Planet Watch: July-September 2023

Solar system showing relative size of (but not distance between) planets.

Keep tabs on our planets with Morrison Planetarium's quarterly guide to planetary activity.

Mercury

The smallest and most elusive of the naked-eye planets begins the season hidden in the Sun's glow, finally emerging into the evening sky. However, due to the shallow angle of the ecliptic (the path of the planets against the stars), Mercury is very low at sunset, even at greatest eastern elongation, which occurs on August 9.

The Moon makes close approaches to Mercury on the evenings of July 19 and August 18, but these pairings are difficult to see in the glow of twilight, as is their predawn encounter on September 19.

Venus

The "evening star" dominates the western sky just after sunset until late-July, clustering with Mars and the star Regulus (the heart of Leo the Lion), even as they gradually drop lower into the twilight. While waiting for fireworks on July 4, impress your friends by pointing them out, with Venus and Mars less than 4° apart (and see if anyone can guess which is which). From July 6-13, Venus, Mars, and Regulus are clustered within a circle of about 6° in diameter (tightly enough for all three to fit in the same field in most binoculars). The combination of apparent size and illuminated extent results in Venus reaching its greatest brilliancy on July 9 (while Mars and Regulus to its upper-left are only a degree apart). As Venus is gradually swallowed by the Sun's glow, it disappears from the evening sky and reaches inferior conjunction on August 13, then reappears in the predawn sky in late-August, rising about an hour after the bright stars Sirius and Procyon and an hour before before sunrise against the stars of Cancer the Crab.

The Moon appears near Venus on the nights of July 19 and 20, when both set an hour after sunset. This may be difficult to see easily in the glow of twilight. Their meeting on August 15 will be completely washed from view by the Sun's glare, but their third meeting of the season on September 11 will be widely spaced but visible in the east before dawn.

Mars

As noted above, Mars puts on a good show for early-evening observers during the first half of July as it clusters with brilliant Venus and the star Regulus (the heart of Leo the Lion) low in the west just after sunset. On July 9-10, Mars is less than a degree from Regulus. Gradually descending into the twilight, the Red Planet becomes increasingly difficult to see and is effectively lost from view by early- to mid-September.

Mars is joined by the crescent Moon on the evening of July 20, with both very low in the west after sunset and separated by about 3.5°—about the width of two fingers held together at arm's length. Coincidentally, this is also the anniversary of the 1969 Apollo 11 landing on the Moon and the automated Viking 1 landing on Mars in 1976. The two meet lower in the sky on the evening of August 18 and set less than two hours after the Sun. Our last chance this season to see them close together in the evening sky is a binocular challenge on September 16, largely obscured by the glow of twilight and setting less than an hour after sunset.

Jupiter

Jupiter dominates the predawn/post-midnight sky against the faint stars of Aries the Ram, rising around 2:15 am on July 1, around 12:30 am on August 1, and around 10:30 pm on September 1.

The Moon joins Jupiter in the morning sky on July 11, August 8, and September 4, all three encounters being within 5° of arc, or less than the width of three fingers held together at arm's length, and certainly in the same field of view in binoculars. If mounted on a steady tripod, the binoculars will also reveal the four largest of Jupiter's 95 presently confirmed moons ("presently"...the number could go up at any time!).

Saturn

Saturn rises at about midnight on July 1 and around 9:40 pm on August 1. Reaching opposition on August 27, when it rises at sunset, it is very low in the southeast just after sunset on September 1. The Ringed Planet lingers against the faint stars of Aquarius the Water-Carrier for the rest of the year.

The Moon is located near Saturn on the nights of July 7, August 3, August 29-30, and September 26. There are two encounters between Saturn and the Moon in August because the length of time it takes for the Moon to travel 360° around to the same background of stars (its sidereal period) is only 27.3 days. This makes it possible to encounter the same background twice in the same month