Skywatch Line for Monday October 11th and Tuesday October 12th, 2021

This is the Dudley Observatory’s Skywatch Line for Monday and Tuesday October 11th, and 12th, written by Joe Slomka.

The Sun sets at 6:19 PM; night falls at 7:53. Dawn begins at 5:32 AM and ends with the Sun rising at 7:06.

Sagittarius houses the Moon on both nights. Monday, the Moon rises at 1:21 PM in the South-southeast, 32 arc-minutes in size, 37% illuminated and 19° high in South by 6:47 PM. Tuesday’s First Quarter Moon rises at 2:42 PM, in the Southeast, slightly smaller, 49% lit and sets at 11:09 PM.

Venus continues to be the “Evening Star”. It rises in Scorpius and, by 7 PM, it blazes with minus 4th magnitude, appears 20 arc-seconds, 57% lit and 9° high in the Southwestern sky. Venus sets at 8:04 PM. Before it gets too low, the observer should look a bit to Venus’ left where the star Antares dwells. Antares is a giant star which serves as the heart of the Scorpion; shining with 1st magnitude, it also is a variable star and was named because it is colored like Mars. Binocular or telescope users can compare and contrast the two before they set.

Once observers are finished with Venus and Antares, they should swing South, where Capricornus contains both Saturn and Jupiter. Saturn rises first, glowing with zero magnitude, 17 arc-seconds, highest at 8:09 PM and sets at 12:56 AM. For months we have saying the Saturn is in retrograde; now it appears to stop dead in its tracks and soon begins an apparent eastward movement.

Jupiter is next, 15° to Saturn’s left, glimmering with minus 2nd magnitude, appearing over twice Saturn’s size, 25° high by 8 PM, highest at 9:11 PM and sets at 2:26 AM. Binocular observers can easily see Jupiter’s 4 moons. Telescope users can witness the moon Io begin to cross Jupiter’s face at 11:09 PM Monday. Io’s shadow creeps into view at 12:16 AM on Tuesday; Io exits at 1:26 AM and the shadow follows at 2:33 AM. However, these last two phases will probably be too low for most local observers.

Third place Neptune lies in Aquarius, glowing with 7th magnitude, a tiny 2 arc-seconds, 18° high at 8 PM, highest at 11 PM and sets at 4:45 AM. It is found 28° to Jupiter’s left. Uranus brings up the rear, in Northeast Aquarius. It rises at 7:17 PM, shines with 5th magnitude, a slightly larger size of 3 arc-seconds, 7° high at 8 PM and sets during daytime. Neptune is up most of the night and Uranus is a naked eye object if the observer is in a dark, rural area.

Note that the Moon, Saturn, Jupiter, Neptune and Uranus form a celestial chain for most of the night.

Monday is Columbus Day. Most people are familiar with the story of Columbus sailing West to reach China. When he landed in the Caribbean, he thought he had found Japan. How could he have made that mistake? Finding latitude is easy, sight on the Pole Star and measure its height above the horizon. But longitude could not be calculated without very accurate sea-borne clocks; which were not invented for another 300 years. Two ancient Greeks measured the Earth. Eratosthenes accurately estimated the Earth’s diameter; Claudius Ptolemy underestimated it. Arab scholars provided other studies of Earth’s size. They used a shorter Arabian mile, which Columbus mistook to be equal to nautical miles. Using “dead reckoning,” a navigational estimation of a ship’s course, it was natural for Columbus to think he circumnavigated the Earth and landed on Japan.

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