This is the Skywatch Line for Monday and Tuesday, October 28th and 29th, written by Joe Slomka.
The Sun sets at 5:54 PM; night falls at 7:29. Dawn begins at 5:49 AM and ends with the Sun rising at 7:24.
There are three challenge objects in the twilight sky. Tuesday, the 4% illuminated Moon lies low in the southwestern sky. At 6:30 PM, it is about 6° high in Libra, sets at 7:13 PM and lies about 5° above Venus, another challenge object. Venus, in Libra blazes with minus 7th magnitude, appears about 11 arc-seconds in size and sets at 6:46 PM. Mercury lies below Venus, shining with zero magnitude, 7 arc-seconds in size and sets with Venus. Binoculars are recommended for those who try to find them amid the setting Sun’s glare.
Jupiter, still in Ophiuchus, shines with minus 2nd magnitude, is about 34 arc-seconds in size and sets at 8:23 PM. It may be spotted early in the evening by eye alone, but will need binoculars or telescope before it sets.
Saturn still trails Jupiter by about 2 hours. It continues to inhabit Sagittarius. It shines with 1st magnitude and appears about 16 arc-seconds in size. Still worth observing with either binoculars and telescopes because of its rings. The Ringed Planet sets at 10:03 PM.
Neptune, in Aquarius, glimmers with 7th magnitude and appears a tiny 2 arc-seconds. It is best observed at 9:33 PM in the southeastern sky. It sets at 3:40 AM. Uranus, in Aries, is brighter with 5th magnitude and appears slightly larger in our instruments. This Opposition is the ideal time to study Uranus, since it rises near sunset and sets near sunrise. Its blue-green tint helps find it amid similar stars.
Mars rises, in Virgo, about 1 ¾ hours before sunrise, at 5:51 AM. The Red Planet shines with 2nd magnitude, about 4 arc-seconds in size, but 99% lit. At Civil Dawn (6:55 AM), it lies about 10° above the eastern horizon. Again, binoculars or telescopes may be necessary to see in the steadily brightening sky.
The Solar System has an unusual arrangement. Rocky bodies (Mercury, Venus, Earth, Mars and the Asteroid Belt) lie close to the Sun. Gas giant planets (Jupiter, Saturn, Uranus and Neptune) comprise a second group. Finally, there are the icy objects (Pluto, Eris and the rest of the Kuiper and Oort Belts). Uranus and Neptune are very alike. Unlike Jupiter and Saturn, they are made of mostly hydrogen compounds like water, methane and ammonia. Methane is what gives these planets their blue tint. These planets are mostly gas with rock and metal cores. Both planets rotate about every 16 or 17 hours. Since they are mostly gas, rotation varies by latitude; the equator spins faster than the poles. Uranus has four medium moons; Neptune has two. Uranus has no obvious weather. Neptune occasionally sports a “Great Dark Spot,” similar to Jupiter’s Great Red Spot. Uranus emits no heat, while Neptune glows with twice the energy it receives from the distant Sun. Both planets have rings, which can only be seen through the Hubble Telescope and passing space probes.