The Night Sky This Month (May 2012)

The Planets Mercury Innermost Mercury is lost in the solar glare until early June, when it re-emerges in the evening sky. Venus Venus, the dazzling Evening Star, outshines all the other stars and planets in the night sky and is in good view in the west as darkness begins to fall. It starts the month only 2° below the second-magnitude star Beta Tauri and slowly

comes closer during the next several days, passing within 0.8° of the star on May 6. By the end of the month, Venus becomes increasingly hard to see in the bright twilight and sets only 40 minutes after the Sun.

For such a dazzling naked-eye sight, Venus is one of the most profoundly disappointing sights in the telescope. The planet's surface is perpetually obscured by an extremely thick atmosphere, so any observations will be limited to details discernible in its rather bland covering - slightly brighter or darker areas, or irregularities in the terminator (the line that separates the illuminated day side and the dark night side of the planet).

It has been found that the top of the atmosphere lies around 250 miles above the surface, and that the upper clouds have a rotation period of only four days. The upper clouds lie at an altitude of 45 miles, and there are several definite cloud- layers, though below 20 miles the atmosphere is relatively clear and calm. The atmosphere's main constituent is carbon dioxide, accounting for over 95-percent of the whole; most of the rest is nitrogen. The clouds are rich in sulfuric acid; at some levels there must be sulfuric acid "rain" which evaporates before reaching ground level.

Finder map (early May) - one hour after sunset, looking west.
Finder map (mid-May) - 30 minutes after sunset, looking west.
Finder map (late May) - 30 minutes after sunset, looking northwest.

Mars

The Hubble Space Telescope recorded the biggest dust storm seen in several decades raging on Mars in 2001. On left, appearance of Mars in June, about the time that the dust storm started. In September, right, the storm obscured all surface features. NASA/STScI/AURA [larger image]

By mid- evening, as Venus sets in the west, Mars stands two-thirds of the way from the southern horizon to the zenith. It spends the month floating 10° or less from Regulus (Alpha Leonis), a slightly dimmer star with which it

contrasts beautifully. Through a telescope, the Red Planet's disk will appear no bigger than 10" across, still rather small even at high magnifications.

Even a disk as small as 10" is worth scrutiny. In a 6-inch telescope on a night of good seeing, you should be able to make out the north polar cap, limb hazes, dark surface markings and occasional white clouds or dust storms.

Mars is never easy to study, and its small diameter this May presents a special challenge. The best telescope for planetary observing is a large apochromatic refractor with first-rate optics, but fine planetary views have also been obtained with Schmidt-Cassegrains. In the end, the limiting factor is atmospheric seeing. Studying the planets means spending a lot of time at the eyepiece, waiting for brief glimpses through steady air. Just as important, the longer you watch, the better trained your eye becomes.

One last thing - color filters. They are an important aid for planetary observing and improve the delicate contrast of Martian features, sometimes can even steady the seeing slightly. The improvements are subtle, but with experience they become important.

Finder map (early May) - 30 minutes after sunset, looking south.
Finder map (mid-May) - 30 minutes after sunset, looking south.
Finder map (late May) - 30 minutes after sunset, looking southwest.

Jupiter

Jupiter is too deep in the solar glare and cannot be observed until mid-June, when it will reappear in the morning sky.

Saturn

Saturn's rings are made up of billions of ice and rock particles, thought to be pieces of comets, asteroids, or shattered moons. NASA/JPL [larger image]

The ringed planet reached opposition and peak visibility in April, but remains a stunning sight through any telescope during May. It shines high in the southeast, among the stars of Virgo the Maiden and close to brilliant Spica, about one hour after sundown. At the beginning of May, Saturn is magnitude +0.3 and its globe is 19"-wide. By late in the month, the disk has shrunk to 18.5" and the planet also glows slightly dimmer, at magnitude +0.5. The rings span 43", more than double the planet's disk, and tilt 14° to our line of sight.

You should have no trouble spotting the Cassini Division, a thin black gap in the rings named in honor of G. D. Cassini, who discovered it in 1675.

Saturn's rings are one of the spectacles of the night sky when seen through a decent telescope. When Galileo trained a primitive telescope on the planet for the first time in 1610, he was misled. From the poorly resolved image in his viewfinder, he believed Saturn to be a triple-system, with a large body in the center and smaller ones on each side.

The rings may be much younger than the planet itself, and great mathematicians have found them worthy of contemplation. Laplace and James Clerk Maxwell calculated that Saturn's rings must consist of many smaller objects, all moving round the planet in the manner of tiny moons. There is no mystery about their composition; they are made up of ordinary water ice.

Finder map (early May) - one hour after sunset, looking southeast.
Finder map (mid-May) - one hour after sunset, looking southeast.
Finder map (late May) - one hour after sunset, looking south.

Uranus

Uranus lies on the border between Cetus and Pisces and is low in the morning sky, just 10° above the eastern horizon by the onset of twilight. Some observers may be able to spot the planet with binoculars, although at magnitude +5.9 it will be a difficult object and will require very transparent skies.

Uranus is so inconspicuous that it was mistaken for a star dozens of times before its accidental discovery in 1781 by German-born British astronomer William Herschel, using a primitive 6-inch Newtonian reflector. The planet is never less than 1,600 million miles from the Earth; it qualifies as a giant, with a diameter of over 30,000 miles, but it is much smaller than Jupiter or Saturn and its composition is quite different.

Uranus is made up largely of "ices", but these need not be in a solid form - there is a mixture of water, methane and ammonia, plus a certain amount of solid matter. The outer atmosphere is made predominantly of hydrogen, and also some methane which absorbs red light and gives the planet its distinctive greenish hue.

Finder map - field width 15°, stars to magnitude +8.

Neptune

A color mosaic of Triton, Neptune's largest moon, taken in 1989 by the Voyager 2 spacecraft. NASA/JPL [larger image]

Distant Neptune can be found among the background stars of Aquarius, low in the east before dawn's first light. The planet glows dimly at magnitude +7.9, much too faint to be viewed with the unaided eye, lying at a mean distance from the Sun of 2.8 billion miles. With a telescope, trying to resolve Neptune into a disk will be a troublesome task. You are going to need at least a four-inch instrument with a magnification of no less than 200-power, just to turn Neptune into a tiny blue dot of light. Denser than the other gas giants, Neptune probably has ice and molten rock in its interior, although rotational data imply that these heavy materials are spread out rather

than concentrated in a small core. The atmosphere is swept by winds moving at up to 2,300 feet per second, the fastest found on any planet. At the equator, the winds blow westwards (retrograde) and beyond latitude 50 degrees they become eastwards. Temperature measurements show that there are cold mid-latitude regions with a warmer equator and pole.

Neptune's thirteen known moons include Nereid, with the most eccentric orbit of any planetary satellite, seven times as distant from the planet at its farthest compared with its closest approach; and Triton, the only large moon in the solar system with a retrograde orbit, which is an orbit in the opposite direction to that of Neptune's.

Finder map - field width 15°, stars to magnitude +8.5.

Pluto

New Horizons, the first mission to Pluto, was launched successfully in January 2006. Its closest approach to Pluto will be on July 14, 2015. NASA [larger image]

The dwarf planet Pluto lies in northern Sagittarius and is highest above the southern horizon just before dawn. Search for it under a dark, moonless sky. Pluto glows at magnitude +14, and as a result, it is a challenge to spot. An 8-inch telescope on a perfect night brings Pluto to the edge of visibility. For a direct view, however, you will want to use at least a 10- inch scope. Pluto was discovered in 1930 as a result of an extensive search by American astronomer Clyde Tombaugh. Astronomers have argued since the late 1990s that Pluto's small size, less than one-fifth the diameter of Earth, and a weird tilted orbit that takes it inside Neptune's orbital path every couple hundred years make Pluto more like a Kuiper Belt body than a full-fledged planet.

On August 24 2006, the International Astronomical Union (the organization responsible for classifying planets) passed a new definition of planet that excludes Pluto and puts it in a new category of "dwarf planet".

The two finder maps below will help you identify Pluto. First locate its general position on the coarse finder map, and after you have identified the 7th-magnitude guide star Tycho 6275-157-1 use the fine finder map which shows the position of the dwarf planet on each night of the month.

Coarse finder map - field width 10°, stars to magnitude +8.5.
Fine finder map - field width 1°, stars to magnitude +14.5.

The Deep Sky

The distribution of galaxies in the night sky is not random. They tend to congregate in groups or clusters, and our own Local Group, which includes the Milky Way, is very far from being exceptional. For instance, the Virgo Cluster, at a distance of around 60 million light-years, contains approximately 1500 members and some (such as M100, a grand design spiral galaxy) are much more massive than our galaxy. M100 can be spotted with large binoculars and backyard telescopes in Coma Berenices, a faint constellation representing the locks of the Egyptian Queen Berenice, which she offered to the gods for the return of her husband from battle. M100 is the brightest and largest spiral galaxy in the Virgo Cluster and has a "grand design", meaning that it has two symmetrically placed spiral arms that extend over most of its visible disk in professional telescopic images.

M100 is a grand design spiral galaxy that presents an intricate structure, with a bright core and two prominent arms. ESO/IDA/Danish 1.5 m/R. Gendler, J.-E. Ovaldsen, C. C. Thone and C. Feron [larger image]

The galaxy is located 8° east of second-magnitude Denebola (Beta Leonis), and 2° northeast of the fifth-magnitude star 6 Comae Berenices. Through an 8-inch scope at low to medium power, look for a soft, round glow about 6' across. You will not see M100's spiral arms until you crank the magnification past 200X, and then only on the best nights.

The arms appear as brighter regions just to the east and west of the nucleus. Through a 12-inch telescope, the spiral structure can be traced twice as far from the core. Two faint dwarf galaxies lie to the north and east. Magnitude +13.9 NGC 4322, to the north, appears to be M100's true companion, while magnitude +13.3 NGC 4328 lies in the foreground to the east.

Long-exposure photography has shown M100 to be far larger than previously believed, with a substantial portion of its mass contained in faint outer regions. The Hubble Space Telescope discovered over twenty Cepheid variables and one nova in M100 and was able to accurately determine its distance as 55 million light-years. Five supernova events have been observed in this galaxy - one as recently as February 2006!

Finder map - field width 15°, stars to magnitude +8.5.

Asteroids

This H chondrite meteorite is probably a sample of the crust of 6 Hebe. Michael P. Klimetz [larger image]

Algieba (Gamma Leonis) is a golden-orange binary system in the constellation Leo, and appears as the brightest star in the well- known Sickle asterism. The component stars are 4" apart, meaning that any small telescope will easily split the pair. This month, Algieba also serves as a handy guide to asteroid hunters. On May 1, the large main-belt asteroid 6 Hebe lies within 1° of the star, and tracks eastwards with each passing night. Glowing at about 11th-magnitude, Hebe looks like an ordinary field star. Although too faint for small

telescopes to pull in under city lights, it is well within their reach from a dark sky when the Moon is out of the way.

Wait until about 10 P.M. local daylight time, when Hebe is highest above the southern horizon, then use the finder map below to hop over to its rough position with your telescope. The tried-and-true method is to detect the asteroid's motion from one night to the next. Make a quick sketch of the star field through which Hebe is passing, and afterwards simply come back a night or two later and compare the field with your sketch to see which dot moved. That is Hebe.

The German astronomer Karl Ludwig Hencke discovered Hebe on July 1, 1847, from his private observatory in Kietz, Driesen. The asteroid measures about 130 miles across and is the thirteenth largest by mass, containing 0.5-percent of the mass of the entire asteroid belt. Hebe is probably the parent body of the H chondrite meteorites, which account for a remarkable 40-percent of all meteorites striking the Earth.

Finder map - field width 7°, stars to magnitude +11.5.

Comets

Comet C/2009 P1 Garradd has graced the Northern Hemisphere sky for over a year now. It is still well placed and an easy target for large binoculars and backyard telescopes. Rolando Ligustri [larger image]

Although great comets like Halley and Hale- Bopp approach Earth only a few times each century, there are always small comets visible in backyard telescopes. This May, there is one to keep an eye on: comet C/2009 P1 Garradd. Comet Garradd has been observable in northern skies for over a year now. Despite the fact that it is currently receding from both the Sun and Earth, it still glows around 8th-magnitude and remains a nice sight throughout binoculars and small telescopes. In early May, Garradd slides through the background stars of the constellation Lynx, but by mid-month it passes the border into neighboring Cancer. For observers located at mid-northern latitudes comet Garradd is highest in the early evening sky and sets around 2 A.M. local daylight time.

If you manage to find comet Garradd with your backyard telescope, do not expect a speeding object. In fact, if you see a comet in the sky, you observe no rapid motion at all. Unless you observe carefully, the comet appears to stay in the same place among the stars, having only the motion across our sky caused by the rotation of the Earth. (It does move among the stars, of course, but very slowly and its motion can be seen over a few days.)

Finder map - field width 30°, stars to magnitude +7.

Meteors

The Eta Aquarids may not be as spectacular as the Perseids in August or the Leonids of winter, but it is fun to think that these meteors are in fact particles of the most famous comets of all - Halley's Comet! The shower gets its name from the area of the sky from which the meteors appear to radiate at the date of the maximum - in this case, a star designated by the Greek letter Eta in the constellation Aquarius the Water Bearer. The Eta Aquarids first appear around April 19, and some can be seen until May 28. The shower's peak occurs around May 5, when up to 20 or 30 meteors can be seen each hour from a dark-sky site. Rates are higher the farther south you are located, and for observers in the Southern Hemisphere the hourly rate climbs to 50. Before and after the maximum, the Eta Aquarids produce only two or three meteors per hour.

The Eta Aquarids are flakes of dust from Halley's Comet (pictured above), which make brilliant shooting stars when they strike Earth's atmosphere. ESA [larger image]

Throughout May, the shower's radiant is found in northern Aquarius - close to Eta Aquarii - and moves daily a little to the northeast. The radiant never gets very high in the sky before dawn, so your observing time is limited. Many Eta Aquarid meteors are bright yellow, and some will likely leave brief smoke trails in their wake.

Map - Eta Aquarids radiant position.

Some meteors do not belong to any known shower. These are the sporadic meteors, caused by random bits of comet debris spread throughout the inner solar system. They appear randomly across the sky all year long.

In this month's night sky careful observers can expect around five sporadics per hour during the morning hours and two during the dark evening.

Observing Aids

Northern Hemisphere's Sky - This map portrays the sky as seen near 40° north latitude at 11 P.M. local daylight time in early May and 10 P.M. in late May.

Southern Hemisphere's Sky - This map is plotted for 35° south latitude. It shows the sky at 8 P.M. local time in early May and 7 P.M. in late May.

Visibility of the Planets - The table provides general information about the visibility of the planets during the current month.

Phases of the Moon - This Moon Phase Calendar shows the Moon's phase for every day in May.

Jupiter's Moons - The diagram shows the positions of Galilean satellites on each day in May at midnight.

Sky Events

May 4 - The Moon is 1.5° south of Spica (Alpha Virginis) at 12:26 P.M. EDT.

May 5 - Full Moon at 10:35 P.M. EDT. The Moon is at perigee, the point in its orbit when it is nearest to Earth. The Eta Aquarid meteor shower is at peak activity.

May 12 - Last Quarter Moon at 4:47 P.M. EDT.

May 13 - Jupiter is in conjunction with the Sun.

May 19 - The Moon is at apogee, the point in its orbit when it is farthest from Earth.

May 20 - New Moon at 6:47 P.M. EDT. An annular solar eclipse will be visible from the Chinese coast, the south of Japan, and the western part of the United States and Canada. A partial eclipse will be visible throughout parts of eastern Asia and most of North America. May 22 - The Moon is 5.2° south of Venus at 3:52 P.M. EDT. May 27 - Mercury is in superior conjunction with the Sun. May 28 - First Quarter Moon at 3:16 P.M. EDT.

The information provided on this page is accurate for the world's mid-northern latitudes. Finder maps for the five naked eye planets are plotted for 40° north latitude, but can also be used from other latitudes close to 40° north. Except the two all-sky maps, all other maps can be used no matter the latitude. Local time (local daylight time during summer) represents the time of the reader.

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