Skywatch Line for Friday, August 10 through Sunday, August 12, 2018

This is Dudley Observatory’s Skywatch Line for Friday, August 10 through Sunday, August 12, written by Sam Salem.

On Friday, the Sun rises at 5:57am and sets at 8:05pm; the waning crescent Moon rises at 4:46am and sets at 7:45pm. The new Moon occurs on Saturday at 5:58am.

Watch Venus, at magnitude –4.4, 45 minutes after sunset. Venus, sits about 10 degrees above the west-southwest horizon. Jupiter shines at magnitude –2.1 and is positioned well past the meridian at dusk. Saturn, at magnitude 0.2, culminates at around 10:15pm. Mars, at magnitude –2.7, is brighter than Jupiter.
Mars is highest in the south around midnight. It’s at a southerly declination (–26°) in southern Capricornus. Mars is still 24 arcseconds wide. The dust in its atmosphere continues to thin gradually, allowing faint, low-contrast views of some dark surface markings.

The annual Perseid meteor shower comes to its peak late Sunday night. You may have noticed a few early Perseids as early as a week or two ago. Conditions are ideal this year as there’s no moonlight. The best time to catch Perseids is after local midnight, when the shower’s radiant is approaching the zenith. You’ll see the most meteors when the shower’s radiant is high from about midnight to dawn. On peak night you may see one or two meteors a minute on average during this time if your sky is very dark. You can see fewer Perseid meteors in the evening hours. The meteors tend to be few and far between at mid-evening. Though this is the best time of night to try to catch an earthgrazer. These are elongated, long-lasting meteors that travels horizontally across the sky. Earthgrazers are rare but most memorable if you’re lucky enough to spot one.

The summer triangle frames the Milky Way between stars Vega and Altair. Deneb, the brightest star in constellation Cygnus, appears in the middle of the Milky Way’s dim band. Look along the band of the Milky Way this weekend, you’ll notice numerous dark rifts and patches. One conspicuous area, known as the Northern Coal Sack, hides in the southeast of 1.2-magnitude Deneb, This feature is part of a much larger dark conglomeration known as the Great Rift, which bisects the summer Milky Way for much of its length. You don’t need a telescope to spot it as just dark skies. Another well-known dark nebula is Barnard’s E, located a few degrees northwest of 0.9-magnitude Altair, the brightest star in constellation Aquila. Use binoculars or a small, wide-field telescope and look for a ragged, E-shaped blackish blob about one degree due west of 2.7-magnitude gamma (γ) Aquilae. Observing the ghostly E takes some effort. But once you train your eye to look for an absence of starlight, you’ll be able to see other dark nebulas you can pick up with binoculars.

Sunday marks the 141st anniversary of the discovery of the first moon of Mars. On August 12 1877, American astronomer Asaph Hall discovered Deimos. Five days later, on August 17 1877, he observed a second moon, which he named Phobos. In Greek mythology, Deimos and Phobos are the sons of Ares (Mars) and Aphrodite (Venus). Deimos is Greek for “panic” and phobos is Greek for “fear.” These two 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. Probably, they are asteroids perturbed by Jupiter into orbits that allowed them to be captured by Mars.

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