Highlights of the Season: October-December 2021

The first full Moon of the season falls on October 6 and is known traditionally as the Hunter's Moon. This name is usually given to the full Moon following the Harvest Moon, which is the full Moon closest to the autumnal equinox. According to conventional wisdom, after fields are harvested, there are presumably fewer places for game animals to hide, making hunting easier, especially for night-prowlers that benefit from the extra light provided by the bright Moon.

Earth passes through the dust trail of Halley's Comet twice each year, the first time being May 1-12, when we see the Eta Aquarid meteors, and the second from October 17-26. This produces the Orionid meteors, featuring some of the swiftest "shooting stars," although not very bright ones. This year, the shower peaks only a day after a full Moon, so they may be difficult to see in the bright moonlight. Although 4-6 sporadic meteors can be seen per hour on any clear, moonless night, the Orionid shower may at its peak on October 20 produce 10-20 per hour radiating from the direction of Orion the Hunter.

Daylight Time ends at 2 am Sunday, November 7, when most of us set our clocks back one hour and return to Standard Time. This means the 1-2 am hour repeats, and we get an extra hour of sleep...theoretically, anyway. Two US states do not observe Daylight Time, so the change does not apply to them. One is Arizona, with the exception of the Navajo Nation in the northeastern corner of the state. This overlaps small parts of Utah and New Mexico, so for consistency across the entire Navajo territory, Daylight Time is observed. Also Hawaii and other US island territories—namely, Guam, American Samoa, the Northern Mariana Islands, Puerto Rico, and the US Virgin Islands—don't observe Daylight Time and aren't affected. Daylight Time is intended to shift working hours to be a better (although still imperfect) match to the slow change in the length of the daylight period, but the adjustment offers no advantage in the tropics, where the Sun rises and sets at approximately the same time through the year, with little change in daylight hours.

The annual Leonid meteor shower is active roughly November 14-20 and even under ideal, moonless conditions is usually a modest display with about 10-15 meteors per hour. These are caused by particles of dust from Comet Tempel-Tuttle burning up in the atmosphere as Earth sweeps them up. However, at intervals of about 33 years, following the passage of the shower's parent comet, we pass through a replenished portion of the dust stream, creating the potential for a spectacle known as a meteor storm, which is defined as a display of more than 1000 meteors per hour, or roughly 17 per minute. The Leonids have produced that and more, including what has gone into the record books as the most spectacular shower in history. This occurred in 1966, with observers reporting bursts of 40 meteors per second!

On November 19, a partial eclipse of the Moon occurs as the full Moon slips into Earth's shadow, but not quite enough to result in a truly total lunar eclipse. At maximum, the dark, inner portion of our planet's shadow (the umbra) doesn't completely envelop the Moon, covering 97% of its diameter. This leaves a tiny but still-bright sliver of the lunar disk in direct sunlight. Ideally timed for the Pacific Ocean, the entire eclipse is visible from the West Coast of the US, starting at 10:02 pm PST, with mid-eclipse at 1:02 am on the morning of the 20th, and ending three hours later at 4:03 am. Observing details for San Francisco can be found at www.timeanddate.com/eclipse/in/usa/san-francisco.

On December 4, as the new Moon passes between Earth and the Sun, its shadow barely skims along Earth's south polar region, cutting across West Antarctica from Marie Byrd Land to the Ross Ice Shelf. From that narrow shadow-path, observers will see a total solar eclipse, during which the Moon completely blocks the Sun's bright disk from view, allowing the Sun's faint outer atmosphere, or corona, to become visible—but only briefly. Totality, when the corona can be seen, lasts less than two minutes. Because of its remote location, however, eclipse-chasers won't find totality easy to observe. Partiality will be visible throughout Antarctica, and (only barely) from the rest of Antarctica.

The Geminid meteor shower is active December 4-16, when Earth plows through the dusty trail of asteroid (or "rock comet," as it's also called) 3200 Phaeton. This produces the most reliable shower of the year, with 80-100 meteors per hour under ideal conditions. These radiate from the direction of the constellation Gemini the Twins, after which the display is named and which is already visible in the east-northeast at nightfall, providing a longer viewing window that other showers, which are usually best observed starting at midnight. However, this year's peak on the night of December 13-14 coincides with a waxing gibbous Moon whose light washes fainter meteors from view until moonset around 3 am. This leaves about 3 hours of dark viewing time before morning twilight starts to grow in the east.

This shower isn't as well-known in the Northern Hemisphere as the Perseid meteors of mid-August which peak when the weather is generally warmer and much more pleasant. The Geminids, on the other hand, take place when the weather is cold and sometimes damp, making it less enjoyable for observers to stay out late at night. We've mentioned our informative "How to Observe a Meteor Shower" video before, and we don't mind mentioning it again as a useful resource for meteor-watchers.

December 21 marks the "shortest day" of the year and therefore winter solstice for the northern hemisphere—but is it the beginning of winter or the middle? Astronomically, the solstices and equinoxes mark very neat points in Earth's orbit where the planet's axis of rotation is at an extreme tilt one way or another (the solstices) or at the points halfway between (the equinoxes). Modern common usage has settled upon using them indicate the beginning and end of the seasons, although it seems a bit anticlimactic to celebrate the first day of winter on the "shortest day" of the year, when day length is immediately going to start getting longer...or, by the same token, the first day of summer (the "longest day") as soon as the days start getting shorter. Meteorologically, however, differences in the rate at which land and oceans warm up and cool down (or their heat capacity) affects atmospheric circulation and weather and delays the warmest and coldest temperatures for up to two months after the solstices (something known as seasonal lag). For that reason, it makes some sense to mark the start of the seasons on the solstices, when the extremes in weather are still on the way.

On the other hand, some people mark the start of various seasons on "cross-quarter days," that fall halfway between the solstices and equinoxes. Those are February 2 (Candlemas or Groundhog Day), May 1 (May Day), August 1 (Lammas), and October 31 (Samhain, Halloween, or All Hallows Eve). This wraps the seasons more symmetrically around the solstices and equinoxes—in fact, in many regions, the solstices are traditionally known as "Midwinter's Day" and "Midsummer's Day."