Questions answered

By Sherry Seethaler

SAN DIEGO UNION TRIBUNE
December 29, 2004

QUESTION: After the winter solstice (Dec. 21), I have noticed that the sunset is indeed getting later. The sunrise, however, seems to be disproportionately lagging in its contribution to the increased daylight hours. The Union-Tribune reports the time of sunrise and sunset daily, and after following it for several days, the sunset is getting later at a faster rate than the sunrise. Can you explain this?

– Darrell Shrader,
Solana Beach

ANSWER: After Dec. 21st, the sunrise should get earlier and sunset later, both extending the amount of daylight. As you point out, while the sun does begin to set later, for several days after the 21st the sunrise also gets later.

This curious phenomenon occurs because we rely on watch time rather than sundial time, and because after the winter solstice, it starts taking longer for the sun to make it from one east-west point in the sky back to that same point.

The Earth takes 23 hours and 56 minutes to spin around on its axis. But the average day is 24 hours long because as the earth is spinning, it is also revolving around the sun. The spin makes the sun appear to move from east to west. The revolution makes the sun appear to move from west to east. So for the sun to make it back to the same east-west point in the sky, the Earth needs to spin a little bit longer than one rotation – for about four more minutes.

Therefore, anything that increases the west to east motion will also increase the length of day (the Earth will need to spin a bit longer for the sun to get back to the same point in the sky). After the solstice, both the tilt of the earth on its axis and the variability in the speed of Earth's revolution around the sun increase the sun's west-to-east motion.

Because of Earth's tilt, as we revolve around the sun, the sun moves from being directly overhead at the Tropic of Cancer (latitude 23.5 degrees north) at our summer solstice to being directly overhead at the Tropic of Capricorn (latitude 23.5 degrees south) at winter solstice.

At the solstices, there is no north-south component of the sun's apparent motion, therefore the west-to-east motion is maximal.

Just after the winter solstice, the Earth, in its elliptical orbit, is closest to the sun. The increased gravitational pull of the sun causes the Earth to revolve faster.

Therefore sun time lags behind watch time, and according to our watches (but not a sundial), the sun rises later.

copyright 2004 The San Diego Union Tribune