Highlights of the Season: January-March 2023

From January 1-5, Earth passes through the dust-stream left behind by asteroid 2003 EH1, a presumed dead comet or "rock comet." First observed in 1825, this shower was named for the then-recognized constellation Quadrans Muralis the Wall-Quadrant, from which the meteors seemed to radiate, located between Bootes the Herdsman and Draco the Dragon. In 1922, when the International Astronomical Union drew up its official list of the 88 constellations that we're familiar with today, Quadrans Muralis was left out, although astronomers continued to use the older name for the shower.

Peaking on the night of January 3-4, this strong display has a narrow peak of about 100 meteors per hour under ideal conditions. However, several factors consistently work against this shower, including typically poor winter weather, a very sharp peak lasting only about an hour around 2 am, and the light of the Moon—this year, a bright, waxing gibbous which drastically reduces the observable number to about 10-15 per hour.

The waxing gibbous Moon passes very near the planet Mars on the night of January 30. At about 9 pm local time, skywatchers in the San Francisco area will see the Moon sweep very close to Mars (within about a minute of arc, or 1/60 of a degree), but for observers located in the southern tier of states, Mexico, and Central America, the Moon will move in front of Mars and briefly block it from view—a lunar occultation of Mars similar to the one that occurred on December 7, 2022. A visibility map can be found here.

Photo credit: Sergio Scauso

February 2 is recognized as a cross-quarter day, halfway between a solstice and an equinox. Some argue that these halfway points—introduced to the English-speaking world by the Celts—should be used to mark the actual start of the seasons rather than the solstices or equinoxes themselves. This is because they then build to the seasonal climax promised by a solstice or an equinox rather than immediately spoil it on the first day of the season. After all, why celebrate the start of summer on the longest day, when the days will only start getting shorter? On the other hand, celebrations on the winter solstice often traditionally focus on the return of daylight, warmth, and the rebirth of the world, which can be observed through the winter, culminating in the spring. However you want to observe them, the cross-quarter days are:

• February 2: Candlemas or Groundhog Day (alternately, Imbolc or St. Brigid's Day on February 1)
• May 1: Beltane or Mayday
• August 1: Lammas
• November 1: Samhain (in modern times replaced by October 31 and Halloween)

In late January and early February, the news media are abuzz with coverage about the "green comet" passing through the inner solar system. Although it's expected to be the brightest comet of the year, most comets are extremely dim, requiring observers to view them through telescopes, and this one is no exception—many skywatchers have had to use binoculars to see it.

Formally designated C/2022 E3 (ZTF), this comet has an orbital period of about 50,000 years, meaning that if it was seen during its previous passage, it would've been by Neanderthals (possibly using binoculars). The greenish color reported is fairly common and is most obvious in photographs. It results from the breakup of carbon atoms in the comet's nucleus due to exposure to the Sun's ultraviolet light. Having just passed between the Big and Little Dippers at the end of January, the comet will continue on a path southward between Orion the Hunter and Taurus the Bull, passing near the bright star Capella on February 5, near Mars on February 10, and near the star Aldebaran (the eye of Taurus the Bull) on February 14. How bright it will be on those dates is uncertain, but its location near those objects (within 2 degrees and easily within the same field of view in binoculars) should help make it easier to find...IF it's still bright enough.

Observations suggest that it will quickly fade through February, and the light of the Moon, which is full on February 5, may also interfere with observing. Comet-watchers should view when the Moon is below the horizon and from dark locations either away from or at least shielded by trees from city lights. Trying to predict a comet's visibility is notoriously difficult, as a comet might break up or lose steam and fizzle out after passing the Sun. A few words of wisdom from famed comet-hunter David Levy are perhaps good to keep in mind: "Comets are like cats. They have tails and they do precisely what they want." Some handy finder charts can be found at aerith.net/comet/catalog/2022E3/2022E3.html.

Standard time ends on March 12, only four months after we returned to it back in November (seems like only yesterday), meaning that we've been observing mean solar time for only four months. With slight variations within each time zone, standard time is based on the Sun, which means that at noon, the Sun is on or near the meridian (the imaginary line that splits the sky, running from due north directly overhead to due south).

At noon, the shadow cast by a sundial's gnomon falls on the "XII" or "12." In fact, the word "noon" itself means midday and is sometimes abbreviated with the letter "m" for meridian. During the morning hours, when the Sun is east of this line, it is said to be "ante-meridiem," or before the meridian (am for short). At noon, it is on the meridian. After that, when the Sun is west of the line and descending, it is "post-meridiem," or after the meridian (or…wait for it…pm). But for ⅔ of the year—during Daylight Time from March through November, when clocks have been adjusted forward—noon is not when the Sun is on the meridian, and sundials are an hour off.

• The vernal (or spring) equinox is sometimes called the March equinox for clarity because of the way the term applies on either side of Earth's equator. While it marks the beginning of spring in the northern hemisphere, it's the beginning of autumn in the southern hemisphere.
• The March equinox can occur as early as March 19 and as late as March 21. This variation results from the fact that successive vernal equinoxes usually occur almost 6 hours later each year, but on leap years, it occurs about 18 hours earlier than the previous year. These two factors balance each other out, keeping the equinox within a day of March 20.
• While it's often said that the equinox is characterized by equal amounts of day and night, this is close, but not precisely the case. Part of this is due to the way sunrise and sunset are defined, whether it means the top of the Sun's ½°-diameter disk or the center is on a theoretical horizon. Because most calculations are made based on a point at the center of the Sun, rather than a disk. This causes a one-minute difference when compared with actual observation. Then, for the latitude of San Francisco (around 40° N), the period of time that some part of the Sun's disk is visible above an ideal horizon is about 8 minutes longer than it is completely below the horizon. This is due to the refractive effects of our atmosphere, which make the image of the Sun appear a few minutes earlier and then linger a few minutes later than the calculated times for sunrise and sunset, respectively.