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22 degree halo

Photo of the Week. The top of the 22-degree halo around the Sun, caused by refraction of sunlight in high ice-crystal clouds.

Astronomy news for the week starting Friday, July 18, 2008.

The Moon spends the entire week in the waning gibbous phase following full, which it passed the morning of Friday, July 18, the gibbous ending at the third quarter, which it will hit on Friday the 25th about the time of Moonset in North America. The entire week is also spent with the Moon climbing northeasterly along (or very close to) the ecliptic, starting out east of Sagittarius and ending in eastern Pisces to the northeast of the Vernal Equinox. As it travels along, the Moon passes just to the north of Neptune on Sunday the 20th, while two days later it similarly runs well north of Uranus, both events out of sight.

For some time now, the lunar orbit has been tilted just right such that the Moon regularly occults (passes in front of) Neptune, and this week's passage is no exception, the event occurring in east Asia and Alaska (during daylight). The farther out from the Sun, the slower the planet. Neptune has been stuck in easTern Capricornus to the north of Deneb Algedi (Delta Capricorni) for some time now, while somewhat faster-moving Uranus, steadily pulling to the east of Neptune, lies in eastern Aquarius just barely south of the border with Pisces.

The sky is becoming bereft of bright planets. Mars and Saturn (Mars now well to the east of the ringed planet, both east of Regulus) now set just after the end of evening twilight, and are harder and harder to find. Venus is not yet high enough for western evening viewing, while in the morning, Mercury rises far too late in twilight to see. That leaves us with the King of the Planets, Jupiter, which -- except for dim Uranus and Neptune -- has the sky to himself. Already well up the southeast at the end of twilight, the giant planet transits the meridian low to the south (for mid- northern latitudes) in northeastern Sagittarius around midnight Daylight Time, and does not set until twilight brightens the eastern sky.

The Season of the Dragon is upon us, as in early evening Draco stands high in the northern sky, most of the huge constellation circumpolar and perpetually visible from mid-northern latitudes. While the Big Dipper's front bowl stars famously point at the North Star (Polaris) at the end of the Little Dipper's handle, on the way they also more or less point to Giausar (Lambda Draconis) at the end of the Dragon's Tail, while Draco's squarish head falls between the Pole and Hercules, which lies just to the west of brilliant Vega.
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