Soon-to-depart Venus reaches peak brilliance in July. With an optical aid, it presents an ever-thinner crescent. Binoculars and a vantage point with unobstructed views toward the west will become increasingly essential to follow Venus, two other planets and a star—down into the evening twilight glow.
Jupiter and Saturn, 62 to 66° apart and well up in the predawn sky, suffer no barrier to observation—except, for some, the early hour. The Summer Milky Way, from a dark site on a clear moonless night, is recommended to all.
In July, Venus takes the plunge: On July 1 at sunset, as seen from Northern Nevada, Venus appears 25° up in the west, and 41° to the upper left of the setting sun. That evening, Venus sets 2.3 hours after the sun. By Aug. 1, Venus will be only 3° up at sunset, and 18° to the upper left of the sun. Venus will then follow the sun below the horizon by only 20 minutes. Venus’ distance narrows, from within 46 million miles of Earth on July 1, to within 29 million miles on Aug. 1. The apparent size of Venus’ disk grows from 35” (arcseconds) to 55”, while the fraction illuminated wanes from 31 percent on July 1, to a slim 5 percent on Aug. 1. The apparent size is large enough for the crescent to be resolved with ordinary binoculars of 7 or 8 power. To succeed, find Venus very soon after sunset. Having the planet near greatest brilliancy at magnitude -4.7 in first half of July makes sightings in the daytime and bright twilight easier. When searching for Venus in daytime, take care to avoid pointing your optics at the sun!
Binoculars are also useful to follow the pairs and trios formed by planets and stars low in the western twilight glow, as plotted on our evening twilight sky map. On June 30, Venus approached within 3.6° to the west (lower right) of Mars in a quasi-conjunction, and now begins to fall back. On July 4, look for a patriotic gathering of red Mars. 3.8° to the upper left of white Venus, and 3.4° to the lower right of blue Regulus, heart of Leo, the Lion. All three fit within a field 7 degrees wide. Compared to Venus, Mars and Regulus are faint, magnitude +1.8 and +1.4.
Venus and Regulus appear within 5° from July 9-25, and reach a minimum separation of 3.5° on July 15 and 16, as Venus passes below Regulus. This is another quasi-conjunction, since Venus approaches within 5° of Regulus without overtaking it. Mars fits within the same 5° field on July 9 and 10, forming a trio, at its most compact on July 9, 4.7° across (from Venus to Regulus). On July 10, Mars appears 0.7° (minimum distance) above Regulus and 4.9° to the upper left of Venus.
Mercury enters the mix by mid-month, but you’ll need binoculars and an unobstructed view, as it remains very low in twilight. During the week of July 16-22, look to the lower right of Venus, by 17° on July 16, and 10° on July 21. As Venus sinks lower into bright twilight, and Mercury is slightly higher daily, you’ll be looking to the upper right of Venus the following week, by 6° on July 24, and 5° (least separation) on July 26. If you can still find zero-magnitude Mercury on July 28, look for fainter Regulus just one-quarter of a degree to its right.
The waxing crescent moon overtakes Mercury, Venus, Regulus and Mars from July 18-21—all twilight gatherings accessible with binoculars! Follow the moon past Spica on July 24 and Antares on July 27-28, until it’s full on Aug. 1.
Saturn (magnitude +0.8 to +0.6), approaching its opposition in late August, rises south of east in the late evening in July, from 3 hours after sunset on the 1st, to 1.25 hours after on the 31st. Bright Jupiter (magnitude -2.2 to -2.4) rises in the east-northeast around 2 a.m. at the start of July, and about a quarter hour after midnight at month’s end.
Predawn skies: Follow the waning moon past Saturn in the south-southeast to south on July 7; Jupiter in the east on July 11-12; and the Pleiades and Hyades star clusters and Aldebaran, in Taurus, on July 13-14. The “V” of Aldebaran and the Hyades points through 2.5-magnitude Alpha and 4.1-magnitude Delta in Cetus, 7° apart, and another 6° in a straight line beyond Delta to the long-period variable star Mira, expected to reach peak brightness near third magnitude this month. An hour before sunrise by late in July, Orion has risen in the east, while the Summer Triangle (Vega, Deneb and Altair) is high in the west.
Stars and the Milky Way: Now that the aforementioned Summer Triangle is in the eastern sky at nightfall, and Scorpius is in the south, we know that the season for evening viewing of the Milky Way has arrived. On dark, moonless nights, look for the Teapot asterism formed by eight bright stars of Sagittarius, the Archer. It follows Antares and the Scorpion across the southern sky, while the Summer Triangle climbs high in the east. The Milky Way looks like a cloud of steam rising out of the spout of the Teapot, and passing through the Summer Triangle, along the neck of Cygnus, the Swan. As we face the Great Sagittarius Star Cloud above the Teapot spout, we’re looking toward the inner regions of our Milky Way galaxy. As we examine the Cygnus Star Cloud, we’re facing ahead of the sun in its revolution around the galactic center, into our own nearby spiral arm, which binoculars readily resolve into countless stars!
The Milky Way is in an ideal position for observation in a dark, moonless sky, with the center of our galaxy highest and due south, in Reno on Sunday, July 9 at 11:35 p.m., getting four minutes earlier each night, until Friday, July 21, at 10:47 p.m.
There are other good viewing times! On earlier dates or earlier in the night, the Milky Way won’t be quite as high in the sky. Later, southern parts of the Milky Way descend closer to the horizon. Avoid bright moonlight or areas with bright artificial lights. On the full moon night of July 2-3, and again on the evening of July 29 (when 90% full), the bright moon is at its southernmost place, in Sagittarius, and creates poor sky conditions for viewing. If your sky is degraded by artificial light, visit the informative website of the International Dark-Sky Association (www.darksky.org) and be encouraged by practical solutions for the protection of dark skies, wildlife, and human health.
The July 2023 Sky Calendar illustrates many of the events described in this column. It, together with a constellation map for the month’s evening sky, is available by subscription at www.abramsplanetarium.org/skycalendar.
Robert C. Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968 and still produces issues occasionally, including May and July 2023. He enjoys being outdoors sharing the beauty of the night sky and other wonders of nature. Robert D. Miller, who provided the evening and morning twilight 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.