Skylights featured on Astronomy Picture of the Day

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Astronomy news for the two weeks starting Friday, June 30, 2017.

The next Skylights will appear Friday, July 14.

We open up with the Moon at its first quarter on Friday, June 30, when the Moon will also be west- northwest of Spica in Virgo. That evening features a special treat, as will also occults the modestly bright star Porrima (Gamma Virginis). Porrima is a double with fairly equal components that is seen as a single star with the naked eye. As the leading dark edge of the Moon sweeps over Porrima, we will see a rare "two step" occultation, as the Moon first sweeps across the western star and then almost immediately hits the other. The first drop is not very great, and is just barely visible to the naked eye. Even a small telescope will separate the pair of stars, allowing the occultations to be seen separately. Occultations are fun to watch. Since a star is close to being a point source of light, it winks out almost instantly. The brief time it takes for the star to disappear behind the Moon allows the angular diameter, then (if the distance is known) the physical diameter, a parameter of immense astrophysical importance.

The night of Wednesday the 5th, look for the rising Moon to be several degrees north of Antares in Scorpius, then on the next night almost immediately above Saturn, which will lie between the classical figures of Scorpius and Sagittarius. By Friday the 7th, the Moon will be sitting almost directly atop Sagittarius's "Teapot." All this time the Moon is brightening in its waxing gibbous phase, heading towards full around midnight the night of Saturday the 8th.

The sky highlights two great freedom celebrations. On the third of July, the Earth passes aphelion, when it is farthest from the Sun, 94,505,901 million miles (152.1 million km) from then Sun, 0.017 percent farther than average. Seeing as it's hot outside, it's pretty clear that distance from the Sun has little to do with the seasons, which are caused almost entirely by the 23.4 degree tilt of the Earth's rotation axis relative to the orbital axis. Since the Moon is at apogee (farthest from Earth, 252,000 miles or 405,500 km), the two apogean dates average out to the Fourth of July, or close enough. Then on Bastille Day, Venus will pass but 3 degrees east of Aldebaran in Taurus, the color contrast quite striking. Continuing through the planets, Mercury sets in twilight as about late as possible for this appearance, Venus rises about as early as possible, an hour before dawn. Jupiter then sets about an hour after Saturn transits the meridian, and Pluto is in opposition to the Sun and in full (though tiny) retrograde motion against the stars of Sagittarius. For the time being, you can forget about Mars.

Feel hot? Then contemplate the constellations of winter, Orion chief among them, the constellation rising roughly along with Venus, the planet making its way between Orion and Auriga farther to the north. Back in the evening, Deneb (at the northeastern apex of the Summer Triangle crosses the meridian about an hour past midnight with the Milky Way streaming overhead, at least once the bright Moon is out of the Way.

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