Skywatch Line for Monday and Tuesday October 9th, and 10th, 2023
This is the Dudley Observatory’s Skywatch Line for Monday and Tuesday October 9th, and 10th, written by Joe Slomka.
The Sun sets at 6:23 PM; night falls at 7:57. Dawn begins at 5:29 AM and ends with the Sun rising at 7:03.
Leo hosts the Moon on both nights. Monday, the 26-day-old Moon rises at 1:38 AM, 29 arc-seconds in size, 22% illuminated and sets at 4:40 PM. Tuesday, the Moon rose at 2:42 AM, same size, but 17% lit, 23° high and sets at 5:02 PM. Tuesday, at about 3 AM, the Moon lies about 2 degrees from Leo’s brightest star – Regulus; also, the Earth reflects some of its light back onto the Moon, called “Earthshine”. Finally, the Moon is also at Apogee (furthest from Earth). The Moon rises at 3:45 AM Wednesday morning.
Mercury and Mars are not visible. Southeastern Saturn remains the evening star, shining with zero magnitude, 18 arc-seconds in diameter, already up in Aquarius, highest at 9:55 PM, 28° high at 8 PM and sets at 3:10 AM. Neptune, in Pisces, lies 24° East of Saturn, 29° at 8 PM, shining with 7th magnitude, 2 arc-seconds, highest at 11:27 PM, and sets at 5:18 AM.
Jupiter shares eastern Aries with Uranus. Gas giant Jupiter shimmers with minus 2nd magnitude, a large 48 arc-seconds, 5° high at 8 PM, highest at 2:30 AM and sets during daytime. Tuesday presents plenty of action. First, the moon Europa’s shadow begins its travel at Midnight. The Great Red Spot (a giant storm) also starts at 12:42 AM, Europa sets off at 1:28 AM, Europa’s shadow ends at 2:30 and finally Europa ends its trip at 3:40. Uranus follows 9° from Jupiter, gleaming with 5th magnitude, 3 arc-seconds, rising at 7:51 PM, 2° high at 8 PM, highest at 3:04 also sets during the day.
Venus brings up the rear, rising in the East at 3:13 AM, blazes with minus 4th magnitude, 43° lit, a moderate 28 arc-seconds, 14° high at 5 AM and sets at 4:26 PM. Note that Venus, the Moon and Jupiter crowd together, Venus 2.5° from Regulus in Leo and 5.5° from the Moon.
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 Earth and landed at Japan.