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28 Jun 2019

July Astronomy: Saturn and Jupiter Are Your Hosts for Evening Sky-Viewing This Month

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In July, watch Saturn follow Jupiter into the evening sky. Witness a complete lunar cycle from beginning to end, as the best season for Milky Way evening viewing gets under way.

On July 9, the Earth overtakes Saturn, and the planet appears at opposition, 180 degrees from the sun, and visible all night: In the southeast at dusk, highest in the south in middle of night, and in the southwest at dawn. We overtook Jupiter 29 days earlier, on June 10. Each is a worthy showpiece for telescopic viewing: Jupiter, with dark cloud belts parallel to its equator, and four bright satellites discovered by Galileo in 1610; and Saturn, with rings now tipped a generous 24 degrees from edgewise. As we overtake them, both planets retrograde—go west against background stars, by just more than 2 degrees in July. So they stay 31 degrees apart all month, with Jupiter creeping closer to Antares, and Saturn backing toward the Teapot of Sagittarius. Saturn passes 1.1 degrees north of the third-magnitude star Pi in the “Teaspoon” of Sagittarius on July 18.

An entire lunation—new moon to new moon—fits into July with a day to spare. In bright dawn twilight on July 1, an old crescent moon, some 31 hours before new, appears in the east-northeast, 5 degrees to the upper right of Venus. We’re getting our last views of Venus before superior conjunction on Aug. 13. On July 1, Venus is 12 degrees from the sun, and rises 50 minutes before sunup; by the 31st, these numbers shrink to 4 degrees, and about 20 minutes.

After a total solar eclipse in the South Pacific, Chile and Argentina on July 2, we in California will see a young crescent moon very low in the west-northwest at dusk on July 3. The moon’s age will be 32-33 hours. You’ll probably need binoculars to see Mars (magnitude +1.8) and Mercury (magnitude +1.4) 3.8 degrees apart, within 2 degrees and 5 degrees of the upper left of the moon. These planets are in their last days of visibility, appearing lower nightly, with Mercury dropping faster than Mars, and fading to equal Mars by July 6. Mercury will pass inferior conjunction, nearly between Earth and the sun, on July 21. Mercury, in its crescent phases, is faint, because, unlike Venus, it is cloudless, and we see its surface, where features cast shadows, dimming Mercury’s total brightness.

Dominating the evening sky this summer and well into autumn are the two giant planets of our solar system. On July evenings, find steady, bright Jupiter in the southeast to south at dusk, with Saturn 31 degrees to its lower left, and twinkling red Antares, heart of Scorpius, 8.5 degrees to 6.8 degrees to Jupiter’s right/lower right. Also, look for the Summer Triangle of Vega, Altair and Deneb in the northeast to east, rising higher as the month progresses; golden Arcturus and blue-white Spica well up in the southwestern quadrant of the sky; and in the west, sinking into the west-northwest twilight glow, Regulus, heart of Leo.

The Milky Way in a dark sky, from the Cygnus Star Cloud along the Swan’s neck within the Summer Triangle, down through the Great Rift—clouds of dust obscuring the stars behind and dividing the Milky Way into two streams, with “clouds of steam” appearing to billow out of the spout of the Teapot of Sagittarius—is an inspiring sight all should experience! The best dates for Milky Way viewing in evening hours this summer are through July 3, after end of twilight; July 4-6, after evening moonset; July 21-Aug. 2, after twilight; Aug. 3-5, after evening moonset; Aug. 20-31, after end of twilight; and Sept. 1-2, after evening moonset.

Follow the moon on July evenings. A waxing crescent moon passes just 2-3 degrees to the upper right of Regulus on July 5. After passing first-quarter phase while below the horizon early on July 9, a slightly gibbous moon appears 7 degrees to the upper left of Spica that evening. On the evening of July 12, find Jupiter 9 degrees to the moon’s lower left, and Antares 8 degrees below a fat gibbous moon. On the next evening, July 13, the moon will appear 4-5 degrees to the left of Jupiter, while Saturn will appear within 27 degrees to the moon’s lower left. On July 14, the moon appears 17 degrees to the lower left of Jupiter and 14 degrees to the upper left of Saturn. On July 15, the nearly full moon will appear only 1-2 degrees to the right of Saturn. On July 16 at dusk, the moon, six hours past full, appears 11 degrees to the lower left of Saturn.

Observe in mornings to follow the moon for rest of July. In brightening dawn twilight on the 16th, the moon, not quite full, appears 2 degrees to the upper left of Saturn, low in the southwest to west-southwest. By July 25, the waning moon has passed last quarter phase, and appears slightly less than half full in the southeast. On July 27, the crescent moon will appear in the east, with the Pleiades cluster 9 degrees to the upper left, and bright Aldebaran, eye of Taurus, within 7 degrees to the moon’s lower left. Look early enough, before the sky brightens too much, to enjoy these fields through binoculars. Stars of the Hyades cluster can be seen between the moon and Aldebaran that morning. By late in July, Orion’s shoulder, red Betelgeuse, and his foot, blue Rigel, can be spotted low in the east, with his three-star belt between them. On July 28, the crescent moon appears 7-8 degrees to the lower left of Aldebaran. On the 29th, a beautiful, thinner crescent moon with earthshine will appear 14 degrees to the upper left of Betelgeuse. On July 30, look for the last, easy crescent low in east-northeast, with the Gemini Twins—Pollux 4.5 degrees below Castor—11-12 degrees to the left of the moon.

In your July predawn forays, look for the Summer Triangle well up in the west to west-northwest; the Mother Goat star Capella in the northeast; and Fomalhaut, mouth of the Southern Fish, in the south to south-southwest. Note the stars, and slow-moving Saturn, are in about the same positions they’ll occupy in the evening sky in December, several months hence.

Abrams Planetarium publishes a monthly Sky Calendar with an evening sky map. The calendar illustrates many of the events described here. Subscriptions are $12 per year at www.abramsplanetarium.org/skycalendar for three printed issues mailed quarterly.

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), will have its next monthly sessions starting at dusk on Saturdays, July 27 and Aug. 31.

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 e-newsletter 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.

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