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Twilight cloud

Photo of the Week. Just a cloud at twilight...

Astronomy news for the week starting Friday, June 29, 2012.

We start our week with the Moon in the waxing gibbous phase as it approaches full on Tuesday, July 3, during daylight hours so that it will rise that night just barely past the phase. It thereafter gibbously wanes until it reaches third quarter next week. Just before full phase, the Moon goes barely south of Pluto, passing in front of the distant planet as seen from Antarctica, not that anyone will actually SEE it happen. Consistently, on Friday, June 29, Pluto is also opposite the Sun, almost lost among the myriad stars of northern ]sag-p.html">Sagittarius. Then, only two days before full, on Sunday the 1st, the Moon goes through perigee, where it is closest to Earth, the proximity in time helping to raise larger tides on the coasts.

Earth, on the other hand, does just the opposite, as on the night of Wednesday, the fourth of July, it passes aphelion, its farthest distance from the Sun on its elliptical orbit, 94.5 million miles (152.1 million kilometers), about 1.7 percent farther than average. Obviously, given July's heat, distance from the Sun has little to do with the seasons, which are caused almost entirely by the tilt of the Earth's rotation axis of 23.4 degrees relative to the orbital perpendicular.

Other planetary action is vastly better than that involving poor Pluto. Both rising around the beginning of morning twilight, Jupiter and brighter Venus again make a fine pairing, giving us something of a repeat of their marvelous evening conjunction last March. Not so close as before, Jupiter will stand above Venus for some time, the two gradually drawing apart. Look the morning of Sunday the first to see (from bottom to top) Aldebaran and the Hyades of Taurus, Venus, Jupiter, and above them all, the Pleiades.

On the other side of the sky, Mercury goes through greatest eastern elongation relative to the Sun on Saturday, June 30, the little planet setting just before the end of evening twilight. Transiting the meridian before sunset, Saturn is in the western celestial hemisphere when it becomes visible still to the north of Virgo's Spica, the two making a most notable pair. Farther over to the west, Mars, in western Virgo, falls to the southeast of Regulus, between it and Spica- Saturn, the red planet setting around midnight, Saturn going down an hour later.

The Moon interferes with our admiration of the constellations. But once it is out of the way, look far below the Saturn-Spica pair to find the bright stars of sprawling Centaurus, which blend in smoothly with those of Lupus (the Wolf) to the east, which in turn howls to the southwest of striking Scorpius.
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