Skywatch Line for Monday July 19th and Tuesday July 20th, 2021

This is the Dudley Observatory’s Skywatch Line for Monday and Tuesday July 19th, and 20th, written by Joe Slomka.

The Sun sets at 8:29 PM; night falls at 10:34. Dawn begins at 3:30 AM and ends with the Sun rising at 5:35.
The 10-day-old Moon rises at 4:10 PM in southwestern Libra, is highest at 9 PM, 70% illuminated, and sets at 1:53 AM.

Tuesday finds the Moon in Scorpius’ claws, rising at 5:27 PM, 32 arc-minutes in size, 80% lit and sets at 2:37 AM.
Last week’s close encounter of Venus and Mars has dissolved. Venus continues to be the “evening star,” blazing with minus 3rd magnitude, a moderate 12 arc-seconds 85% illuminated, 8° above the western horizon, and setting at 9:56 PM. Venus closes in to Leo’s bright star Regulus on Monday and Tuesday. Mars lies about 4° below Venus, glowing with first magnitude, a tiny 3 arc-seconds, 5° above the horizon and sets at 9:45 PM. Both are low and may require an unobstructed horizon; binoculars will help find both amid the bright sunset.

Saturn, in Capricornus, rises at 9:08 PM, in the East, while Venus and Mars are still up. The Ringed Planet shines with zero magnitude, is a moderate 8 arc-seconds and is highest at 2 AM; it is 13° high by 10:30 PM and 23° by Midnight.
Aquarius houses Jupiter and Neptune. Jupiter rises at 10:02 PM, glows with minus 2nd magnitude, a large 47 arc-seconds and is highest at 3:18 AM; it is 10° high at 11PM and 19° high at Midnight. Tuesday, telescopic observers can witness the Jovian moon Io being eclipsed at 10:31 PM and at 11:50 PM see Europa reappear from being occulted. Wednesday at 1:33 AM, observers can experience Io’s reappearance. Neptune rises at 10:53 PM, glimmering with 7th magnitude, a small 2 arc-seconds and highest at 4:41 AM. Uranus, in Aries, rises at 12:53 AM, shines with 5th magnitude, 3 arc-seconds and sets during daytime.

About Midnight, a constellation, shaped like a stick drawing of a house, Cepheus, points toward the North Star – Polaris. In Greek legend, Cepheus was king-husband of Cassiopeia and father of Andromeda. Cepheus houses one star that became the prototype of a whole class of stars. Cepheids are variable stars that have relatively short periods, usually days. These stars vary their light due to processes within the stars. In 1893, Henrietta Swan Leavitt worked as a human computer for Harvard Observatory. She was engaged in a project when she made a discovery. Leavitt noticed that the variation period of this class of star was in direct proportion to its intrinsic brightness. The Cepheid became a “standard candle.” If you know how bright the star is, and you see it from Earth as dimmer, one can estimate distance to the star. Miss Leavitt made it possible to reckon distances to star clouds and galaxies. All one had to do is find a Cepheid star and note its period. Ms. Leavitt’s discovery made possible the amazing progress of astrophysics in the Twentieth Century.

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