The year’s longest day is coming

Saturday, Dec 11, 2021 4:00 AM

Greetings, stargazers.

As strange as it may sound, we are approaching the longest day of the year this month. At least we are if we consider a day to be the time between one local noon and the next. In other words, a solar day.

This is an interesting puzzle that is related to something I get asked about more often – why don’t the sunrise and sunset times both have their respective maximums and minimums on Dec. 21, the first day of winter, and the day with the fewest daylight hours?

The Earth rotates at a very steady rate. It takes 23 hours, 56 minutes, and a bit over 4 seconds (a sidereal day) for the Earth to rotate exactly 360 degrees. That is how long it takes for a distant, “fixed” star to return to its location of the previous night. However, for the sun to get back overhead, the Earth must rotate almost 361 degrees, because during that one day, the Earth has traveled almost one degree in its orbit around the sun.

Local solar noon is the time of day that the sun crosses the meridian. That means the shadow on your sundial would be pointed due north. However, this doesn’t happen at the same time every day. Twenty-four hours is only the average time between noons. I have written previous columns discussing the analemma – that strange figure eight that you might have seen printed on a globe – but here it is again in a nutshell. If you put a mark on the base of your sundial where the tip of the shadow is every day when your phone says it is noon, the dots would form that figure eight.

The biggest change in where the noontime sun is during the year is simply because of the tilt of the Earth’s axis. The sun is much higher in the sky in the summer. However, there is also a change because the elliptical orbit causes the Earth to sometimes move faster in its orbit around the sun. This means sometimes local solar noon is earlier than average and sometimes it is later.

If the Earth is moving farther in its orbit around the sun, that means it will need to rotate a bit more to get the sun back overhead. While the local solar noontime is getting later from one day to the next, the solar days are longer than 24 hours. The faster the change, the longer the day, and the longest day happens to be Dec. 22. And while the local noon is shifting, the local sunrise and sunset are shifting by the same amount.

For the many of you who look forward to the sun setting a bit later in the evening, you are in luck, because the earliest sunset of the year already occurred on Dec. 6. Unfortunately, if you are waiting for the sun to start rising a bit earlier in the morning, you will have to wait for the latest sunrise to occur Jan. 5.

This month

Comet C/2021A1 (Leonard) was discovered earlier this year and should end up being the brightest one visible in 2021. But don’t expect anything as bright as Comet Neowise was last year. Most likely, Leonard will be only barely visible to the naked eye, and most easily seen using binoculars. This fast-moving comet was in the morning eastern sky before dawn early in December, but starting Dec. 14 it will be in the evening sky right after sunset. On Dec. 17, it should be right below Venus, so that might be an easy day to try to find it.

While you are out looking for Comet Leonard, Venus, Jupiter and Saturn continue to dominate the southern sky, and still make excellent binocular or telescope targets.

The Geminid meteor shower is often one of the better ones of the year, and it peaks on the night of Dec. 13. But for the second month in a row, an almost-full moon will make the best meteor shower of the month less spectacular than it might be other years.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.