This is Dudley Observatory’s Skywatch Line for Friday, August 11 through Sunday, August 13, written by Sam Salem.
On Friday, Sun rises at 5:58am and sets at 8:03pm; Moon rises at 10:19pm. Last quarter Moon occurs on Monday at 9:15PM.
One hour after sunset, magnitude –1.8 Jupiter hangs just a little more than 10 degrees above the southeast horizon, while Saturn, magnitude 0.3, is already at the meridian and starting to lose altitude. In the morning sky, Venus, at magnitude –4.0, continues its long-running apparition. It rises well before astronomical twilight begins and is a dominant sight at dawn. Venus will continue as “morning star” all through summer and to the end of fall.
Friday and Saturday nights present the peak nights of the Perseid meteor shower. Watch from late evening till dawn on both nights. The greatest number of meteors typically fall in the hours before dawn, and on a moonless night you can often spot 50 or more meteors per hour. This year the bright waning gibbous Moon lights up the morning hours, intruding on the annual Perseid meteor display. Perseids are well worth watching because a good percentage of these meteors should be bright enough to overcome moonlit glare. The Perseids are known to be colorful meteors. According to NASA’s Meteoroid Office, the Perseids have more fireballs than any other major shower. The constellation Perseus is the radiant for the annual Perseid meteor shower. But you don’t need to know the constellation Perseus to watch the Perseid meteor shower. The Perseids fly every which way across the starry heavens. The radiant sits low in the northeast sky at evening and climbs upward throughout the night. The higher that the radiant is in your sky, the more Perseid meteors you’re likely to see. The radiant is highest before dawn. The earliest historical account of Perseid activity comes from a Chinese record in 36 AD, where it was reported that “more than 100 meteors flew in the morning.” The Perseid meteors happen around this time every year, as Earth in its orbit crosses the orbital path of Comet Swift-Tuttle. Dusty debris left behind by the comet smashes into Earth’s upper atmosphere, lighting up the nighttime as fiery Perseid meteors.
The faint constellation Ophiuchus, the Serpent Bearer appears in the south to southwest sky on August and September evenings, above the bright star Antares, the brightest in the constellation Scorpius, the Scorpion. Planet Saturn appears near Antares, somewhat brighter than Scorpius’ brightest star. Saturn shines in front of the constellation Ophiuchus and will continue to do for several more months. Ophiuchus’ brightest star, Rasalhague, marks the of Ophiuchus’, the Serpent Bearer, Head. Rasalhague is from the Arabic word (ras-al-hawwa) which means the Head of the Serpent Collector.
Saturday marks the anniversary of the discovery of the first of two moons of Mars. On August 12 1877, American astronomer Asaph Hall discovered the first of two moons of Mars. He named the first Deimos. Five days later, on August 17 1877, he observed a second moon, which he named Phobos. In Greek mythology, these are the sons of Ares (Mars) and Aphrodite (Venus). Deimos is Greek for “panic” and phobos is Greek for “fear.” These moons are composed of carbon-rich rock like C-type asteroids and ice. Their densities are so low that they cannot be pure rock. Both are heavily cratered. They are probably asteroids perturbed by Jupiter into orbits that allowed them to be captured by Mars.