The Night Sky This Month (May 2013)

The Planets Mercury Mercury moves to May's evening sky, but will be hard to spot until late month. On May 25, the planet will set almost ninety minutes after the Sun and will be easily identified sitting 10° above the western horizon, close to brilliant Venus. Through a telescope, on the 25th, Mercury shows a 6"-diameter disk that is 80-percent illuminated. By the 31st, the planet appears 6.5" across and 65-percent lit.

Mercury is an unusual planet physically, with a blasted hot surface of 900° F basalt, a very eccentric orbit of 88 days, and a rotational period of only two-third of the year. It has a globe that is denser than that of any other planet apart from the Earth. There seems to be an iron-rich core about 2250 miles in diameter (larger than the whole of the Moon), containing about 80-percent of the total mass; by weight Mercury is 70-percent iron and only 30-percent rocky material.

The planet's surface is heavily cratered - the formations range from small pits up to colossal structures larger than anything comparable on the Moon. It is believed that many of these craters were created four billion years ago, around the time our solar system was formed. Back then, the solar system was full of debris varying in size from tiny pebbles to massive boulders several miles across.

Finder map (late May) - 30 minutes after sunset, looking west.

Venus

Venus' thick atmosphere was photographed above in ultraviolet light by the Hubble Space Telescope. NASA/JPL [larger image]

Venus is often considered the Evening Star or the Morning Star, depending on which time of day it is up and dominating the twilight. For example, until early January, 2014, Venus will appear as a brilliant yellow star in the evening sky, right after sunset. Located 10° above the western horizon half an hour after sundown, it remains on view until after 9:30 P.M. local daylight time. At the beginning of May, Venus spans 9.9" across and shows a disk 99-percent lit. By late in the month, the disk has grown to 10.3" and the phase has shrunk to 95-percent illumination. The planet shines at a stunning -4 magnitude, about ten times brighter than the brightest star Sirius, and by far the brightest celestial object after the Sun and Moon. Venus is so bright due to a combination of factors. Venus is covered with an opaque layer of highly reflective clouds of sulfuric acid. These clouds reflect 70-percent of the sunlight that hits them. For comparison, the Earth reflects 36- percent and Mars and the Moon around

15-percent of the sunlight striking them. Venus is also rather large, being only a bit smaller than the Earth - its radius is 95-percent as large as Earth's. The final piece of Venus' brightness puzzle is its close distance to Earth. Right now, the planet is 1.7 astronomical units (160 million miles) from Earth.

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

Mars

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

Jupiter

Two of Galileo's original telescopes, on display at "Istituto e Museo di Storia della Scienza" in Florence, Italy. The instruments provided a magnification of about 20X. [larger image]

Jupiter lies in Taurus the Bull, just above Aldebaran, the brightest star in the constellation and one of the brightest in the nighttime sky. It glows brightly at magnitude -2 and is visible in the west as evening twilight fades. The size of a planet is not apparent to the unaided eye, but long before the invention of the telescope, Nicolaus Copernicus had deduced that Jupiter was larger than Venus, even though Venus at its brightest is brighter than Jupiter. From the two planets' relative brightness and distances, he concluded that since Jupiter is so much farther away, if it shines only by reflected light it would have to be much larger to appear so bright. Galileo was the first to observe the planets' angular sizes with a primitive telescope and was able to determine that Jupiter was indeed larger, for he could use their angular sizes and relative distances to calculate their relative sizes. Today we know that Jupiter is the largest planet in the solar system, appropriately named after the king of the Roman gods.

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

Saturn

Pictured above, Saturn's famous rings are visible along with two of its largest moons, Dione and Rhea. NASA's Voyager 2 took this "true color" photograph on July 21, 1981. NASA [larger image]

Saturn reached opposition last month, on the night of April 28. The planet glows low in the southeast as twilight fades, to the lower left of Spica and farther lower right of Arcturus. A small telescope will reveal Saturn's system of rings which span 42", surrounding a disk about 19" in diameter. The rings are tilted 18° to our line of sight, the widest open they have been since the year 2006. The planet's ring system is unique, and quite unlike the obscure rings of Jupiter, Uranus and Neptune. Six major rings all lying in the equatorial plane of Saturn have been identified, of which three, in addition to the Cassini division and a subtler demarcation called the Encke division, can be seen from the Earth with a good telescope. Saturn's rings are made up of many small particles, all moving round the planet in the manner of tiny moons. There is no mystery about their composition; they are made primarily of 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 bluish hue.

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

Neptune

Neptune is the last of the hydrogen and helium gas giants in our solar system. NASA [larger image]

Seek out Neptune just before dawn, in central Aquarius, 1° northwest of the 5th-magnitude star Sigma Aquarii. The distant world lies 2.8 billion miles from Earth and glows dimly at magnitude +7.9. A 4-inch diameter telescope is probably the minimum required to see the planet and resolve its disk, only 2.4" across. Neptune takes almost 165 years to complete one journey round the Sun, so that it was discovered about one "Neptunian year" ago. Like Uranus, Neptune shows a light bluish color (from methane in the atmosphere). The main constituents are molecular hydrogen and helium; methane makes up a minor amount. Neptune's atmosphere is marked by cirrus clouds and large storms, most

notably the Great Dark Spot. This darker bluish-gray region circling the planet's Southern Hemisphere rotates counterclockwise in a few days. A region of high pressure, the Dark Spot is surrounded by white clouds of ammonia and is similar to storms on Jupiter.

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

Pluto

Hubble Space Telescope's sharpest picture yet of the dwarf planet Pluto. NASA/JPL [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 6th-magnitude guide star HD 173425 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

With the passing of the March equinox comes the best time of the year for observing galaxies. On spring evenings, if the skies are clear, we can look beyond the veil of stars in our own Galaxy, the Milky Way. As we peer out into deep space we see countless other galaxies, most of them in the direction of the constellations Virgo and Coma Berenices. We are in fact looking at a genuine grouping - the famous Virgo-Coma Cluster of Galaxies,

The central region of the Virgo-Coma Cluster, the closest and best-studied cluster of galaxies. Rogelio Bernal Andreo (DeepSkyColors.com) [larger image]

a huge aggregation of systems at a mean distance of around 60 million light-years. The cluster straddles the boundary between Coma Berenices and Virgo, with the weight in the Bowl of Virgo (outlined by Epsilon, Delta, Gamma, Eta and Beta Virginis, and Denebola or Beta Leonis).

The Virgo-Coma Cluster is probably the most thoroughly studied cluster of galaxies by virtue of being nearest to our own Galaxy. Observations of it have had an important role in the study of several astrophysical problems. The cluster covers about 10° in the sky (twenty times the diameter of the Full Moon), which implies that its physical diameter is some 10 million light-years.

Astronomers estimate between 1,500 and 2,000 galaxies populate this cluster, which forms the heart of the Local Supercluster. The Local Group, of which our Milky Way is part, is another collection of galaxies within the Local Supercluster. The Virgo-Coma Cluster's mass measures some 1.2 quadrillion (1.2 x 10¹⁵) suns, so massive that it influences the Local Group gravitationally; we are moving toward the Virgo-Coma Cluster.

For backyard telescope users, there are sixteen Messier objects in the cluster and several other galaxies brighter than 11th magnitude. From a dark site, a 6-inch scope will show about 140 galaxies in this region! However, to find and identify them means using a detailed chart and, preferably, a telescope equipped with a GoTo mount. Do not confuse the Virgo-Coma Cluster with the open star cluster of Coma Berenices (Melotte 111), which lies close to it in the sky but is a relatively near neighbor in our own Galaxy.

Finder map - field width 90°, stars to magnitude +5.

Asteroids

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

Lambda Ophiuchi, which has the traditional name Marfik (meaning "the elbow" in Arabic), is a binary star in the constellation Ophiuchus. It shines at 4th-magnitude, from a distance of roughly 166 light-years. This month, the star serves as a handy guide to asteroid hunters. In early May, the large main- belt asteroid 6 Hebe lies within 2° of Marfik, and tracks westwards with each passing night. Glowing at about 10th- 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 after midnight, 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 10°, stars to magnitude +10.

Comets

Comet PanSTARRS passed close to the Andromeda Galaxy, M31, during the first week of April. Brendan Alexander [larger image]

Comet C/2011 L4 PanSTARRS was discovered in June 2011, using the PanSTARRS telescope located near the summit of Haleakala, on the island of Maui in Hawaii. At the moment of discovery it had an apparent magnitude of +19, but preliminary calculations soon made it clear that this new object had the potential to become a comet of considerable interest for observers in the Northern Hemisphere. By early May last year, the comet had brightened to

magnitude +13 and could be seen visually with large amateur telescopes. In the months since then, comet PanSTARRS grew even brighter and was spotted with the naked eye in early February this year, by Southern Hemisphere observers.

On March 10, PanSTARRS passed closest to the Sun and many observers glimpsed it with just naked eyes. Throughout April, the comet's rapid northward motion gradually carried it away from the Sun and higher into the evening sky. At the same time, PanSTARRS grew fainter; at magnitude +4 it was only visible through small telescopes or binoculars.

Now, in May, the comet is circumpolar for observers located at mid-northern latitudes. It never sets and can be viewed all night, in the constellation Cepheus the King. PanSTARRS glows at 7th magnitude and appears as a bright, round fuzz ball roughly 5' across, with little hint of a tail. The comet's orbit is bringing it by the Sun for the first time; in the coming months it will fly back out again, never to return.

Finder map - field width 40°, stars to magnitude +6.5.

Comet C/2012 F6 Lemmon put on a nice show for Southern Hemisphere observers, from January and well into March. It approached the Sun down to 0.73 astronomical units (68 million miles) on March 24, when it brightened up to 5th magnitude. On April 20 the comet crossed the celestial equator, and in early May it will become observable from mid-northern latitudes.

You can search for it low in the morning sky, just above the eastern horizon. Begin by sweeping slowly with binoculars about one hour before sunrise, looking for a fuzzy "star" with a short stub of a tail. On May 6, a thin crescent Moon will pass a short distance south of Comet Lemmon, providing a helping hand.

In the coming months, the comet will get progressively higher in the sky. It will pass within 10° of the north celestial pole on August 10, only to head back out into the outer fringes of the solar system once again. Comet Lemmon will not return for another 11,000 years!

Finder map - field width 40°, stars to magnitude +6.5.

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 2 - Last Quarter Moon at 6:14 A.M. EDT.

May 5 - The Eta Aquarid meteor shower is at peak activity.

May 9 - New Moon at 7:29 P.M. EDT. An annular solar eclipse is visible from Southern Asia, parts of Australia, the Pacific and the Indian Ocean.

May 11 - Mercury is in superior conjunction with the Sun.

May 12 - The Moon is 2.9° south of Jupiter at 8:03 A.M. EDT.

May 13 - The Moon is at apogee, the point in its orbit when it is farthest from the Earth.

May 17 - First Quarter Moon at 11:35 P.M. EDT.

May 22 - The Moon is 0.1° south of Spica at 5:34 A.M. EDT.

May 23 - The Moon is 3.7° south of Saturn at 4:55 A.M. EDT.

May 24 - Full Moon at 11:25 P.M. EDT. Penumbral lunar eclipse; due to the small entry into the penumbral shadow the eclipse will be impossible to detect visually. May 25 - The Moon is at perigee, the point in its orbit when it is nearest to Earth. May 27 - Mercury is 2.4° north of Jupiter at 1:06 A.M. EDT. May 28 - Venus is 1° north of Jupiter at 12:30 P.M. EDT. May 31 - Last Quarter Moon at 1:58 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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