October sees a parade of planets, meteor showers and more.
October is one of my favorite months for astronomy. Not only are temperatures cooler in the northern hemisphere, but nights are getting longer: no waiting until past 10 PM for dark skies.
The October Sky: Up north, the summertime Milky Way recedes to the west at dusk, while the Summer Triangle asterism of Vega, Altair and Deneb still rides high. To the east comes the large vacant spaces on Pegasus and Pisces, and with it the promise of galaxies beyond.
Meanwhile in the southern hemisphere sky, the great triple leap of bright stars Fomalhaut, Achernar and Canopus are gaining prominence, along with those famous two satellite dwarf galaxies orbiting our own: the Large and Small Magellanic Clouds.
Fun fact: back in the 19th century before the advent of astrophotography, most asteroids were discovered visually, looking like faint moving ‘stars’ slowly changing position from one night to the next. During this bygone era, more asteroids were discovered visually in the September-October season than any other time of the year. The reason for this is that the relatively vacant area of the ecliptic through Pisces made it easy to spot asteroids that just happened to be near perihelion at the time, versus the star-studded Milky Way regions running high at dusk in the summer and winter sky.
The Moon in October 2021: The Moon reaches New phase on October 6th, and Full on October 20th. This is the Full Hunter’s Moon, providing huntsmen with extra hours of illumination to stalk their prey before winter sets in. Unfortunately, there aren’t any interesting eclipses or lunar occultations in October, though we’re due for the beginning of the second and final eclipse season of 2021 next month, starting with a deep partial lunar eclipse on November 19th.
The Planetary Rundown in October 2021: You can’t miss Saturn and Jupiter this month, riding high to the east at dusk. Venus rules the western sky after sunset, reaching greatest elongation 47 degrees east of the Sun on October 29th, shinning at a brilliant magnitude -4.5. You can actually nab Venus in the daytime… if you know exactly where to look for it against the blank blue sky. A good time to complete this feat of visual athletics is on October 9th, using the nearby waxing crescent Moon as a guide.
Meanwhile, Mercury emerges in the dawn, and reaches greatest dawn elongation one last time for 2021, 18 degrees west of the Sun on October 25th. Only bashful Mars is absent from the sky show this month, reaching solar conjunction on the farside of the Sun on October 8th.
Meteor Showers: Early October is Taurid fireball season, as bolides from Comet 2P Encke pick up in activity. The peak in 2021 is set for October 10th, with the waxing crescent Moon mostly absent from the sky scene. The International Meteor Organization also notes that we may be in for a surprise outburst from the southern constellation of Ara the Altar on the night of October 7th, courtesy of periodic Comet 15P Finlay.
Comets: Speaking of comets, Comet 29P/Schwassmann-Wachmann 1 surprised observers with a quadruple outburst in late September, bringing it up to around magnitude +11. One more outburst ‘could’ bring the comet up into the +10th magnitude or higher range, into the grasp of a small telescope or binoculars. Comet 29P is currently in the constellation of Auriga the Charioteer, and rides high in the pre-dawn hours for northern observers.
Deep Sky Highlight (Northern Hemisphere): Messier 31 – If you look back at star charts older than a century, you’ll see a curious object marked as the ‘Andromeda Nebula.’ this smudge in the constellation of the same name has been noticed since pre-telescopic times, and is in Charles Messier’s deep-sky catalog as M31. Astronomer Edwin Hubble make his classic discovery of a Cepheid variable in Andromeda in 1923, allowing him to calculate its stupendous distance, along with a startling revelation: at 2.5 million light-years distant, M31 is a galaxy, an ‘island universe’ in its own right. Along with nearby M33 in Triangulum, M31 is one of the most distant objects you can see with the naked eye. Though its bright core fills up a telescope’s field of view, its extensions actually cover three full degrees, the size of six Full Moons across.
In about four billion years, the Milky Way and the Andromeda Galaxy will merge into one massive ‘Milkomeda’ mega-galaxy.
Deep Sky Highlight (Southern Hemisphere): When Ferdinand Magellan made his historic trip around the world in 1519, his crew noticed a pair several luminous fixed ‘clouds’ in the deep southern sky. Though cultures had noticed them long before, the names for the two—the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC)—stuck. Today, we know that the ‘clouds’ are actually irregular satellite galaxies of our own Milky Way.
The LMC is is 163,000 light-years distant in the constellation Dorado, and the SMC is 200,000 light-years away in Tucana. Both ride high in the dusk southern sky in October, and are highlights for many a northern astronomer to check off of their life list, when they head south.
Challenge Object (Northern Hemisphere): In 1995, astronomers announced the first discovery of a planet orbiting a main sequence star. 51 Pegasi b (51 Peg) was found using the radial velocity method, watching for tell-tale tugs on the primary star induced by an unseen companion. And though you won’t see the exoplanet, 51 Peg is a decent +5.5 magnitude star easily visible for a small telescope in October, about mid-way between Alpha and Beta Pegasi along the edge of the Square of Pegasus asterism.
Challenge Object (Southern Hemisphere): What’s the closest star to our solar system? It isn’t Alpha Centauri, shinning bright to southwest in October at dusk. It’s actually the pair’s third companion, the +11th magnitude red dwarf star Proxima Centauri about two degrees away. 4.2 light-years distant, Proxima orbits the Alpha-Beta pair once every half and million years.
As of 2021, we know of two exoplanets orbiting Proxima: a 1.6x mass ‘hot Jupiter’ in a speedy five day orbit, and a seven Jupiter mass world in a five year orbit. Be sure to check out Proxima and ponder whether humans will one day make the interstellar leap to these tantalizing worlds.
Top Astronomy Events for October 2021
20-Full Hunter’s Moon
25-Mercury at greatest (dawn) elongation
31-Halloween/DST ends (UK/EU)
29-Venus at greatest (dusk) elongation