Answers: Sunrise times after the winter solstice
By Jack Williams, USATODAY.com
Q: I live in Indianapolis, Ind., and want to know why the sun waits until January before it starts to rise earlier each day. It begins setting later on Dec. 22, but doesn't begin rising earlier for a long time.
A: Most of us know that the days (in the Northern Hemisphere) begin growing longer after the winter solstice, which falls on Dec. 21 or 22 most years in most time zones. (Related question and answer: Does winter really begin on Dec. 21 or 22?)
But, few people notice something unusual, sunrise does not occur earlier until well after the solstice. That is, until well into January the days grow longer because sunset is a little later each day while sunrise might actually occur later or hold at the same time for several days.
Exactly what happens depends on your latitude; that is, how far north of the equator you are.
The U.S. Naval Observatory table for sunrise and sunset times in Indianapolis shows that the sun rises there at rises at 8:03 a.m. and sets at 5:24 p.m. on Dec. 22. By Dec. 25, the sun rises at 8:04 and sets at 5:26. On Dec. 26, 27, 28, and 29 the sun rises at 8:05 but the sun rises a minute later each day. From Dec. 30 through Jan. 10 the run rises at 8:06 a.m. each day, but the time of sunset changes from 5:29 to 5:39 during this period. Finally, on Jan. 11, the run rises a minute easier than the previous day and rises earlier just about each day from then through June 20. (Related resources: Sun, moon, stars, time)
Why this occurs goes back to a couple of things.
• First, the Earth is nearest to the sun in January and is moving faster in its orbit around the sun than at other times of the year.
• Second, our 24-hour day is based on the average time it takes the Earth to spin once on its axis as measured from when the sun is highest in the sky (local noon) one day until noon the next day.
In very simple terms, at different times of the year, time as we measure it by clocks and the movement we see of the sun across the sky grow out of alignment. In effect, this is most noticeable - to those who notice such things - around this time of the year.
Instead of trying to explain the details of what's going on, I'll send you to some Web sites that have good answers. Just follow the links below.
• NASA Ask a High-Energy Astronomer: The equation of time.
• U.S. Naval Observatory: The dark days of winter.
• Staryskies.com: The latest sunrise and the equation of time
• Keith C. Heidorn, The Weather Doctor:Entering the Dark Days.