Dudley Observatory Skywatch Line for Friday, April 15, through Sunday, April 17, 2016

This is Dudley Observatory’s Skywatch Line for Friday, April 15, through Sunday, April 17, written by Sam Salem.

On Friday, the Sun rises at 6:15am and sets at 7:39pm. Look for the waxing gibbous Moon with 65% of its visible disk illuminated on Friday, increasing to 74% on Saturday, and 82% on Sunday night. The Moon sets at 2:58 am on Friday, 3:34 am on Saturday, 4:06 am on Sunday, and 4:36 am on Monday.

On Friday evening, the Moon forms a curving row with Regulus to its left and then Jupiter.  Look above the Moon after dusk, on Saturday, for Regulus, the bottom of the now-vertical Sickle of Leo.  Regulus is the brightest star in the constellation Leo.  On Sunday, watch as the Moon pays a visit to the brightest planet of the night, Jupiter, as both rise in the southern sky.

Mars begins its retrograde motion westward in the sky, on Sunday.  Mars will continue its backward march until June 30.  For a couple of months every two years, Mars changes the direction of its motion against the backdrop of fixed stars. This apparent reversal is due to the fact that Earth is orbiting closer to the sun than Mars and is moving faster on its orbital track. That means our planet periodically passes Mars, creating the illusion that it has changed course. Seen from Mars, Earth appears to be in retrograde during this time.

Mid-April Mars is 63,236,776 miles away from Earth.  Now is the time to start exploring Mars through the telescope.  It blazes highest in the south before the first light of dawn, to the right of dimmer Saturn and above Antares.  In a telescope Mars grows this week from 13 to 14 arc-seconds in diameter.   By the time of its opposition and closest approach in late May, Mars will triple in brightness and grow to 18.8 arc-seconds wide.

Saturn is also moving retrograde but at much slower speed than Mars.  The pair moves apart from each other then start to converge and pass each other on August 25.

By early dawn, Saturn and Mars stand in the south-southwest.  Saturn, Mars, and Antares form a triangle.  Saturn stands on the left, Mars stands on the right, and the fainter, Mars-colored Antares stands beneath Mars.  Antares is the brightest star in the constellation Scorpius.  It is often referred to as “the heart of the scorpion”.  Along with Aldebaran, Regulus, and Fomalhut, Antares comprises the group known as the “Royal Stars of Persia”.   They were regarded as the guardians of the sky during the time of the Ancient Persians.  Persians believed that the sky was divided into four districts with each district being guarded by one of the four Royal Stars.

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