This is Dudley Observatory’s Skywatch Line for Friday, July 8, through Sunday, July 10, written by Sam Salem.
On Friday, the Sun rises at 5:26am and sets at 8:36pm. The 19% illuminated waxing crescent Moon shines in the west at dusk, about 4 degrees below Jupiter in the evening sky. On Saturday night, the Moon will appear about twice as far to Jupiter’s upper left. The Moon sets at 11:12pm on Friday; 11:56pm on Saturday; 12:10am on Monday morning.
Jupiter is low in the west in twilight and is descending closer and closer to the glare of sunset. Jupiter, of -1.8 magnitude, is still the brightest planet in the summer sky until Venus shines later in the season. Use the Moon, on Friday evening, to locate Jupiter. The four Galilean moons of Jupiter, Io, Europa, Ganymede, and Callisto, are easy to observe through a small telescope.
The Galilean moons enabled celestial navigators to determine longitude on land. Galileo proposed that one could use the Galilean moons positions as a universal clock. A device invented by Galileo called “celatone” was made to observe Jupiter’s moons with the purpose of finding longitude on Earth. The device, in a form of a helmet with a telescope mounted taking the place of an eyehole, was designed to accommodate the motion of the observer on the ship. The Galilean moons couldn’t be used to determine longitude in the rough seas; however, seafarers used the moons to check the accuracy of their chronometers once they hit landfall.
Saturn reached opposition in early June. Despite its southern declination, it still offers good viewing opportunities in July. Saturn’s rings are now open almost as wide as possible, making them an easy target even for a small telescope. The planet sinks lower as the season progresses.
Sunday should be a good night to observe asteroid 7 Iris. Look for Iris 0.5 degrees due south of the magnitude 5.0 star Lambda (l) Libra. The 10th-magnitude asteroid lies due south and at its highest position in the night sky.
The ecliptic is the apparent path the Sun appears to take through the sky as a result of the Earth’s revolution around it. The ecliptic represents the projection of the plane of the Earth’s orbit out towards the sky. The Moon and planets are not positioned exactly on the ecliptic because they’re not located exactly in the same orbital plane as Earth. They lie within several degrees of it and form of narrow strip encompassing the entire sky called the Zodiac. The ecliptic runs exactly along the middle of the Zodiac. Twelve constellations through which the ecliptic passes form the Zodiac. The name “Zodiac” is derived from the Greek, meaning “animal circle,” as most of these constellations are named for animals, such as Leo, the Lion and Taurus, the Bull. Because the Moon and planets are often positioned either just to the north or south of the ecliptic, it allows them to sometimes appear within the boundaries of a number of other non-zodiacal star patterns. There are ten other constellations that occasionally can be visited by the Moon and planets.