Skywatch Line for Monday and Tuesday, April 3rd and 4th, 2017

This is the Skywatch Line for Monday and Tuesday, April 3rd and 4th.

The Sun sets at 7:24 PM; night falls at 9:02. Dawn breaks at 4:53 AM and ends with the Sun rising at 6:32.

The First Quarter Moon dominates the evening sky. By Civil Dusk, it is about 62 degrees high, in Gemini. It blazes at minus 10th magnitude, is half illuminated, is 32 arc-minutes large and is best observed at 7:16 PM. Tuesday finds it in Cancer and a bit larger, brighter and fatter. The Moon’s brilliance will hinder observation of dim distant deep space objects. The Moon sets at 2:44 AM on Tuesday and at 3:31 AM on Wednesday.

Three planets are visible in the evening sky. In the West, Mercury, in Aries, is about 12 degrees high and shines at 0.4 magnitude. Visible in binoculars, it is 8 arc-seconds in size and about one-third illuminated. It sets at 9:05 PM. Mercury is just past “greatest elongation,” which means it is at its best and brightest. Now is the time to observe Mercury before it starts fading and sinking into the Sun.

Mars lies 15 degrees to Mercury’s upper left. The Red Planet, also in Aries, glows at first magnitude and appears as a tiny red dot. It sets at 10:17 PM.

Jupiter, in Virgo, continues to creep away from the bright star Spica in preparation for its Opposition on April 7th. It shines at minus 2.5 magnitude and sets after Sunrise. Telescopic observers can see Jovian moon Io disappear behind the planet at 8:40 PM and reappear at 10:58 PM on Monday; they can also witness Ganymede be eclipsed at 12:38 AM and reappear at 3:14 AM on Tuesday. The Great Red Spot, a giant storm on Jupiter, is visible at 5:34 AM on Tuesday and at 1:25 AM on Wednesday.

Dawn finds Saturn already up in Sagittarius, having risen at 1:19 AM. At Dawn, it is 23 degrees high in the eastern sky and shines at zero magnitude. It is best observed just before daybreak at 5:44 AM. Its creamy white appearance makes identification easy and is always a joy to see its ring system.

Two weeks ago, Venus abandoned the evening sky; now it is beginning its “morning star” apparition. It rises in Pisces at 5:25 AM, about an hour before Sunrise. It blazes at minus 4th magnitude, appears about 4 percent illuminated and is about 6 degrees high in the East. Venus climbs higher and brighter as the month progresses.

When you are done observing Jupiter, turn your attention to the close star Porrima. The Latin name refers to the Goddess of Prophesy. The star lies midway between Spica, in Virgo, and Denebola, Leo’s tail. Porrima is a double star. Both stars are nearly identical. They are about the same brightness, third magnitude, and the same mass, about 1.5 times the Sun. They are sun-like, but significantly brighter and warmer. Like the Sun, Porrima and its companion are main sequence stars, fusing hydrogen into helium. Porrima was among the first double star systems discovered. Sir John Herschel calculated its orbit in 1833. They share a highly elliptical orbit, which make one cycle in about 169 years. An observer, with high power eyepieces in the telescope, can see them about 2.5 arc seconds apart.

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