Highlights of the Season: October-December 2023

• The Sun, the Moon, and Earth come into alignment twice this season, causing eclipses of the Sun and Moon, respectively. The first event on October 14 is an annular solar eclipse, during which observers will see the Moon move in front of the Sun. However, since the Moon is slightly farther away from Earth and appears smaller than during a total eclipse, it doesn't completely cover the Sun from view, as during a total solar eclipse. Instead, a bright ring of the Sun's edge remains visible around the Moon. This annulus is bright enough to wash the faint corona (or outer atmosphere) of the Sun from view. The narrow path of annularity runs from the Pacific Northwest toward the Gulf of Mexico, across the Yucatan Peninsula and into Belize, Guatemala, Honduras, Nicaragua, Costa Rica, Panama, Colombia, and Brazil. Observers in all other parts of the US using safe viewing methods will experience a partial solar eclipse, during which less of the solar disk is hidden from view. Details for this eclipse can be found at https://www.timeanddate.com/eclipse/solar/2023-october-14.
• The October 28-29 event is not visible from most of the US, being centered over Saudi Arabia. Even from there, this is a barely perceptible partial lunar eclipse, during which the full Moon skims the edge of Earth's shadow. Observant skywatchers will see only a tiny nibble of the lunar disk being obscured from view, with Earth's shadow crossing only 12% of the way across the Moon's diameter. https://www.timeanddate.com/eclipse/lunar/2023-october-28.

As the saying goes, "Spring forward, fall back," and most of the US falls back on November 5 and returns to standard time, setting clocks back an hour…except in Hawaii, most of Arizona, and US territories including Puerto Rico, Guam, the Virgin Islands, American Samoa, and the Northern Mariana Islands. Those locations don't observe Daylight Saving Time, so no change in the clocks is called for. Conversion back and forth between Standard and Daylight Time continues to be controversial, and in 2022 the Senate voted to make Daylight Time permanent, but the proposal was not approved by the House of Representatives.

Every comet leaves a dusty trail of sand-size particles (meteoroids) behind it, some crossing Earth's orbit. When Earth orbits through one of these trails, the dust particles (which are just minding their own business) are slammed into by a rocky, 6 billion trillion metric ton planet moving 66,000 miles per hour. Depending on what direction they're moving, they can enter the atmosphere at anywhere between 25,000-160,000 mph, fast enough for ram pressure to compress the air in front of them and heat it up to thousands of degrees. This incinerates the particle and causes the streak of light known as a meteor. There are many sporadic meteors that are not associated with comets, and the greater number seen when Earth passes through a comet's dust trail is called a meteor shower.

Three of the year's four remaining major displays of "shooting stars" are favored by Moon phases that are at first quarter or less. In these phases, the Moon sets by midnight, after which Earth's rotation carries observers to face the direction toward which Earth is traveling into the meteoroid stream producing the meteors. Because of this, more meteors are typically observed closer to morning than just after sunset. (https://imo.net/resources/calendar).

Here's the story of each shower:

• The Orionid shower, named after the constellation Orion the Hunter, is caused by Earth's passage through the trail of dust particles shed by Halley's Comet from October 2-November 7, peaking on the morning of the 22nd. As they get swept up by our planet, observers on the ground see 10-20 meteors per hour appear to radiate from just off Betelgeuse (the shoulder of Orion). Best observed around 2 am—well after the Moon sets at midnight, so moonlight won't interfere.
• The Leonids are usually a fairly lightweight display, active November 3-December 2, but every 33 years, Earth passes through a particularly dense portion of the dust stream left by Comet Tempel-Tuttle. This can produce a "meteor storm," which is defined by a rate of over 1000 meteors per hour, famously noted in 1866 and 1966! Another storm was observed in 2001, so mark your calendars and cross your fingers for 2034! This shower is named for Leo the Lion, from which meteors appear to radiate.
• Of the major displays this season, the Geminids (named after Gemini the Twins) usually put on the best show, caused by Earth's passage through the dust trail of "rock comet" 3200 Phaethon and producing 80-100 relatively slow-moving, often yellowish meteors per hour under ideal conditions. The rate is roughly halved by city lights and moonlight, and even that reduced rate isn't bad. However, many people aren't familiar with the Geminids, since they occur only days from the winter solstice, when people would rather stay inside on cold nights. This shower builds slowly, starting on December 4, peaking on the 14th, then quickly subsides by December 17.
• The Ursids, active roughly December 13-24, are probably the weakest and least-observed of this season's major showers, easily outperformed by the Geminids, which peak a week earlier and overlap the Ursids. This year's display is somewhat hindered by the light of a nearly full Moon that sets 3 hours before sunrise, leaving a very short window for observing. Radiating from a point off the cup of the Little Dipper (aka Ursa Minor the Lesser Bear), this shower is caused by dust from Comet Tuttle. It produces 5-10 meteors per hour under ideal conditions, but in moonlight, this rate is reduced to 1-2 per hour.

Here's our video about how to observe meteor showers: https://www.youtube.com/watch?v=EBF4wFhw2Kg

^{Photo: Ed Sweeney from Los Gatos, CA, USA, 2009 Leonid Meteor (4111291263), CC BY 2.0}

• The December solstice on December 21 marks the start of winter in the northern hemisphere and the start of summer in the southern hemisphere.
• This is when Earth's north pole is tipped farthest away from the Sun, and as seen from the northern hemisphere, this causes our star to follow a low, short arc across the sky during the day (as observed from San Francisco, the Sun's greatest elevation at local noon is 28.8° above the southern horizon). For the northern hemisphere, the Sun's low path results in the shortest daylight period of the year, bringing with it the coldest weather.
• In San Francisco, the Sun on December 21 is above the horizon for 9 hours 33 minutes, and night lasts 14 hours 27 minutes. At the north pole, the Sun doesn't rise over the horizon at all between October and March.