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29 Jun 2018

July Astronomy: Brilliant Heavenly Bodies Light Up the Summer Sky—Including All Five Bright Planets

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It’s planetfest, with five bright planets in July’s evening hours! Three of them far outshine all stars. (But early in the month, you must wait until after nightfall to see Mars.)

Venus, the brightest, gleams at magnitude -4.1 to -4.3 in the west at dusk, and sets more than two hours after sunset. Jupiter glows at magnitude -2.3 to -2.1 in the south to south-southwest at dusk. Mars rises in the east-southeast to southeast two hours after sunset on July 1, one hour after sunset on the 16th, and at sunset on the 31st. Earth overtakes Mars on the night of July 26, passing within 35.8 million miles four nights later, on the night of July 30-31. This is our closest approach to Mars since 2003, and the nearest until 2035. As Earth approaches Mars in July, the red planet attains rare brilliance, kindling from magnitude -2.2 to -2.8, to outshine Jupiter until early September.

Mercury shines at zero magnitude first for the few days and lingers very low in the west-northwest twilight glow, to the lower right of Venus, within 16-17 degrees July 1-15, and widening to 20 degrees away by July 20, when it fades to magnitude +1. Saturn, of magnitude 0.0 to +0.2, ascends through the southeast toward the south-southeast at dusk, reaching its high point in the south about 4.5 hours after sunset on July 1, backing to 2.6 hours after sunset on the 31st.

The most prominent stars, also of magnitude zero, are golden Arcturus, high in the south-southwest to west-southwest, and blue-white Vega, high in east-northeast. Vega is a member of the Summer Triangle along with Altair and Deneb, up all night throughout July.

Morning: Mars is by far the brightest “star” in morning twilight, sinking low in the southwest as the month progresses. In the first week, note Saturn very low in the west-southwest, 34 degrees to Mars’ lower right. The two brightest real stars are Vega in west-northwest to northwest, and Capella, climbing in the northeast. Other stars visible in morning twilight include Altair and Deneb, completing the Summer Triangle with Vega; Aldebaran, eye of Taurus, in the east to east-northeast to the lower right of Capella; and Fomalhaut, in the south to south-southwest. Rising into view late in the month are Rigel and Betelgeuse, right and left of Orion’s belt low in the east; and Castor and Pollux of Gemini, well north of east.

July features four telescopic showpiece planets: Venus in July displays a gibbous phase, 70 to 57 percent full. The view will be more impressive in August through January, as Venus goes through half and crescent phases—larger than now in apparent size, because Venus will be closer to Earth. Jupiter shows cloud belts and up to four bright moons discovered by Galileo. Saturn shows rings still tipped a generous 26 degrees into our view, and satellite Titan in its 16-day orbit. Mars this month shows a prominent south polar cap of frozen carbon dioxide, shrinking with the advance of southern spring—catch it soon!—and surface features, such as Syrtis Major and Hellas Basin.

For events at dawn, begin viewing at least one hour before sunrise. For events at dusk, look one hour after sunset except when noted.

July 1: At dawn, in the south-southwest, find bright Mars within 7 degrees to the lower right of the waning gibbous moon. At dusk, find four planets: Mercury very low in the west-northwest, 17 degrees to the lower right of brilliant Venus; Jupiter well up in the south; and Saturn low in southeast. Tonight, Venus to Saturn span nearly 135 degrees. About two hours after sunset, watch for bright Mars rising in the southeast, and another 40 minutes later, watch for the waning gibbous moon rising 16 degrees to the lower left of Mars.

July 2-12: At dusk, Mercury, our solar system’s innermost planet, stays 16 degrees to the lower right of Venus. If you always look exactly one hour after sunset, Mercury will appear highest, nearly 6 degrees up, on July 4, and will reach greatest elongation, 26 degrees from the sun, a week later, on July 11. But Mercury is fading from magnitude 0.0 on July 1 to magnitude +1.0 on July 20, getting fainter each night as Mercury sinks into bright twilight.

July 5, 1 to 1 1/2 hours after sunset: The star Regulus, heart of Leo, is within 5 degrees of Venus for nine evenings through July 13, fitting within the same field of view of most binoculars. Watch nightly as Venus moves just more than one degree per day, and passes one degree north (to the upper right) of Regulus on July 9. For a few days, you can easily detect Venus’ change in position from one day to the next.

July 9, 1 to 1 1/2 hours before sunrise: Find the waning crescent moon (18 percent) just north of east, with the Pleiades star cluster 10 degrees to its upper left, and bright Aldebaran, eye of Taurus, within 13 degrees to the moon’s lower left. Look the next morning, July 10, at the same hour, to catch the moon within 2 degrees to the lower left of the star, and the Hyades star cluster nearby.

July 9, 2 to 1 1/2 hours after sunset: Venus and Regulus appear closest tonight, just 1 degree apart. Jupiter ends retrograde 2.1 degrees west of third-magnitude Alpha in Libra (Zubenelgenubi), and begins creeping back toward that star.

July 10: See four planets simultaneously in the evening sky. You have two choices; select a viewing site where mountains won’t block your view. 1. About one hour after sunset, look west to west-northwest for brilliant Venus 16 degrees up, with Mercury 16 degrees to its lower right and 5 degrees above the horizon. Jupiter is then 40 degrees up in the south to south-southwest, and Saturn is about 20 degrees up in the southeast. 2. Look nearly two hours after sunset, when Venus and Mars are 5 degrees above opposite horizons. Find Venus low in the west to west-northwest; Jupiter in the south-southwest; Saturn in the southeast to south-southeast; and Mars low in the east-southeast to southeast.

July 11: Your last chance to see the old moon in this cycle. One hour before sunrise, look very low in the east-northeast for the slender 4 percent crescent. The new moon occurs on July 12 at 7:48 p.m., with a partial solar eclipse visible in southeast Australia and the ocean to the south.

Nights of July 12-19: Observe Mars’ south polar cap soon, while it is still large, and before a possible planet-wide dust storm that could render it invisible! The Martian south polar cap, where spring began on May 22, in July is tipped 10-17 degrees toward the sun and a favorable 14 to 11 degrees into our view from Earth. The frozen-carbon-dioxide polar cap is now shrinking from its maximum extent. Using your telescope with as high of a magnification as atmospheric conditions allow, look for a large bright area near the southern edge of Mars’ reddish disk. On the nights of July 12-19, the prominent dark mark, Syrtis Major—first recorded with certainty in early telescopic observations by Christiaan Huygens in 1659—will appear near the center of the Martian disk on July 12 at 11:36 p.m., and about 36-37 minutes later each successive night, i.e., July 14 at 12:12 a.m., July 15 at 12:49 a.m., until July 20 at 3:51 a.m. Around these times, Mars is well-placed in our sky for telescopic observation. Catch views of Syrtis Major up to two hours away from those times, provided Mars isn’t too low.

July 13, in early dusk: About 25 minutes after sunset, look for the slender 2 percent crescent about 3 degrees up in the west-northwest, some 29 degrees to the lower right of Venus and 13 degrees to the lower right of Mercury. Tonight, Venus, Jupiter and Saturn span 120 degrees.

July 14, one hour after sunset: The crescent moon, 6 percent full, is very low in the west to west-northwest, with Venus 15 degrees to the upper left, and Mercury within 3 degrees below and a little to the right. Regulus is within 6 degrees to the lower right of Venus.

Sunday, July 15, at dusk: Don’t miss this spectacular sight! One hour after sunset, Venus is only about one degree to the left of the 13 percent crescent moon. Regulus and Mercury are about 7 and 17 degrees to the lower right of Venus. If you have an unobstructed horizon, keep Mercury in view as it sinks lower, toward the west-northwest horizon. If nothing blocks your view, then you may be able to spot Mars before Mercury sets some 16 degrees north of west. From places near our Coachella Valley, the best time to look is about 1.2 hours after sunset. If you spot Mars before Mercury disappears, that makes five planets in view simultaneously! If surrounding mountains prevent you from doing this, then note Mercury-Venus-Jupiter-Saturn in west to east order in the early evening, before Mercury sets; then wait awhile until Mars rises, and you’ll see Venus-Jupiter-Saturn-Mars spanning 150 degrees.

July 16 at dusk: Face west for the 22 percent crescent moon, and Venus within 13 degrees to its lower right. Mercury is 18 degrees to the lower right of Venus. Using binoculars, try for Regulus 8 degrees to the lower right of Venus and 10 degrees to the upper left of Mercury. If you’re trying for all five bright planets, the best time to look is about 1 hour, 9 minutes after sunset, when Mars will have just risen about 30 degrees south of east; Jupiter will be well up in the south-southwest, and Saturn will be 33 degrees to the upper right of Mars. If you spot Mars before Mercury sets, you’ve got it!

July 17: If you have an unobstructed horizon, you can spot six solar system bodies with the unaided eye about 1.1 hours after sunset. In order from west to east, they are Mercury, Venus, the moon, Jupiter, Saturn and Mars. Otherwise, look earlier to catch Mercury before it sets, or look a few minutes later to see Mars after it rises, and you’ll still see five bodies at once! Mercury is getting fainter nightly and lower in twilight.

July 18: In order from west-northwest to east-southeast, nine bright objects lie along the zodiac and above the horizon just more than an hour after sunset tonight: Mercury, Regulus, Venus, a fat crescent moon with Spica 9 degrees to its lower left, Jupiter, Antares; Saturn and Mars. With mountains surrounding us, we are not likely to see Mercury and Mars simultaneously. Look a few minutes earlier for Mercury, or later to spot Mars after it rises.

July 19: Early this evening, the moon has passed its first quarter phase, so it’s slightly more than half full (54 percent), and more than 90 degrees (94, to be exact) east of the sun.

July 20: One hour after sunset, bright Jupiter is just 3-4 degrees to the lower right of the waxing gibbous moon, nearly two-thirds full. Today is the 49th anniversary of the first manned lunar landing, Apollo 11.

July 21: One hour after sunset, the moon is nearly three-fourths full, just west of due south. Jupiter is about 14 degrees west of the moon. Antares, heart of the Scorpion, is 16 degrees to the moon’s lower left. Venus and Jupiter are 60 degrees apart tonight.

July 22 at dusk: Antares is 8 degrees below the moon.

July 23 at dusk: Reddish twinkling Antares is 14 degrees to the moon’s lower right. Steady, yellowish Saturn is 13 degrees to the moon’s lower left.

July 24 at dusk: Saturn is a little more than one degree below the moon’s lower edge. You’ll have no difficulty seeing it if you block the moon with your hand. Note bright apricot-colored Mars 32 degrees to the moon’s lower left.

July 25 at dusk: Saturn is 11 degrees to the moon’s upper right. Brilliant, tawny-colored Mars, gleaming at magnitude -2.8, is within 21 degrees, to the lower left of the moon.

Night of Thursday, July 26: Mars is at opposition tonight as Earth overtakes the red planet and we look almost directly away from the sun to see it. One hour after sunset, look low in the southeast to find Mars within 10 degrees to the lower left of the nearly full moon. Mars is now several degrees south of Earth’s orbit plane, so an hour before sunrise on Friday, July 27, they’re still 8 degrees apart, 8 degrees up in the west-southwest, with Mars to the moon’s left. The moon isn’t then yet at opposition; there are still several hours to go before a deep total lunar eclipse, when the moon at mid-event will be directly overhead of the Indian Ocean just east of Madagascar. It’s daytime then in California, July 27 at 1:22 p.m., with the moon well below our horizon, so none of the eclipse will be seen in North America. We’ll have our chance to see a total lunar eclipse on the evening of Jan. 20, 2019.

Night of Friday, July 27: The moon, several hours past full, rises 6 minutes after sunset. About 10 minutes later, watch for the rising of Mars about 7 degrees to the moon’s right. One hour after sunset this evening, Mars is just 7 degrees up in the southeast, and 7-8 degrees to the moon’s upper right. By an hour before sunrise on Saturday, July 28, Mars is 7 degrees up in the southwest, 9-10 degrees below the moon. Mars sets about 14 minutes before sunrise.

Night of Monday, July 30: Tonight, at 12:50 a.m. on Tuesday, Mars has its closest approach to Earth since August 2003, and the closest until September 2035. Mars is within 35.8 million miles of Earth, and it takes the light reflected off the surface of Mars only 3.2 minutes to reach us. For sky watchers in Palm Springs, Mars reaches its high point in the sky at 12:44 a.m., while 30 degrees above the southern horizon.

The website of the Astronomical Society of the Desert  has a listing of our evening star parties. Sawmill Trailhead (SMT), our high-altitude site (elevation 4,000 feet) will have a star party starting at dusk on Saturday, July 14.

Abrams Planetarium publishes a monthly Sky Calendar with an evening sky map. Subscriptions are $12 per year. Subscribe or view a sample issue at www.abramsplanetarium.org/skycalendar.

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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