This is the Skywatch Line for Monday and Tuesday, July 23rd and 24th.
The Sun sets at 8:25PM; night falls at 10:28. Dawn begins at 3:34 AM and ends with the Sun rising at 5:37.
The evening sky is full of planets. Mercury lies very low in the West. In Leo, it shines with 1st magnitude and appears about 10 arc-seconds in size and 22% illuminated. However, it is about 2º above the western horizon and needs an unobstructed view before it sets at 9:11 PM. Venus appears about 22º to Mercury’s upper left. Venus, also in Leo, blazes with minus 4th magnitude, appears about twice as large as Mercury, and is 60% lit. It sets at 10:22 PM.
Jupiter, high in the southwest, still inhabits Libra. It also glares with minus 2nd magnitude and is best observed at 7:34 PM. Telescopic observers can witness the Jovian moon Io disappear behind the planet on Monday at 9:22 PM and also see Io’s shadow leave the planet’s face at 10:03 PM. Jupiter recently made headlines. On July 17th, the Minor Planet Center announced that Scott Sheppard (Carnegie Institution for Science) publicized the discovery of 12 new moons for Jupiter. Of the 12, nine orbit Jupiter opposite to the planet’s spin (retrograde). One moon, nicknamed Valetudo, spins with Jupiter, but at a 34º angle to the equator; in addition, Valetudo crosses paths with the other retrograde moons. This brings the total moon count for Jupiter at 79 and 185 for the whole Solar System; only Mercury and Venus are moonless. Jupiter sets at 12:43 AM.
Saturn, in Sagittarius, shines with zero magnitude moderately high in southeastern skies. Wednesday, it lies about 6º south of the Moon. Saturn is best viewed at 11 PM and sets at 3:42 AM.
Mars, in Capricornus, rises about 9:04 PM. It reaches Opposition on Thursday night. The Red planet shines with minus 3rd magnitude, appears about 24 arc-seconds in size, the maximum for this appearance, and about 10º high in the southeast. NASA reports the dust storm still rages, limiting views of the planet. Mars sets after sunrise.
Neptune, in Aquarius, rises about 10:19 PM, and is best studied at 4:02 AM. Uranus, in Aries, glows with 6th magnitude and rises at 12:06 AM. Both require detailed sky charts to locate them.
The 11-day-old Moon is already up by sunset. In Ophiuchus, it blazes with minus 11th magnitude, appears 88% lit and is best observed at 10:07 PM. Tuesday’s Moon, in Sagittarius, is brighter and larger. It is best observed at 10:55 PM. The Moon sets at 2:20 AM Tuesday and at 3:42 AM on Wednesday.
As appropriate for the racing season, two horses appear by midnight. The largest horse is, of course, Pegasus. The smallest is Equuleus. This dim constellation is easy to find. Pegasus flies upside down and is easily identified as a Great Square. Two thin star chains sweep northward from the upper left. If one sweeps across the chains, binoculars show a large hazy oval; this is revealed, in telescopes, to be the Andromeda Galaxy – about two and a half million light years distant, accompanied by two smaller satellite galaxies. You can see it with the naked eye under dark rural skies. Pegasus’ neck flows from the lower right corner and angles up. Equuleus is the small angular line of stars West of Pegasus’ nose. A globular star cluster, M 15, lies exactly halfway between Pegasus’ nose and Equuleus. This too is easily seen in binoculars.