Skywatch Line for Monday and Tuesday, April 16th and 17th, 2018

This is the Skywatch Line for Monday and Tuesday, April 16th and 17th.

The Sun sets at 7:39 PM; night falls at 9:22. Dawn begins at 4:27 AM and ends with the Sun rising at 6:10.

Venus, in Aries, is the brightest object in the darkening western sky. It blazes with minus 4th magnitude, appears about 91% lit and lies about 16º above the horizon. It sets at 9:41 PM.

The one-day-old Monday’s Moon lies 13º below Venus and about 2º above the horizon. It shines with 2nd magnitude and appears as a thin 1º crescent. It sets at 8:23 PM. Binocular and telescopic observers can see this very young Moon if they have an unobstructed horizon. This writer once caught a similarly young moon using both binoculars and telescope. Tuesday’s Moon is brighter, about 5% lit, about 13º high and sets at 9:34 PM.

Jupiter, in Libra, rises about 9:32 PM and blazes at minus 2nd magnitude. It is a best observed at 2:29 AM. Telescopic viewers can see the Great Red Spot (a giant Jovian storm) at 11:19 PM Monday and at 5:06 AM on Wednesday. They can also witness the moon Europa’s shadow begin to cross the planet’s face at 11:36 PM on Tuesday, followed by Europa itself at 12:37 AM Wednesday; Europa’s shadow exits at 1:51 AM Wednesday, followed by Europa at 2:45 AM.

Sagittarius houses three Solar System members. Saturn rises at 1:19 AM, shining with zero magnitude and about 10 arc-seconds in size. Saturn is at aphelion (most distant from the Sun) and also it is stationary in our sky. Note its position. Saturn soon begins retrograde motion, which means that it travels westward. Saturn is also high enough for telescopes to capture its rings and moons.

Mars rises about a half-hour after Saturn. It shines with minus zero magnitude, appears about 10 arc-seconds in size and 88% illuminated. It is best studied before Dawn and large enough to begin to show surface features. Dwarf Planet Pluto, which is too small and faint for most amateur telescopes, lies about 5º east of Mars.

Asteroid 4Vesta is the final inhabitant of Sagittarius. Rising at 12:33 AM, it shines with 7th magnitude about 28º above the eastern horizon. It is best observed at 5:28 AM. However, its small apparent size requires the use of detailed finder charts from astronomical media.

If the astronomer sees meteors streaking from the constellation Lyra, he is witnessing the beginning of the annual Lyrid meteor shower. Although it peaks on April 22nd, meteors may occasionally appear before that date.

Saturn is high in the Dawn sky. 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 when on the other.

Recently, two groups of 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 beneath, 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.

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