The Night Sky of April

Dr. Wayne Wooten
Professor of Astronomy

For April 2012, the Moon will be full on April 6th; this is the Paschal Moon, the first full moon after the Vernal Equinox, which sets the date of Passover and Easter Sunday on April 8th. The waxing gibbous moon passes eight degrees south of fading Mars on April 3rd, and the waning gibbous moon passes six degrees south of Saturn on April 7th, with both rising about an hour after sunset.

The last quarter moon rises at midnight on Friday the 13th. The waning crescent moon passes six degrees north of Mercury on the morning of April 18th; Mercury is then at its greatest western elongation, which makes it as far away from the sun and easy to spot as possible. The new moon is April 21st. The waxing crescent passes 2.5 degrees north of Jupiter on March 22nd, but they will be so close to the sun this will be hard to spot. By April 25, the waxing crescent passes 6 degrees south of Venus, and with Venus near greatest brilliancy, you may be able to spot both with the naked eye in broad daylight perhaps 2-3 hours before sunset with a clear day.

Mercury lies to the west of the Sun in the morning sky this month, but early rises can spot it an hour before sunrise about midmonth, with the waning crescent moon passing 7 degrees north of it on April 18th. Venus dominates the evening sky in the west, with Jupiter now vanishing behind the Sun by the end of the month. She passes just south of the Pleiades cluster on April 4th, a fine grouping for binocular viewers. Through the telescope, Venus now appears about half lit and sets about four hours after the sun at the beginning of April. But she is catching up to the slower moving Earth, and in retrograde motion will set earlier and earlier by month’s end.

In May she appears as a large but thin crescent in binocs and telescopes, and historically on June 5, 2012, starting about 5 PM CDT, she will transit the Sun, passing directly between us and the Sun. She will not do so again until December 2117, so hope for clear skies to enjoy this one! Mars is still bright red in Leo in the eastern sky after sunset, but not nearly as big and bright as it was at opposition in early March. Jupiter rapidly disappears in the sun’s glare low in the west now, to reappear in the morning sky in June. But Saturn is at its best in the east in Virgo, just northeast of bright Spica, rising at sunset on its opposition date of April 15th. This is the best time to observe the most beautiful object in the sky.

When viewed with a telescope, the rings are open 14 degrees or about half as wide as at its solstice in 2017, and Titan and several smaller moons fall on either side of the most beautiful telescopic sight in the sky. In addition to its glorious rings, Saturn hosts a huge moon, Titan, visible in most any telescope, and several smaller ones needing at least a 6" scope to spot. Our photo this month shows about how large scopes will show the planet this month.

Yellow Capella, a giant star the same temperature and color as our much smaller Sun, dominates the northwestern sky. It is part of the pentagon on stars making up Auriga, the Charioteer (think Ben Hur). Several nice binocular Messier open clusters are found in the winter milky way here. East of Auriga, the twins, Castor and Pollux highlight the Gemini. South of Gemini, Orion is the most familiar winter constellation, dominating the southern sky at dusk. The reddish supergiant Betelguese marks his eastern shoulder, while blue-white supergiant Rigel stands opposite on his west knee. Just south of the belt, hanging like a sword downward, is M-42, the Great Nebula of Orion, an outstanding binocular and telescopic stellar nursery. The bright diamond of four stars that light it up are the trapezium cluster, one of the finest sights in a telescope. In the east are the hunter’s two faithful companions, Canis major and minor. Procyon is the bright star in the little dog, and rises minutes before Sirius, the brightest star in the sky. Sirius dominates the SE sky as darkness falls. At 8 light years distance, Sirius is the closest star we can easily see with the naked eye.

To the northeast, look for the Big Dipper rising, with the top two stars of the bowl, the pointers, giving you a line to find Polaris, the Pole Star. Look for Mizar-Alcor, a nice naked eye double star, in the bend of the big dipper’s handle. Take the pointers at the front of the dipper’s bowl south instead to the head of Leo, looking much like the profile of the famed Sphinx. The bright star at the Lion’s heart is Regulus, the "regal star", but brighter still is Saturn, just east of Regulus. Mars gets almost back to Regulus by midmonth, but halts its retrograde motion on April 15th, just a little east of Leo’s brightest star, and starts heading rapidly eastward by month’s end, and will head into Virgo in May.

Now take the curved handle of the Big Dipper, and follow the arc SE to bright orange Arcturus, the brightest star of the spring sky. Recent studies of its motion link it to the Sagittarius Dwarf Galaxy, a companion of our Milky Way being tidally disrupted and spilling its stars above and below the plane of the Milky Way, much like dust falling away from a decomposing comet nucleus. So this brightest star of Bootes the Bear Driver is apparently a refugee from another galaxy!

Now spike south to Spica, the blue-white gem in Virgo rising in the SE. Virgo is home to many galaxies, as we look away from the obscuring gas and dust in the plane of the Milky Way into deep space. To the southwest of Spica is the four sided Crow, Corvus. To the ancient Greeks, Spica was associated with Persephone, daughter of Ceres, goddess of the harvest. She was abducted by her suitor Pluto, carried down to Hades (going to Hell for a honeymoon!) and when Jupiter worked out a compromise between the newlyweds and the angry mother-in-law, the agreement dictated Persephone come back to the earth’s surface for six months of the year, and Mama Ceres was again placated, and the crops could grow again.

As you see Spica rising in the SE, it is time to "plant your peas", and six months from now, when Spica again disappears in the sun’s glare in the SW, you need to "get your corn in the crib"….so was set our calendar of planting and harvesting in antiquity. In the arms of Virgo is a rich harvest of galaxies for modern astronomers. And of course Saturn lies just to the northeast of Spica now, retrograding slowly westward but not quite reaching Spica.

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