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30 Jun 2017

July Astronomy: This Month and Next Are the Best Time of Year to Check Out Jupiter's Moons and Saturn's Rings

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July finds bright Jupiter in the south-southwestern sky at dusk, and Saturn in the southeast, closing to 65 degrees apart by month’s end. Before dawn, Venus gleams in the east, against the attractive background of Taurus the Bull, with its two prominent star clusters and bright reddish star.

On some dates, the moon forms attractive pairings with four of the five naked-eye planets, and four of the five bright zodiacal stars.

In July’s evening twilight, bright Jupiter attracts attention in the southwest, with bluish Spica nearby to its left, and golden Arcturus high above them. Yellowish Saturn glows steadily in the southeast to south-southeast, in contrast to the vigorously twinkling reddish star Antares, heart of the Scorpion, to its right. The star reaches south in mid-twilight near month’s end. The Summer Triangle, topped by its brightest star, blue-white Vega, ascends in the eastern sky. The Triangle’s other members are Altair, its southern point, and Deneb, trailing Vega.

Mercury has a several-weeks-long but very low apparition in the west-northwest to west. It loses nearly three-quarters of its brightness during July, fading from magnitude -1.0 to +0.4, and passes close by fainter, departing Regulus (+1.4) on July 25.

In July’s morning twilight, Venus continues climbing higher until late in the month, despite its having reached its greatest angular distance from the sun, 46 degrees, in early June. That’s because in July, the sun-to-Venus line inclines at an ever-steeper angle to our morning twilight horizon. Before dawn brightens much, watch Venus, shifting a little more than one degree daily against background stars, pass 6 degrees south of the Pleiades star cluster on July 5, and within 3.5 degrees north of Aldebaran on July 14. If you observe each morning at the same time interval, Venus will appear almost stationary during July, while the starry background rises toward the upper right, behind Venus. Note Capella far to the north (left or upper left) of Venus. Before month’s end, Orion’s brightest stars, Betelgeuse and Rigel, will appear in the east (with his striking three-star belt, not plotted, midway between the two), and Gemini’s Pollux, with Castor 4.5 degrees above it, will emerge in the east-northeast. Before mid-August, the Dog stars, Procyon and Sirius, will follow Orion into the eastern sky, completing the Winter Triangle with Orion’s shoulder, Betelgeuse.

On July mornings, the Summer Triangle of Vega, Altair and Deneb is still well up in west, and Fomalhaut crosses south toward southwest.

The moon and planets in July:On the evening of July 1, the gibbous moon appears several degrees north of Spica, with bright Jupiter just to their right; on July 5, the moon passes even more widely north of Antares. On the evening of July 6, the moon passes a few degrees north of Saturn. The moon is full two nights later, on Saturday, July 8. By that date, Mercury can be glimpsed very low in the west-northwestern evening twilight. Look to the lower right of Regulus,by 30 degrees on July 5, 15 degrees on July 14, 10 degrees on July 17, and shrinking to 5 degrees by July 21. We’ll return for another look at Mercury a few days later, after the moon has returned to the western early evening sky.

On July 8, we can catch the full moon rising in the east-southeast shortly before sunset; on the next evening, moonrise occurs within half an hour after sunset. Thereafter, the waning gibbous moon rises a little later and farther north each night. By July 14, the moon rises just before midnight, nearly due east. Instead of staying up late to wait for moonrise, get outdoors before sunrise, and follow the waning moon from July 9 through July 21 or 22. Venus, against a pretty background of the stars of Taurus, provides another reason to arise early, before the sky brightens much. Watch Venus go 6 degrees south of the Pleiades star cluster on July 5; just 3.5 degrees north of reddish-orange Aldebaran on July 14; a wide 7 degrees south of Elnath (Beta Tauri, tip of the Bull’s northern horn) on July 25; and very close to third-magnitude Zeta Tauri, the southern horn, on July 27. (Use binoculars to see the faint star near brilliant Venus on the 27th.) The waning crescent moon adds special beauty to the scene on three mornings: On July 19, find the moon to the upper right of Venus, Aldebaran and the Hyades star cluster. On July 20, the crescent moon will be just a few degrees to the south (lower right) of Venus. An hour before sunrise on July 21, the moon will be low in the east-northeast, well to the lower left of Venus, while Orion’s shoulder, Betelgeuse, will be rising widely to the lower right of the moon.

The moon rises at about the same time as Venus on July 20. Two days later, on July 22, binoculars may be required to spot the very old crescent.Look for the hairline 1 percent moon only 2 degrees up, one-half hour before sunrise, just 21-22 hours before new.

That new moon occurs on July 23, at 2:46 a.m.—one lunar month before the Aug. 21 solar eclipse! (See the May column at for a list of resources regarding that very special event.) The first view of this month’s young crescent moon is expected on the evening of July 24, within a half-hour after sunset. The 4 percent illuminated moon will then be 8 degrees up, 10 degrees north of west, at an age of 41-42 hours after new. As the sky darkens a bit, but before the moon gets too low, look a few degrees to the upper left of the moon for bright (magnitude +0.1) Mercury, with the fainter star Regulus (+1.4) just to the planet’s upper left. By the next evening, July 25, the crescent moon will have skipped to the upper left of the Mercury-Regulus pair, then at their closest, within a degree. Brighter Mercury will appear to the south (lower left) of Regulus. The evening of July 26 finds the moon far to the upper left of the now-widening pair.

On the evening of July 28, bright Jupiter is closely to the lower right of the fat crescent moon; note Spica several degrees to the left of Jupiter. On the next evening, July 29, the fat crescent moon, half a day short of first quarter, appears to the upper left of Jupiter and Spica, while Mercury reaches greatest elongation, very low, just north of west, and 27 degrees from the sun (which is below the horizon while you can see Mercury).

Check the website of the Astronomical Society of the Desert at for a listing of and directions to our star parties at Sawmill Trailhead, our high-altitude site (elevation 4,000 feet—wear warm clothes), starting at dusk on Saturday, July 22, Aug. 19, Sept. 23 and Oct. 14. Also, check out the Sky’s the Limit Observatory and Nature Center in Twentynine Palms, at

Robert C. Victor, formerly a staff astronomer at Abrams Planetarium at Michigan State University, is now retired and enjoys providing sky watching opportunities for school children in and around Palm Springs. Robert D. Miller did graduate work in planetarium science, astronomy and computer science at Michigan State University, and remains active in research and public outreach in astronomy.

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