Skywatch Line for Monday and Tuesday, April 17th and 18th, 2017

This is the Skywatch Line for Monday and Tuesday, April 17th and 18th.

The Sun sets at 7:40 PM; night falls at 9:24. Dawn breaks at 4:25 AM and ends with the Sun rising at 6:08.

The evening sky has two bright planets, Mars, in Taurus, in the West and Jupiter in the East. Mars shines at first magnitude moderately high in the western sky, appearing as a tiny red dot. Note Mars’ position. As the sky darkens, Mars lies about 4 degrees below the Pleiades star cluster. This juxtaposition remains for the next 5 nights. Both set by 10:14 PM.

Jupiter rises at 6:31 PM in Virgo. It flashes at minus 2.5 magnitude, is 16 degrees above the horizon and appears 44 arc-seconds in size. Jupiter continues to shy away from Spica, Virgo’s brightest star. While binoculars afford views of the giant planet, telescopes provide greater details. The Great Red Spot, a giant storm, is visible at 9:09 PM on Monday and 2:56 AM on Wednesday. The Jovian moon Io disappears behind Jupiter at 12:15 AM on Tuesday and reappears at 2:41 AM. It also begins to cross Jupiter’s face at 9:32 PM on Tuesday, followed by its shadow at 9:48 PM; Io leaves Jupiter at 11:43 PM also on Tuesday, followed by its shadow at 11:59 PM. Jupiter is best observed at 12:19 AM and sets at 5:57 AM.

Saturn rises in Sagittarius at 12:24 AM, glowing at zero magnitude and appears 17 arc-seconds in size. Best views of Saturn are at 4:58 AM. The cream-colored planet is obvious. Besides the rings, Saturn has sixty-one moons. One of these, Iapetus, has puzzled observers for centuries. Iapetus is bright when it is on one side of Saturn, but markedly darker on the other.

Astronomers think they have figured it out. Iapetus is tidally locked to Saturn, just like the Earth’s Moon – showing the same side to the planet. The leading side of Iapetus sweeps up debris from a newly discovered (and invisible to amateurs) ring. Thus, one side looks like it was covered in chocolate dust, while the trailing side is as white as snow, really ice. In addition, the dust, warmed by sunlight, melts the ice below, which flows to the trailing side and re-freezes. Iapetus has a 79.3-day orbit, and is visible in amateur telescopes. Astronomy programs and websites assist the observer.
The Moon shares Sagittarius with Saturn, and rises an hour after the Ringed Planet. The 21-day-old Moon blazes at minus 10th magnitude, appears about 60 percent illuminated and rises at 1:27 AM Tuesday and 2:11 AM on Wednesday, when it becomes Last Quarter.

Venus rises in Pisces at 4:40 AM, also in the East. It flashes at minus 4th magnitude, appears 46 arc-seconds in size and is 15 percent illuminated. At the same time, Comet PanSTARRS is also visible in the eastern sky, in Aquarius. Reports are that it went from 8th magnitude to 6th magnitude, within binocular visibility. PanSTARRS may even brighten more, since it is nearing perigee (near Earth) on April 19th. However, this becomes a challenge object, since it is in a rapidly brightening dawn sky.

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