Throughout April, brilliant Venus and slowly fading Mars put on a grand show, moving against a rich background of stars of two adjacent zodiac constellations, Taurus and Gemini.
Venus gleams at magnitude -4 in the western sky at dusk. Mars appears to the upper left of Venus, by 44° on April 1. Venus is now moving just more than one degree daily against the stars, compared to Mars’ more than half a degree, so the gap between them is closing, to 35° by April 15, and 26° at month’s end.
Mars glows at magnitude +0.9 to +1.4, comparable to Gemini’s bright “Twin” stars, Pollux at magnitude +1.1, and Castor at +1.6, just 4.5° apart.
Another bright planet, Mercury, has its best evening apparition of this year. Look low in west to west-northwest, 22° to the lower right of Venus, on April 1, holding at 20° during April 5-13, and reopening to 22° on April 16. Mercury starts off bright, at magnitude -1.0 on April 1, but fades to 0.0 by April 10, +0.9 on April 16, and +1.9 by April 20, when Mercury is low in bright twilight, 26° to the lower right of Venus. Mercury’s fade occurs despite the planet’s decreasing distance from Earth, because Mercury, unlike Venus, has no cloud cover, and its surface features cast more shadow as Mercury moves around to the near side of the sun and displays crescent phases.
The brightest star in the evening sky—not as bright as Venus—is Sirius, the Dog Star, in the southwestern quadrant. Confirm your identification by noting that the Belt of Orion points to it. Extend the belt in the opposite direction, passing just below Aldebaran, eye of Taurus, to reach the beautiful Pleiades star cluster, which Venus will pass closely before mid-April. Binoculars will give wonderful views of the Pleiades and of the many of the events described here.
Evening and morning events involving planets, stars and the moon:
April 1 at dusk: Regulus, heart of Leo, the Lion, appears within 6° to the lower right of the gibbous moon, 85 percent full. During April 1-5, Mars passes within 3° north of third-magnitude stars Eta and Mu in Gemini, marking one foot of Castor.
April 5 at dusk: Spica, the spike of grain in the hand of Virgo, has just risen in the east-southeast, within 9° below the full moon.
April 6 at dawn: Spica, in the west-southwest, is 5° to the left of the full moon. The two brightest stars at dawn are Arcturus, high in the west, 33° to the upper right of Spica; and Vega, not far east/east-northeast of overhead.
April 6 at dusk: Spica appears within 6° to the upper right of the rising 99 percent moon, just past full.
April 7 at dawn: Spica appears 9° to the lower right of the 98 percent waning gibbous moon. Instead of waiting later each night until moonrise, we can follow the moon daily in the morning sky.
April 9 at dawn: Antares, heart of the Scorpion, is within 11° to the upper left of the 88 percent waning gibbous moon.
April 9-12 at dusk: Venus passes about 3° south of the Pleiades (Seven Sisters) star cluster. A splendid view through binoculars!
April 10 at dawn: Antares is 3° to the lower right of the 80 percent moon.
April 11 at dusk: Mercury reaches greatest elongation, 19.5°, from the sun. Also, Mercury attains its greatest altitude above the horizon at mid-twilight for this apparition—and for anytime this year, 10° up, when the sun is 9° below the horizon.
April 13 and 14 at dusk: Mars is very close to third-magnitude Epsilon in Gemini.
Night of April 13-14: Spica is at opposition, as Earth passes between that star and the sun. Look for Spica low in the east-southeast at dusk, high in the south in the middle of night, and low in the west-southwest at dawn.
April 15 at dawn: Saturn is in the east-southeast, 11° to the left of the 26 percent crescent moon. During April, Saturn’s rings are tipped only 9° to 8° from edge-on, our narrowest view since 2011. The rings are heading toward an edgewise presentation in spring 2025.
April 16 at dawn: Saturn is 6° to the upper right of the 17 percent crescent moon. The waning old crescent moon will be visible two more mornings, about 13° farther to the lower left of Saturn each time.
April 18 and 19 at dusk: Aldebaran is 7.4° south (to the lower left) of Venus. Binoculars show stars of the Hyades cluster in the same field as Aldebaran, forming with that reddish star the V-shaped head of the Bull.
April 20: The young moon is very low at evening mid-twilight. Using binoculars, try to spot the 1 percent crescent 30° to the lower right of Venus, with Mercury 4° to the moon’s upper right. Mercury is now very faint, magnitude +1.9, so both will be a challenge.
April 21 at dusk: The 5 percent crescent moon, 17° to the lower right of Venus, is much easier than last night’s moon. Using binoculars, enjoy the Pleiades 5° to the upper right of the moon, and try one last time for faint Mercury, magnitude +2.2, 10° to the moon’s lower right.
April 22 at dusk: Venus is 5° to the upper left of a 10 percent crescent moon. Mars forms an isosceles triangle with Pollux and Castor, 10° from each.
April 23 at dusk: Venus is 6° below a 17 percent crescent moon.
April 25 at dusk: Mars is 3° to the lower left of a 34 percent crescent moon. The stars Pollux and Castor mark the heads of the Twins. Tonight, they appear 7-8° above the moon.
April 26 at dusk: Tonight, the moon is 43 percent full. Pollux, Castor and Mars all appear to the lower right of the moon, by 5°, 9° and nearly 12°, respectively.
April 27 at nightfall: The moon, 53 percent full, is in Cancer, the Crab, and just past first quarter phase, when it’s half full and 90 degrees east of the sun. Wait until dark, and binoculars will show you the Beehive star cluster, 5° to the lower left of the moon.
April 28 at dusk: Regulus, in Leo, appears 9° to the lower left of the 62 percent gibbous moon. Tomorrow night, the moon will be 71 percent full, with Regulus within 6° to its lower right.
April 29 and 30 at dusk: Mars is about 2° north of third-magnitude Delta in Gemini. Venus passes 3° south of 1.7-magnitude Beta in Taurus (Elnath, tip of Bull’s northern horn) on April 30.
The evening twilight chart for April shows a large number of bright stars in the western sky at dusk at this time of year. Not counting planets, seven of the 14 brightest stars of first magnitude or brighter visible from Northern Nevada at sometime during the year are gathered in the western sky, getting set to depart in the next several weeks. Rigel, Aldebaran and Sirius will be among the first to go. As a consequence of the Earth’s revolution around the sun, the stars appear lower in the west each night. It’ll be enjoyable to look for them nightly.
Stars (but not planets) disappear around the same date each year, so you can eventually learn to use the sky as a calendar—just as some ancient cultures did
Illustrations of many of the events described here appear on the April 2023 Sky Calendar. Subscription info and another sample issue are available at www.abramsplanetarium.org/skycalendar.
Robert Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968 and still produces issues occasionally. He enjoys being outdoors sharing the wonders of the night sky.