The bright stars of winter are gathered in the western half of the early evening sky before their annual departures, starting with Rigel’s exit in early May, and ending with Pollux and Castor by early July. Surrounded by these luminaries, well up in the west at dusk in April, Mars slips between the horn-tips of Taurus before mid-month, and passes into Gemini before month’s end.
The first evening of the month of Ramadan offers a chance to view a one-day-old moon just after sunset on April 12. A close pairing of Mercury and Venus before it sets in the west-northwest, barely 35 minutes after sunset on April 25, will also challenge observers. Thereafter, Mercury improves rapidly, reaching its peak altitude in mid-May. Leisurely Venus will peak more than six months later, in late November-early December. On April mornings, in the east-southeast to southeast, Jupiter dominates, with Saturn not far to its upper right.
In evening mid-twilight, as April begins, the brightest objects are Sirius, the Dog Star, in the south-southwest, and Capella, the Mother Goat star, high in the northwest. These stars mark the southern and northern apices of the huge Winter Hexagon. In clockwise order from Sirius, find Procyon, Pollux and Castor, Capella, Aldebaran, Rigel and back to Sirius.
Mars, fading from magnitude +1.3 to +1.6 in April, is well up in the west at dusk all month while moving eastward against the star background by 0.6 degrees daily. Look nightly around dates when Mars lines up with or passes very near stars, and you’ll notice its motion easily: Between April 12 and 13, Mars passes between Beta and Zeta Tauri, stars of second and third magnitude eight degrees apart, marking the tips of the Bull’s horns. Between April 28 and May 2, Mars passes within 2.4 degrees north of Eta and Mu of Gemini, a pair of third-magnitude stars 1.9 degrees apart, marking Castor’s foot. On April 30, Mars will appear 2.5 degrees from each star. The events of April 28-May 2 are best appreciated with binoculars just after twilight ends.
Venus passed superior conjunction on far side of the sun on the night of March 25. Each week, Venus moves only 1 degree farther from the sun, so even at the end of April, it will set at evening mid-twilight, only 43 minutes after sunset. Mercury passes superior conjunction on April 18, following Venus by 24 days—but the innermost planet’s speed makes it emerge into visibility much more rapidly, boosted by its crossing north of Earth’s orbit plane on April 22, and passage through perihelion of its orbit on April 27. Binoculars will be needed to view the conjunction of Mercury and Venus on April 25, just 1.2 degrees apart and only 8 degrees from the sun, with the pair setting only 36-38 minutes after sunset. To observe this challenging event, pick a location with an unobstructed view toward the west-northwest where you can still see the sun when it’s very low, about 3 degrees up some 20 minutes before the calculated time of true sunset. (There are places in the Coachella Valley where you can watch the approaching sunset through San Gorgonio Pass.) Remember the sun’s location relative to the foreground landscape; note the time; and then wait 36 minutes. Venus will then be 2.3 degrees to the right of the sun’s previous position, and Mercury will be 1.2 degrees to Venus’ upper right. By April 30, Mercury will be an easy object to see with the unaided eye, nearly 5 degrees above Venus, setting 67 minutes after the sun, and 24 minutes after Venus.
The new moon occurs on Sunday, April 11, at 7:30 p.m. The sighting of the young crescent moon on the first possible evening this time around, Monday, April 12, marks the beginning of the fasting month of Ramadan in the Islamic calendar. If the sky is very clear soon after sunset, and you select your viewing site carefully, you’ll have a good chance to see it. Choose a spot where a low profile of distant mountains will allow you to follow the sun almost down to the ideal horizon. About 20 minutes before the calculated sunset in our area, the sun will be 9 degrees north of west and 3 degrees above an imaginary unobstructed horizon. Note the sun’s location relative to the landscape profile; return to the same site after 45 minutes, and you’ll find the 1 percent lunar crescent at the same altitude, but about 0.6 degrees farther to the right. Binoculars make the sighting easier. The moon will be within 11 degrees of the sun and 24.2 hours old.
The moon at dusk works its way from the western to eastern horizon early in the evenings of April 12-26, while changing from a thin crescent to full. Moving an average of 13 degrees per day against the background of the zodiac constellations, the waxing moon is seen in Aries on April 13; then moves through Taurus (April 14, 15, and 16) and Gemini (April 18 and 19). The moon is in Cancer on April 20; in Leo April 21 and 22; and in Virgo April 23, 24 and 25). The full moon appears in Libra on April 26.
Morning planets: Jupiter (magnitude -2.1 to -2.2) is the brightest morning “star.” At mid-twilight in the course of April, Jupiter climbs from 16 degrees up to 27 degrees up in the east-southeast to Southeast. Saturn (magnitude +0.8 to +0.7) is 12-15 degrees to Jupiter’s upper right. Both planets are moving slowly east against the background.
Bright stars: Zero-magnitude stars include golden Arcturus, well up in the west, and blue-white Vega, passing only five degrees north of overhead after mid-month. Also look for these first-magnitude stars: Spica, setting in the west-southwest; Altair and Deneb, completing the Summer Triangle with Vega; and reddish Antares, heart of the Scorpion, in the south-southwest to southwest. Fomalhaut rises in the southeast before month’s end.
The moon at dawn: About an hour before sunrise through April 9, the moon makes a trek across the morning sky. On April 1 and 2, the waning gibbous moon appears first to the upper right, then to the upper left, of Antares in the south-southwest. A waning crescent moon appears 5 degrees below Saturn on April 6, and 5 degrees below Jupiter the next morning. The last, thin old crescent moon will be seen very low in the east to east-southeast on April 9.
The moon returns to the morning mid-twilight sky April 26-May 9. On April 26, it’ll be not quite full, within 6 degrees above Spica setting in the west-southwest. On April 29, the waning gibbous moon will be 4 degrees to the upper left of Antares in the south-southwest to southwest.
An outstanding highlight for May sky-watchers will be a total lunar eclipse before dawn on Wednesday, May 26. Details will be provided in next month’s column.
An Introduction to Twilight Sky Maps
For your sky-watching sessions, we suggest that you begin during evening twilight, so you can experience the joy of discovering and identifying the brighter stars as they first appear. You can begin your session as soon as a half-hour after sunset, or even earlier when the moon or brightest planets are visible, and can continue until you have enough dark-sky time to observe fainter sky objects.
If you are interested in a predawn session, you can begin early enough to allow time to observe a selection of fainter sky objects before twilight begins. In that case, start the session up to 2 hours before sunrise, and continue long enough into twilight to watch some of the brighter stars disappear.
My friend and former colleague at Michigan State University, Robert D. Miller, has kindly provided us with monthly sky charts tracking daily locations of the five naked-eye planets and the 16 stars of first magnitude or brighter visible from latitude 34 degrees north. Positions of the stars and planets are plotted each day at the moment the sun is 9 degrees below the horizon, which we have called “mid-twilight.” Locations of the planets are plotted as a separate dot for each day, with larger, labeled dots plotted weekly for the 1st, 8th, 15th, 22nd and 29th. Star positions during the course of the month are plotted as continuous tracks, with all stars drifting westward (left to right on the charts) in the course of the month, owing to the Earth’s revolution around the sun.
For the Coachella Valley, at latitude 34 degrees north, the moment of evening mid-twilight occurs 39 to 47 minutes after sunset, depending on the time of year. Morning mid-twilight occurs a similar interval ahead of sunrise.
Sometimes a star is below the horizon at evening mid-twilight at the start of a month, but makes its first appearance above the eastern horizon during the course of the month—for example, Spica in the east-southeast in evening mid-twilight in April. Notice on April’s morning mid-twilight chart that Spica is visible low in the western sky at the start of April, but sinks below the west-southwest horizon in the course of the month. Spica is at opposition to the sun on April 13, and around that date it is visible all night, from dusk until dawn.
Robert Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968, and still produces issues occasionally. For subscription information and a sample calendar and evening sky map for April 2021, visit www.abramsplanetarium.org/skycalendar. Evening and morning twilight sky maps by Robert D. Miller, who did graduate work in planetarium science and later astronomy and computer science at Michigan State University. He remains active in research and public outreach in astronomy.