Mercury will spend all of July in the eastern pre-dawn sky, but it won’t be easily observable during the final week. Mercury’s position south of the dawn ecliptic will prevent the planet from rising very long before the Sun, making this apparition a poor one for Northern Hemisphere observers, but a good one for those viewing the planet from south of the Equator. Mercury will reach greatest elongation and peak visibility on July 4, when it will stretch to 21.6 degrees west of the Sun. At mid-northern latitudes, the best time to see Mercury will then be at about 4:45 a.m. local time. Mercury will brighten from magnitude 0.8 to -2.1 during July. Viewed in a telescope, the speedy planet will wax in illuminated phase from 28 per cent on July 1 to fully-illuminated at month’s end. Meanwhile, the planet’s apparent disk diameter will decrease from 8.7 to 5 arc-seconds. The old crescent Moon will shine a few finger widths to the northeast of Mercury on July 8.
Extremely bright Venus will increase its angle east of the Sun from 25.5 degrees to 33 degrees during July. The very shallow evening ecliptic will prevent the magnitude -3.87 planet from climbing very high over the western horizon after sunset, so you’ll need an unobstructed view toward the west to see it. During the month, Venus’ easterly orbital motion will counteract the westerly shift of the sky due to our orbit around the Sun, so the planet will set at about 10 p.m. local time all month long. The earlier sunsets in late July will surround Venus with a darker sky. Viewed through a telescope in July, Venus will exhibit an 85-per-cent-illuminated waning gibbous phase and a slowly increasing apparent disk diameter of about 12 arc-seconds. (As always, ensure that the Sun has completely disappeared below the horizon before using binoculars or telescopes to view Venus.) On July 11, Venus will depart Cancer for Leo. Meanwhile, faster-moving Venus will catch up to and pass slower-moving Mars in a close conjunction. Venus will outshine Mars by nearly 200 times! The two planets will share the view in binoculars from about July 4 to 21, but they’ll only be telescope-close from July 11 to 14. The young crescent Moon will hop past the close-together planets on July 11-12, offering a nice photo opportunity.
After spending many months parked halfway up the western evening sky, magnitude 1.8 Mars will spend July descending into the western post-sunset twilight while it reduces its angular separation east of the sun from 32 degrees to 22 degrees. On July 1, Mars will set at about 10:30 p.m. local time; and at 9:15 p.m. on July 31. Telescope views of Mars during July will show a shrinking, only 3.8 arc-seconds-wide disk. Mars’ slow easterly prograde motion will carry it from Cancer into Leo on July 10, just days before faster-moving and 200 times brighter Venus will overtake and pass just 0.5 degrees to the north of Mars, on July 12-13. On the same evenings, the young crescent Moon will hop past the duo. Venus will appear with Mars in binoculars from about July 4 to 21, and the pair will be telescope-close from July 11 to 14.
Throughout July, bright, white Jupiter will be travelling retrograde westward across the faint stars of western Aquarius – with yellowish Saturn shining 19 degrees to its right (or celestial east). As the month opens, Jupiter will rise shortly after 11 p.m. local time. By July 31, the planet will be observable from dusk to dawn, and it will have brightened slightly from magnitude -2.65 to -2.83. Unfortunately, the low summertime ecliptic will prevent Jupiter from climbing more than one-third of the way up the southern sky. Telescope views of Jupiter during July will show its large, banded disk increasing in apparent diameter from 45.3 to 48.4 arc-seconds. The Great Red Spot will be visible crossing Jupiter every second or third night. Single transits across Jupiter by the round, black shadows of its Galilean moons will be commonplace. A double shadow transit event will be visible across Eurasia on July 29. The waning gibbous Moon will shine less than 5 degrees below (to the celestial south of) Jupiter from dusk to dawn on July 25.
Saturn will be ideally positioned for viewing in evening during July as it approaches opposition in early August. On July 1, the ringed planet will rise shortly after 10 p.m. local time. That will advance to 8:45 p.m. at month-end. All month long, Saturn will be travelling retrograde west across the surrounding stars of central Capricornus. But while Saturn’s magnitude 0.4 will outshine all of the stars of that constellation, it can’t compete with 16 times brighter Jupiter shining 19 degrees to its left (or celestial east). The low summertime ecliptic will keep Saturn from ever climbing more than one-third of the way up the southern sky. When viewed through a backyard telescope, Saturn will exhibit its majestic rings, a number of its moons, and a mean apparent disk size of 18.5 arc-seconds. The waning gibbous Moon will hop past Saturn on July 23-24.
During July, blue-green Uranus will be available for observing in the eastern pre-dawn sky, especially near the end of the month, when it will rise around midnight and reach more than 37 degrees altitude before the dawn twilight arrives. The magnitude 5.8 planet will be slowly travelling eastward across the stars of southern Aries, roughly between two stars of comparable brightness, Omicron and Rho Arietis. The waning crescent Moon will pass several finger widths below (or 4.5 degrees to the celestial south) of Uranus on July 4-5.
During July the distant and slow-moving planet Neptune will be travelling retrograde westward through northeastern Aquarius – approximately a palm’s width to the left (or 6 degrees to the east) of the 4th-magnitude stars Psi, Xi, and Phi Aquarii. The blue, magnitude 7.9 planet will be rising just after midnight on July 1 and by 10:30 p.m. on July 31.
Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.