both eyes. In addition, color perception and contrast are enhanced.

Why take my word for it? On the next clear, moonless night, try an easy test. With both eyes open, cover one with your hand and look at the sky. Make a mental note of the faintest stars you see. Now uncover your eye and look again at the same area of the sky. Lo and behold, there are more stars! Indeed, it is not unusual to experience a 10-percent improvement in perception when viewing with two eyes instead of one.

Now repeat this test with a nebulous object, such as the Milky Way’s hazy band stretching across the starry vault. Alternately cover and uncover one eye as before. The contrast between the soft glow of our galaxy’s star clouds and the background sky will appear far more distinct through both eyes than with only one. In fact, many observers enjoy up to a 40-percent increase in the contrast of hazy objects merely by using both eyes. Note that while these two tests have been conducted without optical aid, similar results are achieved through binoculars versus a single equivalent-size monocular or telescope.

How can this improvement be explained? Light entering the eyes is focused by the lens onto the retina. The retina, consisting of rods and cones, converts the image into electrical pulses and sends them to the brain. The brain interprets the pulses and produces the image we see. By having the brain rely on only one set of pulses (i.e., by using just one eye), inconsistencies in the signals will interfere with the final image seen. With two sets of signals to interpret, the brain is able to reduce interference by averaging the pair of electrical messages. The result is the ability to see fainter, lower contrast objects.

There is little doubt that our view of the night sky improves just by using both of our eyes. This, along with their portability, affordability, and ease of operation makes binoculars the instrument of choice to tour the universe with.

2

The Moon

Lying an average of 238,000 miles away, the Moon offers more detail to earthbound observers than any other member of the Solar System. Towering mountains, large plains, abysmally deep valleys, and scores of craters are all easily visible in binoculars.

Most of us can still remember the anxious anticipation we felt just before our first view of the Moon’s surface through either binoculars or a telescope. Its harsh beauty is awesome. Yet, powerful telescopes are not required to see many different lunar features.

Some lunar phases are better for sightseeing than others. Little or no detail may be spotted for the first couple of nights after the New Moon, but as the Moon progresses night after night toward the east, surface features become plainly visible. The later waxing-crescent through First-Quarter phases display a tremendous variety of lunar terrain for observers to marvel at. Dominating the equatorial zone are the vast expanses of the lunar seas Mare Crisium, Fecunditatis, Tranquillitatis, and Serenitatis. To their north are many scattered large craters, while to their south lies the Moon’s no man’s land, that is, very rugged terrain (many mountains, craters, etc.—no manned landings due to ruggedness). The south polar region is awe inspiring in its coarse beauty, with craters so numerous that it is often difficult to distinguish one from another.

It is always fascinating to watch the unusual lighting effects along the Moon’s terminator as the Sun rises and sets across the stark lunar surface. Depending on the angle, sunlight may just strike a crater’s rim, causing the crater to look like a bright, bottomless ring. As the Sun’s elevation increases, light travels down the steep, clifflike walls until it floods the crater’s floor.

Figure 2.1

The one-day-old Moon, as captured on April 6, 1989, by Jeffrey Jones. He employed a 200-mm f/4.5 telephoto lens and a one-second exposure on hypered Technical Pan 2415 film.

As the Sun sinks in the lunar sky, mountain ranges seem to experience metaphysical changes. First, the setting Sun thrusts their bases into darkness. Then, ever so slowly, as the Sun continues to approach the horizon, its diminishing light moves up the mountainside. Just before disappearing entirely, sunlight strikes only the highest peaks, causing them to detach from the Moon and float above the dark leading edge of the terminator.

In the past few years, an informal sport of spotting the extremely thin crescent phases (Figure 2.1) has become popular among amateur astronomers. Binoculars prove invaluable in this quest, since the very young or very old Moon lies extremely low in the bright twilit sky.

The best months for spotting a very young Moon are April and May in the northern hemisphere, and October and November in the southern hemisphere. If you want to find the Moon less than a day before New Moon, then your best chance from north of the equator exists during July and August, while January and February are favored from points south of the equator. It is at these times that the ecliptic is nearly perpendicular to the horizon, placing the Moon higher in the sky. Slowly scan above the Sun’s point below the horizon. If your view is free of trees, clouds, and other obstacles, you just might catch a glimpse of the very thin crescent.

This guide to the Moon presents a wide selection of lunar features that are resolvable through steadily supported (i.e., tripod-mounted) binoculars. The lunar maria, craters, mountain ranges, and miscellaneous features are listed in alphabetical order. The list is divided into two groups on the basis of location to the east or west of the Moon’s central meridian (the imaginary line passing from the lunar north pole, through the middle of the visible disk, to the south pole). Those features found to the lunar east of the central meridian are best seen between New Moon and First Quarter (Figure 2.2), while those found to the lunar west of the central meridian are best seen between Last Quarter and New Moon (Figure 2.3). The quarter phases were chosen as cutoff points because they offer the best compromise between shadow relief and the amount of surface illuminated, while avoiding the overwhelming brightness of the larger phases.

In an effort to keep the finder charts as uncluttered as possible, the features are keyed to numbers or letters, which are also found in brackets at the beginning of each highlighted description. Note that lunar maria are specified by capital letters (A, B, etc.), craters are listed by number (1, 2, etc.), and mountain ranges and other features are denoted by lowercase letters (a, b, etc.).

To help the reader further to view specific lunar features at their prime, two numbers are listed in parentheses after each entry. These numbers signify the nights after New Moon that the particular target is most favorably placed for observation. In most cases, they indicate the time when the lunar feature is on or near the terminator, with the first number referring to sunrise and the second indicating sunset. All of the features thus referenced are actually visible for several nights during each lunar cycle.

Due to the Moon’s brightness, regardless of phase, your eyes will never become fully dilated when observing the Moon. Therefore, binoculars with smaller exit pupils—as small as 2.5 mm—are perfectly acceptable for lunar study. Indeed, the large 7-mm exit pupils of night glasses transmit so much light that the observer’s eyes will be unnecessarily taxed. Consult Appendix A for a discussion of calculation of exit pupils.

Figure 2.2

Map of the First-Quarter Moon showing the locations of features highlighted in the section of the text titled New Moon through First Quarter. Lick Observatory photograph.

New Moon Through First Quarter

Maria

Mare Crisium [A], the Sea of Crises, is a large oval plain measuring 270 miles by 350 miles, with the long dimension running east to west. This is just the opposite of the visual impression we get from Earth because of the foreshortening of the lunar globe. Unlike the other, interconnecting maria, Mare Crisium stands alone. While no manned Apollo mission landed there, the Sea of Crises was visited by three unmanned Soviet spacecraft. Luna 15 was the first to land, on July 16,1969 (only three days before Apollo 11 landed in Mare Tranquillitatis), followed by Luna 23 five years later. The third spacecraft, Luna 24, landed in 1976, drilled 18 inches below the surface, and returned a cylinder of lunar soil to Earth. This mission holds the distinction of being the last in which a spacecraft visited the Moon.

Mare Fecunditatis [B], the Sea of Fertility, is seen to the south of Mare Crisium. Only the twin craters Messier and Messier A blemish its smooth, dark surface. Little attention was paid to the area during the days of the lunar probes. Only Luna 16 nestled down on this barren plateau back in 1970. It became the first unmanned Soviet mission to bring soil samples back to Earth.

Mare Frigoris [C], the Sea of Cold, is most unusual in appearance. Instead of being the typical circular plain, Mare Frigoris stretches for over 700 miles, but is no more than 45 miles wide at points. While no large craters lie within, several striking examples—including Endymion, Atlas, Hercules, Aristoteles, and Plato—surround it.

Mare Marginis [D], as the name implies, is only marginally visible. This oddly shaped mesa measures 210 miles by 290 miles. Most earthbound observers are unfamiliar with Mare Marginis, as only a small portion of it is visible along the eastern limb, and then only during favorable librations. Look for it directly east of Mare Crisium.

Mare Nectaris [E], the Sea of Nectar, spans a 220-mile by 265-mile area, making it one of the smallest of the Moon’s major maria. Found off the southern shore of Mare Tranquillitatis, it is encircled by several prominent craters, notably Theophilus and Fracastorius.

Mare Serenitatis [F], the Sea of Serenity, is thought by some authorities to be the oldest lunar mare of all. Covering 360 miles by 420 miles, it is bound by the Haemus Mountains to the south, the Apennines to the west, the Caucasus Mountains to the north, and the Taurus Mountains to the east. No mission from Earth has ever touched its nearly crater-free surface.

Mare Smythii [G] hides along the Moon’s eastern limb and is almost always a difficult catch. At best, it may be seen as a long, thin patch of darkness south of Mare Crisium and Mare Marginis. Though impossible to tell from our vantage point on Earth, Mare Smythii covers 165 miles east to west by 255 miles north to south.

Mare Tranquillitatis [H], 400 miles by 550 miles in extent, became the most famous lunar sea after Apollo 11 landed there on July 19, 1969. As we look toward the Sea of Tranquility, it is easy to recall Neil Armstrong’s words: Houston, Tranquility Base here. The Eagle has landed. Later, as he became the first person to set foot on another body in space by taking one small step for [a] man … one giant leap for mankind, Mare Tranquillitatis became ingrained in history and astronomy books alike.

Armstrong and lunar module pilot Edwin Buzz Aldrin set their craft down in the southwest corner of the Sea, to the north of the prominent crater Theophilus. Other spacecraft that have visited the Sea of Tranquility include Ranger 6, Ranger 8, and Surveyor 5.

Mare Vaporum [I], the Sea of Vapors, is a small plain wedged between the Haemus and Apennine Mountains. Only the crater Manilius is large enough to mar its surface through most binoculars. The most powerful glasses should also spot the Hyginus Rill crossing the southern portion of the sea. It appears as little more than a thin pencil line, but in reality measures 140 miles long by 2 miles wide.

Craters

Albategnius [1], measuring 81 miles in diameter, is an outstanding crater through binoculars. A noticeable off-center peak rises from the dark floor, some 14,000 feet below the crater’s rim. (7,22)

Aliacenis [2] rides the terminator on the night of the First Quarter Moon. Paired with Werner, it stands out as a deep lunar pothole spanning 46 miles by 55 miles. Its sharp crater walls plummet for nearly 2½ miles to a relatively dark, smooth floor. (7,21)

Aristoteles [3] is a prominent 55-mile-diameter crater found on the southern edge of Mare Frigorus. Its steeply banked walls plummet 12,000 feet to a smooth floor. (6,20)

Atlas [4], at 54 miles across, is one of the more noteworthy craters visible during the waxing crescent phases. A multiple peak ascends 2,600 feet above its 10,000-foot-deep floor. Atlas forms a nice pair with Hercules, a slightly smaller crater to its west. (5,18)

Cleomedes [5] is one of the most visible craters on the waxing crescent. This out-of-round crater, located just to the north of Mare Crisium, measures 81 miles by 92 miles. Its sheer walls descend nearly three miles to a rough floor. (3,16)

Cyrillus [6] is an extremely old crater found to the west of Mare Nectaris. Were it isolated, it would dominate the field, since it is 62 miles across and 11,800 feet deep. However, Cyrillus is overshadowed by Theophilus, a fresher crater that protrudes into its northeastern wall. (5,19)

Endymion [7] is a large, distinguished crater that is most easily seen during the middle of the waxing crescent phases. Found in the northeast quadrant between Mare Frigoris and the lunar limb, it is set apart from the immediate surroundings by its dark floor. Endymion is 77 miles wide, with walls that cascade for 16,100 feet. (4,18)

Fabricius [8] lies in the Moon’s southeast district. It is found squeezed between Metius and Janssen, two larger enclosures. Fabricius must be the most recent of the three, as it juts out into the others. (4,18)

Fracastorius [9], though once a 73-mile-wide crater, appears as a bay on the south shore of Mare Nectaris. Its north wall was washed away when a flood of hot lava from the neighboring maria filled its floor. Through low-power binoculars, it appears as a complete ring, but the greater resolving power of telescopes reveals this to be an illusion caused by a series of hills and mounds. As it remains today, Fracastorius is one of the Moon’s finest examples of a partially destroyed crater ring. (6,20)

Furnerius [10] is noted for its sharply angled rim, which is almost hexagonal in form. Measuring 81 miles across, this crater is one of the key features visible on the very young waxing crescent, yet is all but invisible once the ray system of neighboring Stevinus catches the Sun. (2,16)

Hercules [11] and previously mentioned Atlas form a powerful pair of craters near Mare Frigoris. The smaller of the two, Hercules is 45 miles across with pronounced walls and a distinctive dark floor. (5,18)

Hipparchus [12] is found just north of Albategnius. While Hipparchus is the larger of the two craters, at 83 miles by 89 miles, it proves less noteworthy. The years have been hard on its ancient face, with the 7,500-foot-deep walls bearing the scars of more recent impacts. (7,21)

Janssen [13] is a large, very old crater bordering Fabricius to the south. Though Janssen measures 122 miles by 153 miles, its battered walls mask the crater’s identity, except under ideal lighting conditions. (4,24)

Langrenus [14], found on the southeastern shore of Mare Fecunditatis, is an amazing crater to watch from waxing crescent to Full Moon. As the Sun rises higher in the sky above it, 85-mile-wide Langrenus seems almost to catch fire as its floor is transformed from a dull gray to a brilliant whitish glow. (2 through 16)

Manzinus [15] is found toward the Moon’s southeast limb. Measuring 55 miles by 63 miles, it forms an attractive crater pair with Mutus [16] to the east. (5,19)

Maurolycus [17] spans 73 miles and has walls 16,700 feet high. It is found in the rugged highlands toward the Moon’s southern limb, where it dominates the view. (7,14)

Metius [18] lies in the southeastern region of the Moon, directly adjacent to Fabricius. Together, the two craters, measuring 53 and 48 miles across, respectively, look like twin depressions through low-power binoculars. Careful scrutiny through higher power glasses reveals a strong central peak in Fabricius, while the floor of Metius appears comparatively smooth. (4,18)

Petavius [19], a 99-mile by 110-mile enclosure, is one of the highlights of the young crescent Moon. Look for it to the south of Mare Fecunditatis. When the Sun is at just the right elevation, the 8,200-foot-high central peak of the crater is easily seen in 7 × glasses. (2,16)

Piccolomini [20] sees sunrise about four days after New Moon and is an outstanding crater from then until past First Quarter. Measuring 54 miles across, it exhibits a strong central peak through 11 × and higher power binoculars. Look for Piccolomini to the south of Fracastorius and Mare Nectaris. (5,18)

Posidonius [21] stands out well as a 52-mile by 61-mile chasm. Bordering Mare Serenitatis to the south, it was partially filled with lava during the Moon’s volcanic era some three billion years ago. (5,20)

Rheita [22] is a small, easily overlooked crater in the southeastern lunar highlands. Though only 44 miles across, its presence helps point the way to the Rheita Valley (see Other Features). (4,18)

Stevinus [23] is hardly visible through binoculars when the Sun rises over it on the third day after New Moon. However, a day or two afterwards, the whole region catches fire as the ray system from an unresolvable crater (known as Stevinus A) becomes sunlit. Forty-eight-mile-wide Stevinus appears framed by these brilliant rays. (5 through 9)

Stofler [24] is a large oval crater that rides the terminator at the quarter phases. Measuring 68 miles by 85 miles, its southeastern wall was obliterated millennia ago by the smaller impact crater Faraday. (7,21)

Taruntius [25] blemishes the northwest corner of smooth-surfaced Mare Fecunditatis. Though only 36 miles in diameter, its bright appearance contrasts nicely with the dark surrounding plain. (4,18)

Thales [26], when viewed at or near Full Moon, forms one-half of a pair of spotlights on the northern limb of the Moon. The other beacon is Anaxagoras, described in the section titled Last Quarter through New Moon. Thales, at 22 miles across, is barely discernible at phases other than Full Moon. (13 through 17)

Theophilus [27] is found on the northwest shore of Mare Nectaris and dominates the area. The strong, jagged walls of 65-mile-wide Theophilus cascade down for over four miles to a rough floor. (5,19)

Vendelinus [28] is an old crater found to the south of much more prominent Langrenus and covers 92 miles by 100 miles. Its northeast wall has been demolished by the smaller impact crater Lame. (2,16)

Werner [29] teams with Aliacensis to form an impressive crater duet on the night of the First Quarter. While both stand out well when on the terminator, they are easily lost within a couple of nights as the Sun rises higher in their sky. (7,21)

Mountain Ranges

The Caucasus Mountains [a] are a conspicuous range that divides Mare Imbrium from Mare Serenitatis. With their highest peaks rising 17,400 feet above sea level, these mountains are an especially impressive sight when found on the terminator. (6,20)

The Haemus Mountains [b], a little-known range of peaks that rise to about 10,000 feet, form the southern border of Mare Serenitatis. They appear to merge with the Apennines to create a peninsula that separates the Sea of Serenity from the Sea of Tranquility. (6,20)

Other Features

Sinus Medii [c], a large oval plain measuring 100 miles by 200 miles, is found at the center of the Moon’s near side. The landing site for both Surveyor 4 and Surveyor 6 in the mid-1960s, its smooth surface is unscarred by large craters. (7,21)

The Rheita Valley [d] is a puzzling feature that stretches over 250 miles to the southeast from the crater Rheita. The valley appears as a giant scar on the Moon’s surface and stands out prominently when sunlight has not quite reached its bottom. (4,18)

Lacus Somniorum [e], the Lake of Dreams, appears as an oddly shaped bay extending northward from Mare Serenitatis. Though darker than the enclosing highland area, its surface is noticeably lighter than the maria. Farther north, Lacus Somniorum floods into Lacus Mortis [f], the Lake of the Dead, which in turn passes into Mare Frigorus. Lacus Somniorum measures about 90 miles by 180 miles, while Lacus Mortis spans 100 miles by 110 miles. Both are littered with many islands. (4,18)

Last Quarter Through New Moon

Maria

Mare Cognitum [J] is a small flat plain measuring only 120 miles by 200 miles and found to the south of the crater Copernicus. It is separated from the Ocean of Storms to the west by the Riphaeus Mountains and bordered on the east by a small island of craters. Though difficult to make out through binoculars, the ancient crater Fra Mauro forms the northern edge of this island. Fra Mauro was the site of 1971’s Apollo 14 mission.

Figure 2.3

Map of the Last-Quarter Moon showing the locations of features highlighted in the section of the text titled Last Quarter through New Moon. Lick Observatory photograph.

Mare Humorum [K], the Sea of Moisture, is a nearly circular dark plain to the south of Oceanus Procellarum. Its 250-mile by 275-mile surface appears quite smooth through binoculars, except for a few waves to the east of center. Some experts believe that Mare Humorum is the oldest of the maria.

Mare Imbrium [L], the Sea of Rains, spans 670 miles by 750 miles. Touched by only the Luna 17 mission, it is highlighted by many striking features, such as Sinus Iridum, the craters Archimedes and Eratosthenes, and the surrounding Apennine and Alps Mountains.

Mare Nubium [M], the Sea of Clouds, is notable for its unusually dark floor. Some minor blemishes may be seen through binoculars, but none is large enough to be easily identifiable.

Oceanus Procellarum [N], the Ocean of Storms, is the largest of the lunar plains and dominates the waning phases. It encompasses more than one million square miles of the Moon’s surface. Several noteworthy features, including the craters Aristarchus, Kepler, and Copernicus, are found within its vast borders. Among the many spacecraft to land there were Surveyor 1, Surveyor 3, and Apollo 12 from the United States, as well as the Soviets’ Lunas 5, 7, 8, 9, and 13.

Craters

Alphonsus [29] is one of five large craters that stretch southward from Sinus Medii at the Moon’s center. Alphonsus spans 64 miles by 73 miles. The site of the Ranger 9 impact mission in 1965, the crater has been observed to have, on rare occasions, a cloud of carbon molecules temporarily floating above it. Photographs of this unusual phenomenon taken in the 1950s prove that the Moon is not a dead world. (8,22)

Anaxagoras [30], when on the terminator, is far surpassed by many of its larger neighboring craters. Yet, by the time the Sun has risen high in its sky, this 3 3-mile-wide crater puts on a striking display of bright rays that gives it prominence. Together, Anaxagoras and Thales, described earlier, form an impressive pair of spotlights along the Moon’s northern limb. (13,17)

Archimedes [31] is found in eastern Mare Imbrium and spans 51 miles. A portion of its walls were flooded by lava during the Moon’s volcanic era. (8,22)

Aristarchus [32] is one of the smallest craters identifiable through binoculars. Surprisingly, it is also one of the easiest to identify, as a magnificent system of rays sprays out from its rim into surrounding Oceanus Procellarum. Indeed, these rays are so dazzlingly bright that they can completely overwhelm the crater itself.