The Waxing Moon Slides by Some Stars, Bright Planets Parade all night, and Asterisms Abound!
Asterisms are informal pictures or patterns made up of stars that, with a few exceptions, differ from the 88 officially recognized constellations. Because they are composed of stars, checking out asterisms is a perfect pursuit when the moon washes out the fainter deep sky objects, as it will this week, and next.
Asterisms span a wide range in size. The Winter Hexagon, among the largest, is enormous, combining stars from multiple constellations. At the present time, the eastern half of the Winter Hexagon is arcing over Venus and setting in the west during late evening. If you face west, from left to right, the stars include white Procyon in Canis Minor (the Little Dog), warm colored Pollux and Castor in Gemini (the Twins), and yellow-tinted Capella in Auriga (the Charioteer).
In the middle-range size we find asterisms that are made from only part of a single constellation. Located about halfway up the southwestern sky, the Sickle (or backwards question mark) asterism takes up the front (western) portion of Leo (the Lion). It is anchored at the bottom by the constellation’s brightest star, Regulus. High in the southeastern sky, the Kite asterism takes up most of the constellation of Boötes (the Shepherd). The bottom corner of the kite is marked by bright Arcturus, while the “wind” blows the rest of the kite upwards and to the left.
Look about four fist diameters to the left (northeast) of Arcturus for the distinctive Keystone asterism in Hercules. It’s about a palm’s width across. Corona Borealis (Northern Crown) sits between them, a case where the entire constellation forms the asterism, an nearly complete ring of stars.
In late evening, the Summer Triangle rises in the east, starting with bright white Vega. The white stars Deneb and Altair sit to its lower left and lower right respectively. The Big Dipper asterism sits near the zenith in late evening. Its seven bright stars are only a portion of a large constellation called Ursa Major (the Big Bear). Fainter stars located east of the dipper form the bear’s head and neck, and its front and rear legs. The bowl of the dipper resembles a saddle on the bear’s back. The dipper’s handle doubles as the bear’s tail. Astronomers believe that five of the stars in the dipper — Mizar, Alioth, Merak, Megrez, and Phecda, formed together about 300 million years ago, and are still travelling through space together as siblings — hence their similar distances of about 80 light-years from Earth.
Small asterisms are perfect for binoculars. These include the Coathanger, located midway between Vega and Altair, and cute little Delphinus (the Dolphin), a complete constellation that is best seen in the summer months. At the tiny end of the size scale, asterisms that need a telescope include one of my favorites, the Stargate, located in the constellation of Corvus (the Crow). To me, this asterism looks like the flux capacitor from Back to the Future. This asterism is observable this time of year. It’s located in the lower third of the southern sky about a fist’s width to the right of bright Spica and the same distance below the bright star Porrima, both in Virgo (the Maiden). The Stargate is also only one degree to the lower right of the Sombrero Galaxy, so look for that at the same time.
The Moon and Planets
The moon reaches its First Quarter phase on Monday evening, the point in its orbit when it makes a 90° angle with Earth and the sun. First quarter moons always rise during the afternoon and move into a perfect position for early evening moon-gazing.
On any clear night this week, head outside with your binoculars, or telescope of any size, and view the lunar terrain extending from pole to pole along the terminator — the boundary between the lit and dark sides. As the sun slowly rises over the moon’s near side, the terminator shifts towards the lunar west (our east). Every night during the ten days or so that precede the full moon, slanted rays of sunlight will illuminate the mountain peaks and crater rims which in turn cast deep black shadows into low-lying areas — with something new to see each night.
While the moon waxes fuller, it also slides eastward along the ecliptic and zodiac. In late evening on Monday, the moon will be situated just above (north of) Regulus, the brightest star in Leo (the Lion). The stars marking the lion’s neck and head form a backwards question mark that extends upwards starting at Regulus. The moon and the star will appear together in the field of view of a small telescope at low power. By observing the relative positions of the two objects between dusk and moonset several hours later, the moon’s eastward orbital motion will be obvious. Minimum separation occurs about 11:15 pm. EDT.
On Thursday evening, the moon will be gibbous (nearing full) and land just to the upper left (north) of another prominent star named Porrima, the brightest star in Virgo (the Maiden). In binoculars and telescopes, Porrima splits into a closely spaced pair of stars. The same trick for watching the moon slide eastward will apply — if you’re willing to stay up late!
Finally, next Sunday evening, the very bright moon will start a tour of the major planets — landing less than a palm’s width to the left of Jupiter and accompanying it as it crosses the night sky.
Venus continues to gleam in the western evening sky this week. It sets well after 11 pm local time. Tonight (Sunday), Venus, situated among the toes of Gemini (the Twins), will arrive within a finger’s width to the right of the bright open star cluster designated Messier 35. The distant star cluster and nearby planet will appear together within the field of view of a low power telescope or your binoculars. If Venus’ 400 times brighter brilliance is overwhelming the cluster’s stars, try hiding the planet just outside the edge of your field of view. Next Sunday evening, our sister planet will end up extremely close to a modestly-bright star named Mebsuta which marks the waist of Castor, the westerly twin.
Jupiter is still visible all night long this week. Look for it as a very bright object in the southeastern sky after dusk. It will reach its highest elevation (about three fist diameters) above the southern horizon around 12:30 am local time, and then descend into the southwestern sky as the sun rises. The bright star sitting just to the upper right of Jupiter is Libra’s (the Scales) brightest star, Zubenelgenubi. In binoculars or a small telescope, it splits into a closely spaced pair of stars.
On Monday between 1:05 and 2:48 am, the black shadow of Jupiter’s moon Ganymede and its little round black shadow will cross (or transit) Jupiter’s disk near the planet’s northern pole. On Tuesday morning, May 22 the moon Io and its shadow will transit between 2:45 am and 4:54 am. On Wednesday evening, May 23, Io and its shadow will transit between 9:13 pm and 11:23 pm. Finally, on Sunday morning, May 27 the moon Europa and its shadow will transit between 1:50 and 4:05 am. All times are given in Eastern Daylight Time (EDT). A reasonable backyard telescope will show the black shadows, but a very good telescope is needed to see the moons themselves.
The Great Red Spot takes about three hours to cross Jupiter’s disk. But the planet’s 10-hour rotation period (i.e., its day) means that the spot is only observable from Earth every 2–3 nights. If you’d like to see the GRS, use a medium-sized telescope (or larger). You’ll have your best luck on evenings with steady air — when the stars are not twinkling too much. The best times to try this week are: Sunday, May 20 at 9:25 pm, Tuesday, May 22 at 11 pm, Friday, May 25 at 12:41 am and again at 8:32 pm, and Sunday, May 27 at 10:10 pm. All times are given in Eastern Daylight Time (EDT), so adjust for your local time zone. Try to look within an hour before or after the times I’ve given.
This week, yellowish Saturn will rise in the east at about 11 pm local time. It will be spending this year on the eastern edge of the Milky Way, just above the Teapot-shaped star pattern that forms part of the constellation Sagittarius (the Archer). You should be able to see Saturn until almost 5:45 am, when it will sit about two fist widths above the southern horizon.
Reddish Mars, now noticeably brighter than Saturn, will be rising about 1.5 hours after Saturn this week. This puts it 2.3 fist widths to the lower left of the ringed planet. Mars continues to steadily brighten and increase in size as the Earth’s faster orbit brings us closer to the red planet this summer. (We will pass it on the “inside track” in late July.)
Distant blue Neptune, still among the modest stars of Aquarius (the Water-bearer), has become observable in telescopes in the pre-dawn eastern sky after it rises about 2:30 am local time.
Astronomy Skylights for this week (from May 20th, 2018) by Chris Vaughan.
Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.