Daylight dwindles to a few wan hours during December in the Adirondacks. For astronomers, it's an exciting time of long nights, but most people look forward to the return of the light. Solstice means "Sun pause" and indicates the day when the sun stops its southward motion and begins moving north to bring us mud season and summer. If you watch the sun rise or set on this day at the latitude of Tupper Lake, it will cross the (theoretical flat) horizon 34 degrees south of due east or west. Though the position of sunrise and sunset will start moving north after that, it moves so slowly that the sun seems to pause at this endpoint and the northern one in June.
The winter solstice will occur at 6:38 p.m. on Dec. 21 this year. Before that event, we'll be treated to a total eclipse of the moon. There hasn't been a lunar eclipse on the winter solstice since 1638, so it might be worth stepping out for a look. At 1:32 a.m. on Dec. 21 (the night of Dec. 20-21), the moon will enter the darkest part of Earth's shadow, the umbra, beginning the partial eclipse. The entire moon will be in the umbra, totally eclipsed, from 2:41 a.m. to 3:53 a.m. It will then be partially eclipsed until 5:01 a.m. as it moves out of the umbra. For more information on eclipses go to the NASA Eclipse page at eclipse.gsfc.nasa.gov/eclipse.html.
Curiously, though the winter solstice is the shortest day of the year with 8 hours and 51 minutes of daylight, sunset on the solstice is not the earliest sunset of the year. In fact, the evenings are already getting longer! The earliest sunsets occurred on Dec. 9 and 10 at the Adirondack Public Observatory location. On those days, the sun set at 4:18:52 p.m. Tonight it will set at 4:19:20 p.m., and it will set at 4:21:47 pm on the solstice. Though we're gaining light in the evening, we continue to lose it in the morning. If you're a morning person, the darkness you face will continue to deepen into the new year. The latest sunrises won't occur until Jan. 2 and 3 at 7:33:36 a.m.
The difference in the occurrence of the earliest sunset, shortest day and latest sunrise are due to the fact that the solar day is not 24 hours long but varies through the year. Solar noon is when the sun crosses the meridian, the line from due north through the zenith (directly overhead) to due south. The time from one solar noon to the next is the solar day and is sometimes longer and sometimes shorter than 24 hours. Thus solar noon sometimes occurs before and sometimes after clock noon. If you were to photograph the sun at clock noon (disregarding daylight saving time) for a year, its position relative to the meridian would from a figure-8 pattern known as the Analemma and shown in Figure 1.
Sunrise and sunset are symmetric about solar noon, not clock noon, so their times relative to clock noon vary with solar noon's. Figures 2 and 3 show the Analemma at sunrise and sunset. In December, the sun crosses the meridian early so solar noon occurs before clock noon. Since it's already west of the meridian at noon, sunset will also occur early. On Dec. 29 (this year) solar noon will occur at clock noon. In January, solar noon will occur after clock noon and sunrise will also be late. The lengthening days will make us less aware of this phenomenon, but you can note it again in June when the earliest sunrises at APO occur on June 15 and 16, the longest day occurs on June 21, and the latest sunsets occur on June 26 and 27.
If you have any questions about
astronomy, please visit the Adirondack Public Observatory website at
apobservatory.org or email Aileen at email@example.com.