The Sky This Week, 2013 December 3 – 10 — Naval Oceanography Portal

December 7th marks the beginning of the series of phenomena associated with the winter solstice. This is the evening of the year’s earliest sunset, which in the Washington, DC area occurs at 4:46 pm EST. From this evening onward Old Sol will set a little bit later on successive nights. The change is very incremental at first, but by the time the solstice occurs on the 21st sunset will be four minutes later. By the end of the year sunset will occur at 4:58 pm. The trade-off comes with the time of latest sunrise. That won’t occur until January 4th, 2012, when the Sun peeks over the horizon at 7:27 am. The shortest day of the year still falls halfway between these dates on the solstice itself, marking the astronomical beginning to the winter season. The reason for this seeming discrepancy has to do with the “equation of time”, which is the formula used to correct “sundial” or “apparent” solar time to “mean” solar time. This is graphically displayed on Earth globes as the “figure-8” diagram, the “annalema”, that’s usually printed over the expanse of the Pacific Ocean. The rate of change of the equation of time reaches its maximum near the time of the winter solstice, causing the times of sunrise and sunset to be “skewed” in the weeks surrounding the solstice itself.

The Sky This Week, 2013 December 3 – 10 — Naval Oceanography Portal