In the Sky This Month

If you’re south of about Dallas, look low in the southwest in the early evening twilight for Mercury. The little planet looks like a bright star, but you need a clear horizon to spot it.

3I/ATLAS, a visitor from beyond the solar system, will make its closest approach to the Sun tomorrow, at 126 million miles. After that it will head back toward interstellar space. The comet is hidden in the Sun’s glare now, but will return to view in December.

The Summer Triangle is low in the west and northwest in late evening. Its three bright stars stand high overhead during the short nights of summer, but slip from view as we move toward the longer nights of winter.

The Big Dipper is plunging toward the northern horizon as night falls. If you line up the stars at the outer edge of its bowl and follow that line to the upper right, the first moderately bright star you come to is Polaris, the Pole Star or North Star.

Earlier this year, scientists in Europe beamed “The Blue Danube” waltz toward the Voyager 2 spacecraft. Voyager is passing near Rasalhague, the brightest star of Ophiuchus, the serpent bearer. The star is in the west-southwest at nightfall, and is easy to see.

Mirach, the second-brightest star of Andromeda, has passed through the prime phase of life, and now is in the red-giant phase. It’s puffed up to about 85 times the diameter of the Sun, making it shine about 1700 times brighter than the Sun.

For skywatchers in the far-southern United States, the constellation Pictor, the painter’s easel, is barely in view, low in the south, before dawn.

Alderamin, the brightest star of Cepheus, the king, shines high in the northern sky. It stands directly above Polaris, the North Star, a couple of hours after sunset. Alderamin is a white star, about 50 light-years from Earth.

Mira, a star in the constellation Cetus, brightens and fades dramatically every 11 months. At its brightest, it’s fairly easy to see. At its faintest, it’s visible only through a telescope. It climbs into view in the east by 8:30 or 9 p.m.

Two comets, Lemmon and Swan, are sweeping across the western and southwestern evening sky this week. Lemmon is closest to Earth on October 21, and will appear to the upper right of the bright star Arcturus, which is in the west-north, low in the sky, in the evening twilight. The comet is a good target for binoculars and could become visible to the eye alone. Comet Swan is farther toward the southwest, to the upper left of teapot-shaped Sagittarius on the 21st. It, too, is a good binocular target. Both comets will slide to the left along the horizon during the rest of October.