CSTA Classroom Science

California Skies, June/July 2021

By Robert C. Victor and Robert D. Miller

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° across on July 21. Three hours after sunset on July 1 (backing to one hour at month’s end), bright Jupiter rises in ESE, and chases Saturn across the sky for the rest of the night, ending in SW at dawn. Late in July, it becomes possible to see the two brightest planets, Venus and Jupiter, simultaneously, above opposite horizons.

In July’s evening twilight [S202107P]

In order of brightness, the actors in July’s evening “celestial traffic jam” are brilliant Venus, of mag. –3.9; Regulus, at mag. +1.4, the faintest of all the first-magnitude stars; and Mars at mag. +1.8, as faint as it gets. Binoculars are recommended for viewing the two fainter members of the gatherings low in bright twilight.

As July opens, Venus, very low in WNW, is the lowest of the three bodies, with Mars within 7° to its upper left, and Regulus 17° to the upper left of Mars. The three objects span 24° on July 1.

Venus moves closer to Mars each evening until July 12. In the first “narrow miss,” they’ll appear within 1.0° apart on July 11; 0.5° to 0.6° apart on July 12 and 13; and about 1.0° 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° lower right of Venus on July 10, will be a challenge, but on the next two nights a beautiful waxing crescent Moon will hopscotch past the Venus-Mars pair. 

On July 11, the 5-percent crescent Moon will appear within 5° to the lower right of Venus, and on July 12, the 10-percent Moon will appear 8° upper left of the Venus-Mars pair, and 5° 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 SW 6° to the upper right of Spica on July 16. Next, a waxing gibbous Moon will appear 7° to the upper right of Antares in SSE to S at dusk on July 19, and 9° 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° north (upper right) of Regulus. That evening, Mars will be 5.0° 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 SE, 9° upper right of Saturn. On the next night, July 24, wait until about 1½ hours after sunset to see the Moon 8° lower left of Saturn, with Jupiter, just risen, 14° to Moon’s lower left. On July 25, look about two hours after sunset to catch the 94-percent waning Moon, just risen, within 5° lower right of Jupiter and 21° lower left of Saturn.

The final “narrow miss”, the faintest, lowest, and most difficult to observe, will be the conjunction of Mars with Regulus on July 29. Using binoculars, look within 10° lower right of Venus. Faint Mars will appear just over 0.6° to the upper right of Regulus.  

Other stars not yet mentioned so far in this account of July’s evening sky: Castor and Pollux, very low in WNW to NW twilight glow at the start of July, require binoculars and very clear skies. They’ll be in solar conjunction, on the far side of the Sun, July 12-15. Arcturus is very high in southwestern sky at dusk. The Summer Triangle of Vega, Altair, and Deneb, climbing higher in 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 evening: Saturn rises in ESE within two hours after sunset on July 1, and rises at sunset on the 31st. Saturn will be at opposition to the Sun on August 1, as Earth passes between the Sun and Saturn. Jupiter rises in ESE about an hour after Saturn, and will reach opposition on August 19.

On July 6, Venus and Saturn are at mutual opposition, 180° apart in celestial longitude, and appear less than 1° above opposite horizons, about 1.6 hours after sunset, a few minutes before the end of evening twilight. On July 18, about 1.2 hours after sunset, each will be 5° up. On what evening will you first see Venus and Saturn simultaneously?

On the 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° above opposite horizons 1.4 hours after sunset, and by August 4, each will be 5° up about 1.2 hours after sunset. On what evening will you first see Venus and Jupiter simultaneously?

In July’s morning twilight  [S202107A]

Planets: Jupiter, at mag. –2.7 to –2.8, is the brightest morning “star”. Find it in south to southwest at morning mid-twilight, with Saturn, of mag. +0.4 to +0.2, between 19° and 20° to Jupiter’s lower right. The giant planets reach a maximum distance apart, 19¾° 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° apart.

You can detect the slow retrograde motion of each planet by carefully noting its position in relation to a background star. During July 2021, Jupiter goes 2.4° west, decreasing its distance from 4.3-mag. Iota in Aquarius from 3.2° to 1.1°. Saturn goes 2.1° west, directly away from 4.1-mag. Theta in Capricornus, and increases its distance WSW of Theta, from 1.8° to 3.9°. In a dark predawn sky on July 13, you can catch each planet equally distant, within 2.6°, from its respective background companion star.

In coming years, the usual trend will be for the two giant planets to move farther apart, as both planets move eastward, Jupiter moving farther ahead of Saturn. As seen from Earth, they’ll appear in mutual opposition, 180° apart in celestial longitude, five times in 2029-31, and have their next conjunction, a single event in Virgo, on October 31, 2040.

The other naked-eye morning planet is Mercury, visible very low in ENE for about the first three weeks of July. On July 1, the innermost planet, of magnitude +0.8, is 9° lower left of similarly bright Aldebaran, eye of Taurus, the Bull. Brightening, Mercury reaches its greatest elongation, nearly 22° west of Sun, on July 4. On July 6, a 12-percent waning crescent Moon appears 6° above Aldebaran and 7° below the Pleiades. On the morning of July 8, Mercury, brightened to zero magnitude, appears within 5° upper right of a 3-percent last old crescent Moon. Mercury reaches its highest elevation, nearly 7° above the morning mid-twilight horizon, on July 8-10. Mercury brightens to mag. –1.0 by July 18, and to mag. –1.5 by July 24, but by the latter date it is barely above the horizon at mid-twilight. Mercury passes superior conjunction August 1. On what morning before then will you last see it?

Also on July 24, the Moon returns to the morning sky. Low in SW just past Full, it is 6° lower left of Saturn. On the next morning, July 25, the Moon is within 11° upper left of Saturn and 11° lower right of Jupiter. On the morning of the 26th, the 93-percent Moon appears 6° 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 abramsplanetarium.org.

The brightest stars on July mornings are Vega sinking in WNW to NW, and Capella ascending in NE.

Altair and Deneb complete the Summer Triangle with Vega. Fomalhaut, mouth of the Southern Fish, is in S to SSW, 20° lower left of Jupiter. Aldebaran is ascending in ENE to E, 31° lower right of Capella. After mid-month, watch for the rising of Orion’s two bright stars, Betelgeuse about 10° north of east, and Rigel about 10° 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° apart, with Castor upper left of Pollux. The Twins aren’t out of view for long around the dates of their conjunctions with the Sun, because they’re north of the Earth’s orbit plane and above the horizon longer than the Sun. Orion, south of the ecliptic (Earth’s orbit plane), has been absent from our skies much longer than the Twins, ever since he departed from our early evening sky, in May.


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Written by Robert C. Victor


Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs, CA. Robert is a member of CASE.

Author

Written By Robert D. Miller


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.

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