In April 2017, bright and steady Jupiter is likely the first “star” to be spotted after sunset, south of east. Twinkling, blue-white Sirius, in the southwest quadrant, is next. Jupiter is at opposition to the sun, and above the horizon all night, on April 7-8; you can estimate the hour by noting the planet’s location—low in the eastern sky at dusk, high in the south in the middle of night, and low in the western sky at dawn.
As morning twilight brightens, we find an even more spectacular planet, Venus, rising in the east, as Jupiter slinks off toward the western horizon. Before dawn brightens too much, locate Saturn in the south.
Our evening sky chart plots daily positions of the brightest objects in the current month’s sky at mid-twilight. As April begins, the most-prominent objects, in order of brightness, are steady yellow-white Jupiter, climbing in the east to southeast as the month progresses; twinkling blue-white Sirius in the south-southwest to southwest; golden Arcturus ascending in the east-northeast to east, to the far left of Jupiter; and Capella, high in the northwest.
Sirius and Capella mark extreme south and north points of the huge Winter Hexagon, encircling Betelgeuse and Orion’s three-star belt inside. The belt points the way to Sirius, the Dog Star, and in the opposite direction, to Aldebaran, eye of Taurus, the Bull. In clockwise order around the hexagon from Sirius, locate Procyon, Pollux (with his fainter twin brother Castor nearby, not plotted), Capella, Aldebaran, Rigel and then back to Sirius.
Preceding the Hex across the sky in April 2017 is faint Mars, itself preceded by Mercury, as bright as zero-magnitude Arcturus on April 1, but fading to first magnitude by April 6, and very sharply thereafter. The trailing Pollux-Procyon side of the Hex crosses due south into the western half of the sky in twilight at the start of April. Following is Regulus, heart of Leo, reaching south at month’s end. Still farther east in April 2017, we find Jupiter, with Spica close by, and Arcturus.
The moon in evening sky: On the evening of April 1, the crescent moon, about one-third full, appears several degrees to the upper left of Aldebaran. The waxing moon, moving eastward, remains within the Winter Hexagon for two more evenings, but by April 4, the now-gibbous moon has exited (in a maneuver known as Hexit), appearing to the left of the Pollux-Procyon line. On April 6, the moon passes closely south of Regulus. On the evening of Sunday, April 9, the moon appears about 9 degrees above Jupiter, and on Monday, April 10, the full moon appears 3-4 degrees to the lower left of Jupiter. That evening, the moon rises about 17 minutes before sunset. Using binoculars, can you spot Jupiter in daylight to the moon’s upper right? As the sky darkens, notice Spica 7 degrees to the lower right of the moon-Jupiter pair.
After the full moon of early spring, the moon rises later and farther south nightly. On Tuesday, April 11, moonrise occurs at 7:52 p.m.; on April 12, at 8:47 p.m.; on April 13, at 9:41 p.m.; on April 14, at 10:34 p.m.; and on April 15, at 11:25 p.m.
The moon in morning sky: From the full moon onward, it may be more convenient for most folks to follow the moon at dawn. On Sunday, April 10, one hour before sunrise, find bright Jupiter low in the west-southwest to west, with Spica 7 degrees to its left. The nearly full moon will be 7 degrees to Jupiter’s lower right. On the next morning, April 11, the moon, just past full, will appear 7 degrees to Jupiter’s upper left. By Saturday, April 15, the waning gibbous moon will appear in the south-southwest, about 10 degrees above Antares. On Easter Sunday, April 16, the moon will appear closely to the upper right of Saturn in the south, and on April 17, farther to Saturn’s upper left. While you’re up early enjoying the moon and Saturn those two mornings, add to your total of solar system objects by spotting brilliant Venus low in the east before Jupiter sets in the west-southwest to west. Our morning twilight sky chart shows all these stars and planets, plus Arcturus in the west, to the upper right of Jupiter, and Vega, passing just north of overhead. High in the eastern sky, Altair and Deneb complete the Summer Triangle with Vega.
By the morning of April 19, the moon will reach last quarter phase, half full and 90 degrees or one-quarter of a circle west of the sun. After April 19, the moon is a waning crescent, closer to the sun each day.
On Saturday, April 22 there won’t be much moonlight to interfere with the peak of the Lyrid meteor shower, best in the hour or two before first light of dawn. To see the greatest number of meteors, observe from a dark place, and get most of your viewing in before 4:30 a.m.
On Sunday, April 23, moonrise occurs at 4:27 a.m., with sunrise at 6:05 a.m. Look an hour before sunrise, and you’ll see a 12-percent crescent moon very low in the east to east-northeast, 7-8 degrees to the lower right of Venus. Slight optical aid will reveal Venus itself is a crescent, 20 percent illuminated that morning.
The Venus-moon pairing on April 23 isn’t particularly close—the next one, on May 22, will be much tighter and more impressive—but this is the first of eight monthly predawn Venus-moon pairings through November.
On Monday, April 24, moonrise occurs at 5:06 a.m., with sunrise at 6:04 a.m., so look about 40 minutes before sunrise, at 5:24 a.m., to catch the last old moon, a 5 percent crescent, 3 degrees up just south of due east, and 11 degrees to the lower left of Venus.
Some special sights for binoculars:
1. Venus in April 2017 appears in crescent phase, less than 3 percent full on April 1, while nearly one arcminute across. One arcminute is 1/60 of a degree, and the moon is about a half-degree in apparent diameter, so a magnification of just over 30-power used to observe Venus at the start of April makes it appear as large as the moon is with the unaided eye! Even 7-power binoculars reveal a crescent if Venus is observed in bright twilight or in daylight. As weeks pass, Venus recedes from Earth and shrinks in apparent size, as the crescent begins to fill.
2. Star clusters, including the Hyades with Aldebaran in the foreground. Aim binoculars at Aldebaran, placing that bright star in the upper left part of the field, and other stars within the field will complete the V-shaped head of Taurus, the Bull. Those fainter stars belong to the Hyades, about 150 light years away, compared to Aldebaran at 67 light years. Also, find Pleiades, or Seven Sisters: This compact star cluster, located about 400 light years away, can be found in April’s evening sky about 14 degrees to the lower right of Aldebaran.
3. Jupiter’s Galilean moons: As many as all four of Jupiter’s satellites discovered by Galileo in 1610 can be spotted in binoculars very close to the planet. The brightest and easiest to see is Ganymede, the biggest moon in our solar system, larger than Mercury! In its seven-day orbit, this month find it farthest west of Jupiter on weekends, April 1, 8 and 9, 15 and 16, 22 and 23, and 30. Fainter, wider-ranging Callisto, in its 17-day orbit, appears farthest east of Jupiter on April 2 and 19, and farthest west on April 10 and 27. Slower-moving, it remains easy to see for about two days before and after these dates.
4. The thin crescent moon with earthshine. The moon returns to evening sky on Thursday, April 27. Your first chance to see the waxing crescent moon of the next lunar month will be at dusk on April 27, when the moon of age 39 hours will be very low, between the west and west-northwest. About an hour after sunset, look for the 4-percent lunar crescent to the lower right of Aldebaran, and lower left of Mars and the Pleiades. On Friday, April 28, the thicker 10-percent lunar crescent will appear a few degrees to the upper left of Aldebaran. On both evenings, and perhaps for a few more, note the faint, bluish illumination on the dark, non-sunlit portion of the moon. This is earthshine, from sunlight reflected from Earth.
Star parties provide wonderful opportunities to join with other folks who love to share their interest in observing the sky, and to get great views of astronomical objects through a variety of binoculars and telescopes. The Astronomical Society of the Desert will host the last of the season’s monthly public star parties at the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument, on Saturday, April 1, from 7 to 10 p.m., and on Saturday, May 17, from 8 to 10 p.m. The Visitor Center is on Highway 74, within four miles south of Highway 111 in Palm Desert. Check www.astrorx.org for listings of our regular star parties and our high altitude star parties at Sawmill Trailhead starting at dusk. The next high-altitude star party (at 4,000 feet; wear warm clothes!) will be held on Saturday, April 22. Follow links to maps and directions to both star party sites.
Also, check the link to our “Impromptu Star Parties,” which could be announced on short notice at any time.
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. Robert D. Miller did graduate work in planetarium science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.