Skywatch Line for Monday and Tuesday, October 12th and 13th

This is the Dudley Observatory Skywatch Line for Monday and Tuesday, October 12th and 13th written by Joe Slomka.

The Sun sets at 6:18 PM; night falls at 7:52. Dawn begins at 5:32 AM and ends with the Sun rising at 7:06.

The Moon officially turns “New” on Monday night; Tuesday sees a very young Moon very low on the western horizon, which sets at 6:49 PM.

The twilight sky reveals only one bright planet, Saturn. Saturn is now low on the western horizon and sets at 8:21 PM, two hours after sunset. It becomes lower daily. Our turbulent atmosphere is now smearing the view of the famous rings. The best time to observe Saturn is as soon as possible after the Sun sets.

Nightfall reveals three dimmer Solar System members. Neptune still resides in Aquarius. At magnitude 7.8, it requires dark skies, telescope and detailed sky charts. It sets after 10 PM.

Uranus rose in Pisces shortly before Sunset; it is brighter at magnitude 5.7. It is visible in binoculars, for those who know where to look. Uranus is best observed at about 12:39 AM, and sets at Sunrise.

Finally, as we mentioned last week, sixth magnitude asteroid Vesta can be found in the constellation Cetus. It lies near the star Iota Ceti; it remains in this neighborhood all month. It is best observed around Midnight.

All three can be found with the aid of astronomy magazines, websites and apps.

Tuesday, the bright star Algol, in Perseus, dims at 1:06 AM. A much dimmer companion eclipses the main star every 2.87 days.

Dawn bring four bright planets onto the scene. Venus rises first at 3:14 AM.  At magnitude minus 4.5, it is the brightest object in the sky, appearing beneath Leo’s belly.

The Red Planet Mars is next to rise at 3:47 AM, appearing about eight degrees to Venus’ left. Finally, Jupiter rises at 3:58 AM, and is about two degrees to Mars’ left. These three planets are slowly converging toward conjunctions this month.

Mercury rises in Virgo at 5:34 AM. At minus 1 magnitude, it should be visible about eleven degrees above the eastern horizon.

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; such clocks were not invented for another 200 years. Two ancient Greeks measured the Earth. Eratosthenes accurately estimated the Earth’s diameter; Claudius Ptolemy underestimated it. Arab scholars provided other approximations of Earth’s size. They used a smaller Arabic 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 mistake the island of Jamaica for Japan.

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