In July 2021, evening twilight gatherings of brilliant Venus, faint Mars and the star Regulus include close pairings on July 11-14, 21 and 29; a crescent moon passing through on July 11 and 12; and a gathering of three bodies in a field just 5 degrees across on July 21.
Three hours after sunset on July 1 (lowering to one hour at month’s end), bright Jupiter rises in the east-southeast, and chases Saturn across the sky for the rest of the night, ending in the southwest at dawn. Late in July, it becomes possible to see the two brightest planets, Venus and Jupiter, simultaneously, above opposite horizons.
In order of brightness, the actors in July’s evening “celestial traffic jam” are brilliant Venus, of magnitude -3.9; Regulus, at magnitude +1.4, the faintest of all the first-magnitude stars; and Mars, at magnitude +1.8, as faint as it gets. Binoculars are recommended for viewing the two fainter members, low in bright twilight.
As July opens, Venus, very low in the west-northwest, is the lowest of the three bodies, with Mars within 7 degrees to its upper left, and Regulus 17 degrees to the upper left of Mars. The three objects span 24 degrees on July 1. Venus moves closer to Mars each evening until July 12. In the first “narrow miss,” they’ll appear within 1 degree on July 11; 0.5-0.6 degrees apart on July 12 and 13; and about 1 degree apart again on July 14.
Even the moon gets into the act! A very thin, 1 percent crescent moon—setting in bright twilight some 16 degrees to the lower right of Venus—on July 10 will be a challenge to see, but on the next two nights, the beautiful waxing crescent moon will hopscotch past the Venus-Mars pair: On July 11, the 5 percent crescent moon will appear within 5 degrees to the lower right of Venus, and on July 12, the 10 percent moon will appear 8 degrees to the upper left of the Venus-Mars pair, and 5 degrees to the upper right of Regulus. On both evenings, the non-sunlit portion of the lunar disk will be attractively illuminated by earthshine.
A nearly half-illuminated moon, almost at first-quarter phase, will appear in the southwest, 6 degrees to the upper right of Spica, on July 16. Next, a waxing gibbous moon will appear 7 degrees to the upper right of Antares in the south-southeast to south at dusk on July 19, and 9 degrees to the left of that star on July 20.
The second “narrow miss” of the “traffic jam” will occur on July 21, as Venus passes 1.1 degrees north (to the upper right) of Regulus. That evening, Mars will be 5 degrees to their lower right, creating the most compact gathering of the three objects. All three bodies will fit within the field of view of most binoculars magnifying no more than 10 times.
On the last date of the moon’s visibility in evening mid-twilight, July 23, the full moon will appear low in the southeast, 9 degrees to the upper right of Saturn. On the next night, July 24, wait until about 90 minutes after sunset to see the moon 8 degrees to the lower left of Saturn, with Jupiter, just risen, 14 degrees to the moon’s lower left. On July 25, look about two hours after sunset to catch the 94 percent waning moon, just risen, within 5 degrees to the lower right of Jupiter, and 21 degrees to the lower left of Saturn.
The final “narrow miss”—the most difficult to observe—will be the conjunction of Mars with Regulus on July 29. Using binoculars, look within 10 degrees to the lower right of Venus. Faint Mars will appear just more than 0.6 degrees to the upper right of Regulus.
Other stars: Castor and Pollux, very low in the west-northwest to northwest twilight glow at the start of July, require binoculars and very clear skies. They’ll be in conjunction, on the far side of the sun, July 12-15. Arcturus is very high in the southwestern sky at dusk. The Summer Triangle of Vega, Altair and Deneb, climbing higher in the eastern sky, is visible all night throughout July. On clear, dark moonless nights, you won’t fail to notice the Milky Way, with a prominent star cloud within the Summer Triangle, and another appearing as a puff of steam above the spout of the Teapot of Sagittarius, in the southern sky to the left of Antares and Scorpius.
Later in the evening: Saturn rises in the east-southeast within two hours after sunset on July 1, and rises at sunset on the 31st.
On July 6, Venus and Saturn are at mutual opposition, 180 degrees apart in celestial longitude, and appear less than 1 degree above opposite horizons, about 1.6 hours after sunset, a few minutes before end of evening twilight. On July 18, about 1.2 hours after sunset, each will be 5 degrees up. On night of July 21, Venus and Jupiter are at mutual opposition, and appear barely above opposite horizons 1.6 hours after sunset, very near the end of evening twilight. By July 29, these two brightest planets appear 3 degrees above opposite horizons 1.4 hours after sunset.
Morning twilight planets: Jupiter, at magnitude -2.7 to -2.8, is the brightest morning “star.” Find it in the south to southwest at morning mid-twilight, with Saturn, of magnitude +0.4 to +0.2, 19-20 degrees to Jupiter’s lower right. The giant planets reach a maximum distance apart, 19 3/4 degrees, on July 11, when Jupiter’s rate of retrograde (westward) motion against background stars begins to exceed Saturn’s. The distance between the giants will then close until Oct. 24, when they’ll be 15.4 degrees apart.
You can detect the slow retrograde motion of each planet by carefully noting its position in relation to a background star. During July, Jupiter goes 2.4 degrees west, decreasing its distance from 4.3-magnitude Iota in Aquarius from 3.2 degrees to 1.1 degrees. Saturn goes 2.1 degrees west, directly away from 4.1-magnitude Theta in Capricornus, and increases its distance west-southwest of Theta, from 1.8 to 3.9 degrees. In a dark predawn sky on July 13, you can catch each planet equally distant, within 2.6 degrees, from its respective background companion star.
In the coming years, the usual trend will be for the two giant planets to move farther apart, as both planets move eastward, with Jupiter moving farther ahead of Saturn. As seen from Earth, they’ll appear in mutual opposition, 180 degrees apart in celestial longitude, five times in 2029-31, and have their next conjunction, a single event in Virgo, on Oct. 31, 2040.
The other naked-eye morning planet is Mercury, visible very low in the east-northeast for about the first three weeks of July. On July 1, the innermost planet, of magnitude +0.8, is 9 degrees to the lower left of similarly bright Aldebaran, eye of Taurus, the Bull. Brightening, Mercury reaches greatest elongation, nearly 22 degrees west of the sun, on July 4. On July 6, a 12 percent waning crescent moon appears 6 degrees above Aldebaran and 7 degrees below the Pleiades. On the morning of July 8, Mercury, brightened to 0 magnitude, appears within 5 degrees to the upper right of a 3 percent old crescent moon. Mercury reaches its highest elevation, nearly 7 degrees above the morning mid-twilight horizon, on July 8-10. Mercury brightens to magnitude -1.0 by July 18, and to magnitude -1.5 by July 24, but by that latter date, it is barely above the horizon at mid-twilight. Mercury passes superior conjunction Aug. 1.
Also on July 24, the moon returns to the morning sky. Low in the southwest just past full, it is 6 degrees to the lower left of Saturn. On the next morning, July 25, the moon is within 11 degrees to the upper left of Saturn, and 11 degrees to the lower right of Jupiter. On the morning of the 26th, the 93 percent moon appears 6 degrees to the left of Jupiter and a little higher.
All the lunar and planetary groupings mentioned here are illustrated on the Sky Calendar for July 2021. To subscribe for $12 per year (for three printed monthly issues mailed quarterly), or to view a sample issue, visit www.abramsplanetarium.org/skycalendar.
The brightest stars on July mornings are Vega, sinking in the west-northwest to northwest; and Capella, ascending in the northeast.
Altair and Deneb complete the Summer Triangle with Vega. Fomalhaut, mouth of the Southern Fish, is in the south to south-southwest, 20 degrees to the lower left of Jupiter. Aldebaran is ascending in the east-northeast to east, 31 degrees to the lower right of Capella. After mid-month, watch for the rising of Orion’s two bright stars: Betelgeuse, about 10 degrees north of east; and Rigel, about 10 degrees south of east. When the sky is dark enough, look for the three-star belt midway between them.
Farther north, almost northeast, watch for the rising of Castor and Pollux, 4.5 degrees apart, with Castor to the upper left of Pollux. The Twins aren’t out of view for long around the dates of their conjunctions with the s un, because they’re north of the Earth’s orbit plane and above the horizon longer than the sun.
Robert C. Victor was a staff astronomer at Abrams Planetarium, at Michigan State University. When the coast is clear, he looks forward to sky-watching sessions, in time for the fine display of three planets in the evening sky in autumn 2021. Robert D. Miller, who provided the twilight charts, 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.