Galileo, more than four centuries ago, observed and described phenomena you can witness in the evening sky this summer, including the phases of Venus, the moons of Jupiter, the Milky Way, details on the surface of the moon, and more. The Discoveries and Opinions of Galileo, a selection of the scientist’s writings translated by Stillman Drake, is fascinating reading.
Venus and Jupiter are still easy to spot during evenings for most of July 2015, because they far outshine all nighttime stars. Venussinks lower in the evening sky in July while getting ever more interesting for telescopic observation: As Venus draws closer to Earth, it displays an ever thinner, more backlit crescent.
Jupiter lingers close to Venus during the final weeks of their joint appearance in evening sky. First, Jupiter appears to right of Venus, by 0.6 degrees on July 1, and 1 degree on July 2. On July 4, they’re still within 2 degrees, and on July 9, within 4 degrees, with Jupiter to the lower right of Venus. On July 13, Venus and Jupiter appear just 5 degrees apart. Even at the end of July, Venus and Jupiter are still within 6.5 degrees, but Venus will then set in bright twilight, very soon after sunset. The crescent phase of Venus can be resolved with just a pair of binoculars, by avoiding the planet’s glare against a darkened sky. The best occasions in 2015 are in the late afternoon or around sunset from mid-June until late July/early August, and around sunrise or soon afterward from late August until mid-October.
Saturn appeared at opposition to the sun and was up all night on May 22, as Earth passed between that planet and the sun. Now in the months following, Saturn remains visible in the evening sky, reaching its high point in the south about half an hour earlier each week, crossing due south at dusk in July. In this year’s observing season, steady Saturn appears not far from the reddish twinkling first-magnitude star Antares, heart of the Scorpion, and outshines that star by as much as a magnitude. Through a telescope, we get fine views of Saturn’s rings, now tipped 24 degrees from edge-on.
The moon can be followed one hour after sunset daily as it waxes from a thin crescent, through first-quarter phase (half full and 90 degrees east of the sun), to full, July 18 through Aug. 1.
In evening sky on July 14, Venus approaches to within 2.4 degrees below Regulus. That evening, Jupiter is 5.2 degrees from Venus, narrowly missing forming a trio (three objects within a 5-degree field). Do all three fit within the field of view of your binoculars? On July 17, 30 minutes after sunset, the young moon is very low, north of west, 11 degrees to the lower right of Venus and 8 degrees to the lower right of Jupiter. Venus and Jupiter are 5.8 degrees apart. On July 18, before sunset from California, during the 6 p.m. hour, Venus is 3/4 of a degree from the moon’s northern cusp (the upper right point of the crescent), and at sunset is still within 0.9 degrees of the moon’s northern cusp. Can you see Venus in the daytime? On July 19, note the triangle Venus-Jupiter-Regulus, 13 degrees to 18 degrees to the lower right of the moon. On July 22 and 23, look for Spica near the moon. The moon reaches first-quarter phase, half full and 90 degrees from the sun, on July 23. On July 25, the moon is 3 degrees to the upper right of Saturn, while Antares is 13 degrees to Saturn’s lower left. On July 26, Saturn is 10 degrees to the moon’s right, and Antares is 9 degrees below the moon.
In the morning sky in July, Mercury can still be spotted very low in the east-northeast an hour before sunup in first half of month. Look to the lower left of Aldebaran, by 9 degrees on July 1, increasing to 22 degrees by July 9. On July 11, two hours before sunup, find the waning crescent moon in the east, 11 degrees right of the Pleiades star cluster, itself a pretty sight for binoculars. On July 12, 75 minutes to 2 hours before sunrise, find the lunar crescent in the east to east-northeast, with Aldebaran, eye of Taurus, 3 1/2 degrees to 3 degrees to the lower left. Binoculars show, in the same field, the fainter more distant stars of the V-shaped Hyades star cluster, with the moon just within the southern point of the sideways “V”. On the next morning, July 13, 75 minutes before sunup, a thinner crescent moon appears 10 degrees to the lower left of Aldebaran. On July 14, catch a last view of the thin old crescent moon about 3 degrees up in the east-northeast, 45 minutes before sunrise. Using binoculars, try for Mercury rising 9 degrees to the moon’s lower left. Also on its way out, Mercury will pass superior conjunction, invisibly on the far side of the sun, on July 23.
Replacing Mercury by the third week of July will be Orion’s two brightest stars: Betelgeuse rising north of east, and Rigel rising south of east. Using binoculars, you spot them as early as July 14; they’ll get easier to see later in the month, as they rise nearly four minutes earlier each morning. Before month’s end, these two bright stars will be easy to see in the east to east-southeast an hour before sunrise, with Orion’s belt appearing as a vertical line of three stars midway between. On July 29 and 30, look much farther north and lower, in the northeast to east-northeast, for the Gemini twins: Castor and Pollux 4 1/2 degrees to its lower right. On those two mornings, fainter Mars (magnitude +1.7) passes 5 3/4 degrees south (lower right) of Pollux.
See them all on July 18, at the Astronomical Society of the Desert “star party” at Sawmill Trailhead in the Santa Rosa Mountains. Visit www.astrorx.org for more information.
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs.
Robert D. Miller, who provided the twilight charts and the planet orbit charts, 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.