Saturday night is a full moon. We are also less than a month after the summer solstice. The combination of a full moon and fewer hours without the sun means that there will be hardly any darkness for you to experience deep sky objects for the next few days.
We celebrated Independence Day on July 4. The Fourth is also the day of Earth’s aphelion, or the day that the Earth is farthest from the sun in its elliptical orbit. You might not be able to tell, but that means the sun is moving more slowly with respect to the distant stars. A convenient way to paraphrase Kepler’s second law is to say that planets move faster when they are closer to the sun and slower when they are farther away. So aphelion is the time of year that the sun would move the slowest through the sky.
Solar noon is the time when the sun is at its highest point in the sky. The average time between one solar noon and the next, or mean solar day, is 24 hours long. However, the length of a solar day varies slightly throughout the year. There are two things that make the length of a solar day vary. The first is the just mentioned eccentricity of the Earth’s orbit. The second is a geometric effect that has to do with the tilt of the Earth’s axis. The combined result is that the shortest day, by just more than 21 seconds, is Sept. 16. The longest, by almost 30 seconds, is Dec. 22, which happens to be very near the winter solstice.
Actually, the mean solar day is a couple of milliseconds longer than 24 hours because the Earth is slowing down from tidal interactions with the moon. Besides the extra day we get every leap year, we periodically have a leap second – an extra second added to a day to keep Coordinated Universal Time close to the Mean Solar Time. The most recent leap second was last Dec. 31.
The shorter and longer days are bunched together during the year, so that the solar noon can vary from the average value by up to 15 minutes. The average solar noon in Durango is about 12 minutes after the clock noon. The extra 12 minutes are from our longitude of almost 108 degrees west, which is 7.2 hours different from the prime meridian, rather than an exact 7 hours. Durango is lumped with Denver into the Mountain Time zone, but that doesn’t change when solar noon would be here. Solar noon in Durango on Sunday (July 9) is 1:17 p.m. The extra hour in the offset is the result of daylight saving time. If you use a clock and mark the 12 o’clock shadow of a sundial every day for a year, your marks will make a figure eight as the sun gets ahead of and then falls behind the average time. This figure is called an analemma.
With all the variations on the motion of the sun in the sky, it is quite impressive that historical eclipses could be predicted as well as they have been. I will talk more about the upcoming solar eclipse next month.
This monthThe summer Milky Way is one of the most spectacular things to see in the night sky. And with the Four Corners being among the darkest places in the country, you shouldn’t miss your chance to get out and see it.
Next week as the moon progresses from full to third quarter, the evenings will be getting quite dark again. Because of daylight saving time, it won’t get astronomically dark until after 10 p.m., but the view of the Milky Way as it rises through zenith is worth the wait. Binoculars, or very low-power, wide-field telescopes, are the best option for scanning the Milky Way.
This is an excellent time of year for planets. Jupiter and Saturn are favorite targets in the early evening, even before it gets totally dark. The moons of Jupiter and the rings of Saturn are easily visible with very low magnification but look great through high magnification. Venus is still the brightest thing in the morning sky and can be seen during daylight hours.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at firstname.lastname@example.org.