Highlights of the Season: October-December 2019

The full Moon occurring on October 13 is the first after September's Harvest Moon, which was the full Moon closest to the autumnal equinox. In traditional lore, after the harvest, there are fewer places in the fields for small game to hide, making them easier prey for hunters. Hence, the name Hunter's Moon.

Meteor showers can occur if Earth's orbit is close enough to the dusty trail left behind by a comet that we pass through it. As the particles slam into the atmosphere at tremendous speed, they heat to thousands of degrees and burn up, causing occasional, brief streaks seen against the night sky. Comets were once thought to be random celestial apparitions, but in 1705, Edmond Halley suggested that comets that were observed in 1531, 1607, and 1682 were all the same object returning to the inner solar system every 75 or 76 years. He successfully predicted its return in 1658, and the comet was named in tribute to him.

It's rare that Earth gets two meteor showers from the same comet, but the orbits of our planet and Halley's Comet intersect twice—producing the Eta Aquarid shower in May and this month's Orionid meteor shower, which peaks on October 21-22. Under moonless conditions and away from city lights, observers might see 20-25 meteors per hour, radiating from the constellation Orion the Hunter, for which they're named. Orionid meteors are among the faster meteors and can leave glowing trails, or "trains," that linger behind them. However, during the peak, the light of the last quarter Moon might wash these trains and fainter meteors from view, reducing the number seen to about 5-10 per hour. For the best chance at seeing a shower, don't limit yourself to looking just on the night of the peak—try the nights before and after, just to be sure.

On November 11, weather-permitting and using safe solar-viewing techniques, skywatchers can observe the smallest of the planets slowly cross in front of the Sun. This is a rare event called a Mercury transit during which Mercury appears as a tiny dot moving across the Sun's disk—so tiny that it would take nearly 200 of them to span the Sun's diameter.

Since the transit lasts about 5-1/2 hours from start to finish, some portion of it will be visible from most of Earth's daytime side, except central and eastern Asia, Indonesia, and Australia. For observers in San Francisco, the transit is already underway at sunrise (6:46 am local time), with Mercury nearly halfway across the Sun's disk. Mid-transit occurs at 7:20 am, when the Sun is quite low—only about six degrees above the horizon. The end of the eclipse is at 10:03 am, when the Sun is about 30 degrees above the horizon (times are PST and will vary for other time zones).

Transits of Mercury happen about 13 or 14 times each century, with the last one seen from Earth in 2016, and the next one in 2032, although it will be centered over Madagascar and not visible from the US (the next Mercury transit visible from San Francisco will occur in 2049).

On November 17, one of the most famous meteor showers of all time peaks, although it won't display the same rate that made it famous. The Leonid shower is a variable display, most sources listing an average rate of 20 meteors per hour under ideal conditions, radiating from the constellation Leo the Lion. However, every 33 years or so, when the shower's parent comet—55P/Tempel-Tuttle—returns through the inner solar system to replenish the reserves of dust along its path, a meteor storm can occur as Earth passes through. The key word here is "can"...the shower has not always delivered as expected, but the greatest known Leonid displays occurred in 1833, 1866, and 1966, when observers estimated bursts equivalent to a rate of more than 1000 meteors per hour. Such a spectacular display isn't expected to be possible until around 2033.

This year's peak coincides with a waning gibbous Moon that rises around 9:45 pm PST and whose bright light will interfere, washing fainter meteors from view.

The most reliable meteor shower of the year—surpassing the performance of even the Perseids of August—peaks on the night of December 13 and into the morning hours of the 14th.

Under ideal conditions, the Geminids are often described as producing more than 100 meteors per hour, radiating from the constellation Gemini the Twins. However, this year's display will be diminished by the light of a waning gibbous Moon, which will be bright enough to wash the fainter meteors from view. Under these conditions, observers should then expect to see about 20-30 meteors per hour. Most people in the northern hemisphere aren't as familiar with the Geminids as they are with the Perseids of August, because the former are a winter display, occurring when it's often cold out and most people prefer to be inside.

The Geminid dust stream is associated with solar system object 3200 Phaethon, which is described as a "rock comet" that approaches the Sun closer than any other named asteroid. Phaethon's orbit is very comet-like, although the object is clearly depleted of the characteristic ices that comets are made of.

The winter solstice occurs on December 21, when the tilt of Earth's axis causes the north pole to lean farthest away from the Sun. For observers in the northern hemisphere, this makes the Sun rise and set farthest south on the horizon, making a low, short arc across the sky. This results in the shortest daylight period for the year, usually along with the coldest weather. Points farther north get less sunlight, and north of the Arctic Circle—66.5 degrees north latitude—direct sunlight does not reach Earth's surface at all. On the other hand, for the southern hemisphere, this is the first day of summer with the longest daylight period of the year.

On December 26, the Moon moves directly in front of the Sun, and eclipses it mostly from view. Unlike during a total solar eclipse, the Moon is a little farther away in its orbit from Earth. This makes it appear ever-so-slightly-smaller than the Sun so that it never completely covers the solar disk.

At mid-eclipse, a bright ring (or annulus) of the Sun's disk is still visible around the silhouette of the Moon. Thus, this event is known as an annular solar eclipse and is visible from parts of the Middle East, India and Sri Lanka, Malaysia, Indonesia, Singapore, and Guam. All of Asia and about half of Australia will see a partial solar eclipse, depending on how close observers are to the annular path.