Evenings this month feature the gas-giant planets, Jupiter and Saturn, and on dark moonless nights, the Milky Way. Predawn skies include the bright stars we’ll meet again on winter evenings—and a brief visit by our solar system’s smallest planet, Mercury.
In the evening, bright Jupiter gleams in the south to south-southwest at dusk, while Saturn is in the southeast to south-southeast, 31 to 29 degrees east (to the left) of Jupiter. Note Antares, heart of Scorpius, twinkling to Jupiter’s lower right. Jupiter lingers 7 degrees from this red supergiant star from mid-July through first week of September; their least separation of 6.7 degrees occurs Aug. 8-15 as Jupiter ends retrograde on Aug. 11.
Follow the moon at dusk Aug. 2-15. On Aug. 2, the thin crescent moon is easy to see, though very low, a little north of due west. Don’t miss a striking pairing of the moon and Jupiter, within 3 degrees, at dusk on Aug. 9, and the moon and Saturn, just 3 degrees apart on Aug. 11. By the evening of Aug. 12, the moon has moved 9 degrees east of Saturn. On Aug. 15, the moon, past full, is rising in the east-southeast. The moon returns to the evening sky as a crescent low in the west on Aug. 31.
The year’s best Milky Way viewing after evening twilight occurs through Aug. 2, and Aug. 20-Sept. 1. Go to a dark place to enjoy these with the unaided eye and binoculars: the Cygnus Star Cloud, along axis of the Northern Cross or neck of the Swan, within the Summer Triangle; the Great Sagittarius Star Cloud, above the spout of the Teapot; the Great Rift, a dark lane of dust dividing the Milky Way into two streams southward from Cygnus; Lagoon Nebula M8 (Messier 8), above the tip of the spout; star clusters M7 and M6, near the end of the tail of Scorpius; and much more.
As for the mornings: Mercury makes a brief visit to the a.m. sky. It brightens past first magnitude (to +0.9) by Aug. 5 to become visible low in the east-northeast morning twilight. During Aug. 5-10, it lingers 9 degrees to the lower right of Pollux while brightening another magnitude to -0.1. Mercury reaches greatest elongation, 19 degrees west of the sun, on Aug. 10, and on Aug. 11, a line from Castor to Pollux, 4.5 degrees apart, extended nearly 10 degrees, locates Mercury. Late in the month, Mercury sinks into bright twilight; superior conjunction beyond the sun will occur Sept. 3. The “Dog Star” Sirius, the brightest star, emerges in the east-southeast morning twilight before mid-August. Sirius rises 4 minutes earlier each morning, climbing higher as the month progresses. Note Orion’s belt points almost directly to Sirius.
Follow the moon in morning sky Aug. 15-29: On Aug. 15, the moon is full, setting in the west-southwest. The moon passes last quarter phase on Aug. 23, while 9 degrees south of Pleiades. On Aug. 24, the fat crescent moon is 2 degrees north of Aldebaran, eye of the Bull. On Sunday, Aug. 25, the 30 percent crescent moon occults third-magnitude Zeta Tauri, tip of southern horn of Taurus. From the Coachella Valley, the star disappears behind the bright edge of the moon at 3:46 a.m., and reappears from behind the dark, earthlit side at 4:41 a.m. It’s best viewed with a telescope.
Absent in August are Venus, in superior conjunction on night of Aug. 13, and Mars, in conjunction on Sept. 2. When passing beyond the sun, Venus and Mars remain out of sight for longer than other planets, because their motions most closely match that of Earth; Venus is the next planet inward from Earth, and Mars is the next planet beyond Earth’s orbit. Venus goes faster than Earth and so emerges from beyond the sun into the evening sky. Mars goes more slowly and emerges into the morning sky. During the 2019-20 school year, each participates in a spectacular gathering: Venus, with Jupiter and Saturn at dusk in late November-early December; and Mars, with Jupiter and Saturn at dawn in late March 2020. Stay tuned!
Abrams Planetarium publishes a monthly Sky Calendar with an evening sky map. The calendar illustrates many of the present and future events described here. Subscriptions are $12 per year at www.abramsplanetarium.org/skycalendar for three printed issues, mailed quarterly.
For a preview of evening and morning planet viewing through August 2020, see Jeffrey Hunt’s two-page graphic summary of moon and planet setting and rising times, relative to the times of sunset and sunrise, exact for Palm Springs, below. The graph of evening planet setting times shows the departure of Jupiter and Saturn in December 2019, and Venus improving until its very favorable evening appearance in March 2020, when it will set 3 1/2 hours after sunset, followed by its rapid departure from the evening sky in May. The graph of morning-planet-rising times shows the gradual emergence of Mars before dawn in autumn 2019; the emergence of Jupiter and Saturn in January-February 2020; and the sudden rise of Venus into predawn prominence in June-July 2020. The charts also make it easy to pick out the best dates to see Mercury in evening and morning skies.
Check the website of the Astronomical Society of the Desert at www.astrorx.org for dates and times of our evening star parties. Sawmill Trailhead, our high-altitude site (elevation 4,000 feet—wear warm clothes), will have its next monthly sessions starting at dusk on Saturdays, Aug. 31 and Sept. 28. Our primary, more-accessible location is at the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument (on Highway 74, within four miles south of Highway 111 in Palm Desert). Monthly sessions there will resume on Saturday, Oct. 5.
But don’t wait until then! You can pre-register for one of the Stargazing Parties offered several times each month at the Rancho Mirage Library Observatory at www.ranchomiragelibrary.org/observatory.html. (Click on Stargazing Parties, then sign up for their eNewsletter to receive registration information.) If you go farther afield, check out the Sky’s the Limit Observatory and Nature Center in Twentynine Palms, at www.skysthelimit29.org.
Wishing you clear skies!
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing sky watching opportunities for a variety of groups in the Coachella Valley. 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.