The Moon enters last quarter. After midnight, observers will catch the Moon in its “quiet phase,” a time when the vast, dark lava plains of Mare Imbrium and Oceanus Procellarum are illuminated by the low-angle Sun. Binoculars and telescopes show the bright arc of the Apennine Mountains near the terminator. Craters Aristarchus, Kepler, and 93-kilometre-wide Copernicus are illuminated from lunar west, throwing into stark relief the walls and central peaks of these gigantic impact features. The quiet phase is a superb sight, too, in a mid-morning sky. Try looking in daytime for the bright ray systems of Kepler and Copernicus on the dark background of the lunar plains.
October 22 to 25
Halley’s Comet doesn’t return to the inner solar system until 2061. Every autumn, however, we can see bits and pieces of it burn in our upper atmosphere as our planet passes through the famous comet’s dusty trail.
On nights from October 21 to 24, head out to see the peak of the annual Orionid meteor shower. Although active from early October to early November, this week marks the shower’s peak rate of 50 to 70 meteors per hour. The waning Last-quarter Moon may wash out some fainter meteors, but there’s always the possibility of first- and zero-magnitude displays; long-lasting, glowing smoke trails; and a brighter fireball or two. Be sure to compare meteor brightness to known stars. Look, too, for colourful meteors. These colours are caused by materials, such as nickel and calcium, in the rocky comet debris that is heated to fluorescence by friction with our atmosphere.
The point in the night sky where the shower appears to originate—its radiant—rises in the east by 10 p.m. Look for the Orionids’ radiant between the stars Betelgeuse in Orion and Alhena in Gemini (Gamma Geminorum). As always with meteor showers, a dark-sky location is best.
October 24 and 25
As mighty Jupiter and Saturn fall to the west, our solar system’s most distant major planet becomes a prime telescopic target. At magnitude +7.8, Neptune isn’t visible to the unaided eye, but we can still find its position on the sky: between a pair of fourth-magnitude stars in Aquarius beneath the Circlet of Pisces. In the autumn of 1846, astronomer Johann Galle found Neptune near one of these two stars, Phi Aquarii, using the mathematical work of John Couch Adams and Urbain Le Verrier. Three decades ago, interplanetary probe Voyager 2 sailed by our solar system’s “god of the sea.”
In late October, telescopic observers can find Neptune about 1.5° west of Phi Aquarii. Just like Johann Galle, you’ll see a tiny bluish disc, 2.3 arc minutes wide, distinct from the points of distant background stars. You can search for Neptune’s large moon, too: late evening on the 25th, look for a magnitude 13.5 speck some 16 arc minutes west of Neptune. That’s Triton!
October 21 to 28
You’ve seen “the Sirius of Summer.” Now see the “Vega of Autumn!” Fomalhaut is the luminary of Piscis Austrinus, the Southern Fish, a smaller and fainter constellation than its zodiacal cousin, Pisces. For Canadian observers, Fomalhaut stands out as a lone first-magnitude star in the deep south of an October evening. Around 9 p.m., look for a twinkling, modestly bright, blue-white star due south in the chill sky. Keep in mind how infrared observations reveal that this hot, main-sequence star has several dusty rings with possibly a Neptune-mass planet fronting an outer stellar debris disk. Telescope and binocular users can hunt for a magnitude +6.48 yellow-orange companion star about 2° below Fomalhaut.