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Windblown clouds

Photo of the Week. The Sun hides behind wind-blown clouds.

Astronomy news for the week starting Friday, June 27, 2008.

The Moon runs through the latter part of its phases this week, starting just after third quarter (which took place last Thursday, June 26), so that we get to watch the crescent wane to new, that phase taking place on Wednesday, July 2, the waxing crescent not readily visible until the evening of Friday, the Fourth of July. As it travels near the ecliptic, the Moon passes well south of little Mercury the morning of Tuesday, July 1, but otherwise visits no other planets. That same day, Mercury reaches greatest western elongation, making it modestly visible in eastern morning twilight.

The first of the weeks's two "big events" is the passage of Mars less than a degree to the north of Regulus in Leo the night of Monday, June 30, the sight as good the following evening just as the sky is turning dark. With Saturn to the east of the pair, the trio will make a fine sight. Then watch over the next few evenings as Mars pulls in between the ringed planet and Regulus, the motion quite obvious (Mars and Saturn passing their own conjunction the night of July 10th). Look early, as the two now set in the west just after 11 PM Daylight Time, less than an hour after the end of evening twilight.

The second of the two events is the passage of the Earth through its aphelion point the morning of Friday, July 4 (helping celebrate the day), when the Earth is farthest from the Sun, at a distance of 94,513,144 miles (152,104,160 km), 1.7 percent farther than average. Given the obvious heat of the day (or the usual heat at least), the distance between the Earth and the Sun has little to do with the origin of the seasons, which are caused instead by the 23.4 degree tilt of the terrestrial rotation axis relative to the orbital perpendicular. A bit over two days before aphelion passage, the Moon goes through its perigee , where it is closest to the Earth.

Among the ancient planets, that pretty much leaves us with Jupiter. Beautifully set just to the northwest of classic Sagittarius, the giant planet is nearing its opposition to the Sun on July 9. Now rising in bright twilight, by the time the sky is dark, Jupiter dominates the southeast, crossing the meridian to the south a bit after local midnight (1 AM Daylight). Among the outer planets, Uranus begins retrograde motion on Friday, June 27.

Tracing the sky from Jupiter and Sagittarius to the west, first find wonderful Scorpius, and then a bit to the northwest much fainter Libra, the Scales, only two of its stars (Zubenelgenubi, the southern one, and Zubeneschamali) readily visible. Libra transits the meridian around 9 PM Daylight shortly before the sky is fully dark. Farther to the northwest lies Virgo (made notable by the bright star Spica), which some 2500 years ago took the Autumnal Equinox away from Libra as a result of the 26,000 year precession -- wobble -- in the Earth's rotation axis.
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