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Photo of the Week. End of day.

Astronomy news for the week starting Friday, September 20, 2013.

We begin with the Moon just past full, which took place on Thursday, September 19. During the week, we thus get to watch the waning gibbous phase as the Moon heads toward third quarter the night of Thursday the 26th roughly about the time of Moonrise in North America. Only a few hours later, the Moon passes its apogee where it is farthest from the Earth. The night of Thursday the 26th (more the morning of the 27th), the Moon also invades southern Gemini, and will appear nicely to the west of bright Jupiter, with Castor and Pollux farther north.

Moving more or less in opposite directions relative to the Sun, Venus and Saturn are finally crossing paths, the two setting about the same time about as twilight ends (8:30 PM Daylight Time). Look the night of Tuesday the 24th to find them on a line parallel to the twilight horizon, much fainter Saturn to the right. With Jupiter rising in the northeast about midnight local time (1 AM Daylight), the morning planetary show is better. Wait two hours until 3 AM Daylight to see Mars rising in eastern Cancer between the bright constellations of Gemini (with Castor and Pollux) and Leo (with Regulus). At dawn, the two make a glorious sight Jupiter on top with the star Sirius down and to the right.

The big event, though, belongs to Earth, when on Sunday the 22nd at 3:44 PM Central Daylight Time (4:44 PM EDT, 2:44 MDT, 1:44 PDT) the Sun crosses the Autumnal Equinox in Virgo and autumn begins in the northern hemisphere (spring in the southern). On that day the Sun rises due east, sets due west, and days and nights are equal, ignoring twilight each about 12 hours long. The Sun also technically sets at the north pole and rises at the south pole. Go down there and watch, but bring a good coat.

In early evening, look toward the meridian to the south and upward to find Altair in Aquila, the star easily recognizable by its two flanking outliers, Alshain (Beta Aql) and Tarazed (Gamma). Look a little bit to the northeast to find Delphinus, the Dolphin, which looks like a hand with a finger pointing back to the southwest. On August 14, Delphinus was found to host a modest "nova" that just barely reached fourth magnitude and is now not visible to the naked eye. In a nova, a star in a double system feeds matter through tidal action onto a dense stellar remnant, a white dwarf. The new surface layer eventually explodes, enormously increasing the brightness of the system, which eventually returns to normal.
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