Mars, 2024 November 24, 03:30 UT
The Moon waxes as she climbs through the autumnal constellations this week, passing near the bright evening planets along the way. First Quarter occurs on the 30th at 9:37 am Eastern Standard Time and marks the second time this phase falls in November. Look for Luna southeast of Saturn on the evening of the 29th. On December 1st you will find her just to the south of bright Jupiter. She closes out the week drawing a bead on ruddy Mars high in the eastern sky.
We are now entering the nearly month-long sequence of events that characterize the winter solstice “season”. The earliest sunsets of the year begin on December 2nd and last until December 12th. Here in Washington Old Sol sinks below the horizon at 4:46 pm EST during this interval, but by the 13th he gradually begins to set a tad later each night. Yet the year’s shortest day is still some three weeks off, occurring with the winter solstice on the 21st. So why does the time of sunset start to move later well before the solstice? The reason is that the time of latest sunrise won’t occur until the beginning of January. Right now sunrise occurs at 7:06 am EST, but by January 1st sunrise won’t occur until 7:27 am. As with the times of sunset, these late rising times will stick around for about 10 days before everything starts to shift to longer lengths of day.
These phenomena are a result of our need for precise timekeeping. Before accurate mechanical clocks were developed most people who cared about time used sundials to track the hours. The most important measurement to them was noon, when the Sun crossed the meridian at its highest altitude. This “apparent Sun” crossed the meridian at noon every day, setting the times of sunrise and sunset at the same interval of time before and after noon transit. However, as precision clocks were developed, it was quickly discovered that the Sun’s noon transit didn’t always occur at noon. The eccentricity of the Earth’s orbit causes the planet’s orbital speed to vary during the year, and this affects when the Sun appears to transit. Sometimes it’s “fast”, occurring before noon on the clock, sometimes it’s “slow”, transiting later. To compensate for this, timekeepers devised a “mean Sun” to define noon transit, and the difference between “apparent Sun” and “mean Sun” was the solution to the “equation of time”. Our clocks are attuned to the “mean Sun”, but sunrise and set phenomena are tied to the “apparent Sun”., causing the seeming discrepancy in rise and set times around the solstices. Look on almost any globe map of Earth and you will see a figure-8 diagram over the Pacific Ocean. That is the “analemma”, a graphic representation of the equation of time. Use it to compensate your sun dial and you’ll be in step with the rest of the world.
As the Moon waxes this week she climbs steadily northward along the ecliptic to join the bright stars of the rising Great Winter Circle. Her brightening glow does little to overcome the bright stars of the season, which all center on the distinctive figure of Orion, the Hunter. Within the bounds of the Circle you will find nine of the twenty-five brightest strs in the sky, and for added cheer this year there is the distinct ruddy glow of Mars.
Saturn is now best viewed shortly after sunset, where you will find the ringed planet in the southwestern sky. His low declination means that you only have until around 8:00 pm local time to get a good look at him in the telescope before he begins to get mired in the turbulence of Earth’s atmosphere. We have a few more weeks to enjoy his mellow glow. By the year’s end he will start to be swallowed up in fading twilight.
Jupiter is the showpiece of the early evening sky, appearing high in the south just after sunset. I often observe the giant planet during twilight, when the brighter sky background helps to bring out a bit more contrast in his dark cloud belts, and he should provide lots of interesting views into the late evening. While his apparent disc has lost about 15 percent of its size since opposition, his generous girth still offers the best planet views for smaller telescopes. On the evening of December 2nd you can watch the Great Red Spot rotate across the disc, and at 8:10 pm EST watch the moon Europa disappear behind the planet. One hour later Io will emerge from Jupiter’s shadow.
Ruddy Mars draws your attention away from Jupiter later in the evening as he beams his ruddy glow down from the “horns” of Taurus, the Bull. The red planet reaches opposition on the 8th, but his closest approach to Earth occurs on the 1st. While his apparent disc is just over 17 arcseconds in diameter, his high northerly perch means that larger telescopes won’t have to peer through dense layers of Earth’s atmosphere.