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Photo of the Week. Evening twilight.

Astronomy news for the two weeks starting Friday, April 13, 2012.

The Moon starts the week off, on Friday, April 13th, in its third quarter, after which it fades through waning crescent until it disappears into morning twilight, new phase finally taking place the night of Friday the 20th. As it goes, the Moon passes several degrees north of Neptune on Monday the 16th, then similarly to the north of Mercury and Uranus the night of Wednesday the 18th, not that any of these are readily visible, but perhaps it's nice to know.

The planets themselves more than make up for the lack of lunar passages. Bright Mars, high to the south near the meridian at 10 PM in Leo to the east of Regulus, ceases its"> retrograde motion on Sunday the 15th. It thereafter once again resumes its normal easterly trek against the stellar background. You can watch the red planet until it finally sets around 4:30 AM. Just six hours after the Martian event, Saturn goes through opposition with the Sun, when it rises at sunset, crosses the meridian at local midnight (1 AM Daylight), and sets at sunrise. Thus up all night, the ringed planet makes a fine sight coupled to Virgo's Spica, which lies somewhat to the southwest.

Next up is brilliant, unmistakable Venus, which continues to dominate twilight and early evening skies and does not set until 11:30 PM Daylight, the planet making a magnificent spring appearance. The night of Monday the 16th, it passes 10 degrees north of Aldebaran in Taurus. Venus has an almost-exact eight year cycle relative to Earth. We will thus get a similar show in 2020. On the other side of the sky and night, Venus's "inferior" (meaning closer to the Sun than we) partner Mercury goes through a rather poor greatest western elongation relative to the Sun on Wednesday the 18th, the little planet not rising until mid- twilight and essentially invisible. Jupiter, moving out of sight, sets just as twilight ends.

This is a fine season to admire the longest constellation of the sky, Hydra, the Water Serpent. It's raggedly-circular head can be found just to the east of Procyon in Canis Minor, the figure also making the southern apex of a triangle that includes the Sickle of Leo to the east and the bright pair Castor and Pollux in Gemini to the west. The constellation then continues to the southeast, diving beneath ancient Crater (the Cup) and box-like Corvus (the Crow), until it finally ends southeast of Spica (and currently Saturn).
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