Now that winter solstice has passed, most folks expect the days to get longer. The sun should begin to set a little later and rise a little earlier each day. But if you look at at a sunrise-sunset table, you'll find this is only half true until early January. In Portland, the "crossover" date, which I'll explain later, comes on January 5th in 2008. Until then, yes, our sunset is happening later, but sunrise also continues to get later! What's up with that?
The ominous sounding Equation of Time is responsible for the asymmetry in sun rise and set times.
Two factors determine sunrise and sunset time: the tilt of the earth's axis relative to it's orbit around the sun, and the fact that said orbit is not a circle but an ellipse. The elliptical orbit means the earth has to travel faster in it's orbit when it's closest to the sun, which occurs in early January. It's kind of like how a skater spins faster when she or he pulls their arms close in to their body.
But back to the sun. Around the solstices, the angle of the sun (caused by earth's tilted rotational axis) is changing very little day by day. But the earth's position in its orbit around the sun is changing a lot. So this factor dominates in determining the change in sunrise and set.
Why? We have to consider something called the Solar Day. This is the amount of time it takes for the sun to go from one spot in the sky back to that exact same spot the next day. And guess what? It's MORE than 24 hours around the solstices! In fact the time of the solar day varies throughout the year. The 24-hour clock we use is an average over the course of the year, and occasionally has to be adjusted by adding a "leap second".
But let's not make a complex discussion more complicated. Back to sunrise and sunset times. Because the solar day is longer than 24 hours, it pushes the perceived sunrise and sunset times later, starting before solstice! In Portland this year, our sunsets started getting later on December 15. This is the beginning "crossover date", which is the day when the length-of-solar-day portion of the equation of time dominates the tilt-of-the-earth's-axis portion. The ending crossover date is January 5th in 2008, when the tilt of the earth's axis portion of the Equation Of Time begins to dominate the tricky little solar day portion and time progresses the way we all expect it to.
So between December 15 and the solstice, the solar day-dominated equation of time works with later sunrises but against earlier sunsets. Between the solstice and January 5th in 2008, the solar day-dominated equation of time works with the expected later sunsets, but against earlier sunrises, making the sunrises later too, until the tilt of the earth's axis component of the equation of time begins to dominate.
Phew! It's a complicated little system.. this whole earth-sun thing. And it works the same way for summer solstice, but it's not as pronounced, because the earth is then farthest from the sun and moving slower (skater's arms held wide) so the length of the solar day, while longer than 24 hours, isn't as long as in December.
Attention Weather Geeks
If you want to meet fellow weather geeks in the Portland area, here's your chance:
What: Fun meeting of weather blog folks and Oregon Chapter of the AMS members of all ages and a trip over the the National Weather Service Office in PDX.
When: Saturday, Dec 29th @ 12 noon. Meet at the Stark Street Pizza Company located at 9234 SE Stark Street. Take the Stark St exit off of I-205 (near Rockey Butte). Click here for a map and directions.
After pizza, around 2pm, the weather posse heads for The National Weather Service office in NE PDX for a fun tour of the facilities up there.
Merry Solstice and Happy Holidays to all,
KGW Chief Meteorologist