Yesterday, December 21, was the Winter Solstice here in the northern hemisphere. On that day, the Earth was in the part of its orbit where its north pole was tilted as far away from the Sun as it will get this year. The Sun appeared as far south in the sky as it will. That means that those of us in the northern hemisphere had out shortest day of the year. Where I live, in Texas, the Sun rose at 07:28 and set at 17:27 (Central Standard Time). That make the day just under ten hours long. The night, of course, was about fourteen hours long. After the equinox, the days begin to get longer. Granted, today we will get only seconds longer sunlight, but eventually that will add up. By the time that the Summer Solstice arrives in June, we will be getting about fourteen hours of daylight and ten hours of night (the reverse of the Winter Solstice). Of course, with the north pole tilted away from the Sun, the south pole it tilted towards the Sun. That means that the southern hemisphere is having their longest days and shortest nights right now, just like we will at the end of June.
Here in the United States, we declare that winter starts at the Winter Solstice and ends at the Vernal Equinox. Autumn starts at the autumnal equinox and ends at the winter solstice. I’ve written about the seasons before, and I think that this is perhaps not the best way to declare the seasons, but I suppose that it works well enough. Given that the solstices and equinoxes are well defined points in time, they make convenient dates to mark on the calendar. That gives the television weather people something to talk about. Really, of course, the weather is not significantly different on average on December 20 from what it is on December 22. But, as I said, these are convenient dates to mark. But, there is one point that they often get wrong. Almost every weather forecast on the Winter Solstice says that the solstice marks the shortest day of the year. This year, that was actually true, because the solstice occurred at 17:45 UT (that is 11:45 AM Central Standard Time). Sometimes, though, when the solstice occurs during the night, early morning, or late afternoon, it is actually the calendar day before or the day after the date of solstice that is actually the shortest day. The calendar date of the solstice is the shortest if the actual moment of the solstice is near the middle of the day. If you want to determine how long the day is at your location, the US Naval Observatory has a really nice online utility to give sunrise and sunset data for one day at a time or even for one year. If you look at the data using that utility, one finds that for where I live, the Sun rose on December 21 at 7:28 AM and set at 5:27 PM. But, you find the same sunrise and sunset times for today, December 22. But, at this time of year, the rate at which the length of daylight changes is very slow. In fact, the slowest changes in the length of daylight occur near the solstices, and the fastest changes in the length of daylight occur near the equinoxes.
Frequently, when the weather reporters are talking about the solstices, they make a mistake. Almost every year I hear one of them saying that the solstice marks the shortest day of the year and the latest sunrise. Hey, that would just seem to make sense, right? After all, if it is the shortest day of the year, that would seam mean that the sun rises latest and sets earliest. Unfortunately, that is not correct. If you look at the sunrise/sunset utility that I mentioned before, you’ll find that the sun rises tomorrow at 7:29 AM and sets at 5:28 PM. The day is still just about as long, but the sun is rising later than today or the day of the solstice! In fact, you find that time of sunrise slips a bit until it is rising at 7:33 AM on January 7. Sunset on January 7 occurs at 5:38 PM, so that means that the day is longer on January 7 than it is now, but the sun still rises later. What gives?
The answer to this mystery resides in the motion of the Earth around the Sun.
In the diagram above, the Earth is rotating as it moves around the Sun. Imagine the Sun at some point well below the bottom of the computer screen. The Earth moves a little less than one degree around its orbit each day. That means that it must rotate a little more than one degree in order for a point on the Earth facing the Sun to face the Sun once again. This is the difference between what we call the sidereal day (the time that it takes to make one complete rotation) and the synodic day (the time that it takes to go from the Sun highest in the sky until the Sun is again at its highest in the sky). The sidereal day is just over 23 hours 56 minutes long. The synodic day is closer to 24 hours. So, the Earth must rotate almost 361 degrees in order for one solar day to occur. The stars, on the other hand, rise every 23 hours 56 minutes. That means that the stars will appear to rise and set about 4 minutes earlier every day.
But, it gets a little more complicated than that. Earth’s orbit is a little bit elliptical. Earth was farthest from the Sun on July 4 of this year (2009), and it will be closest to the Sun January 3, 2010. Note that it is the tilt of the Earth’s axis, not its distance from the Sun that causes the seasons! But, as a planet moves around the Sun in an elliptical orbit, it speeds up and slows down. The planet moves quickest at perihelion (closest to the Sun) and slowest at aphelion (farthest from the Sun). So, Earth is currently moving a bit faster in its orbit than average. That means, however, that in order to complete one synodic day, or solar day (a point facing the Sun to once again face the Sun), the Earth will have to turn a little more near perihelion than it would near aphelion. That makes the days longer. Indeed, the length of the synodic day changes over the year, being longest near perihelion and shortest near aphelion. The synodic day (solar day) can range from about 23 hours 59 minutes 38 seconds to about 24 hours 29 seconds. That isn’t much difference, but it adds up. It would be a bit confusing if the clocks had to run at different speeds at different times of the year. Thus, clocks run at a constant rate set by the average length of the solar day over the year: 24 hours. The actual day isn’t far off of that, but with a difference of nearly half a minute per day, the discrepancy adds up. After a couple days, the clock will be a minute off. That is one reason that the sun doesn’t always appear highest in the sky (local noon) at the same time of day every day. Clock time and solar time are off a bit. To correct between solar time and clock time, you need to know the equation of time, which will tell you how much to add or subtract from a sundial’s time to find the clock time. With the solar (synodic) days out of sync with clock time, then solar events like sunrise, sunset, solar noon, etc, tend to be out of sync with the clock. They slip a little each day at this time of year. That is the reason that the latest sunrise does not occur at the Winter Solstice. If the Earth had a perfectly circular orbit, then the latest sunrise and the solstice would occur at the same time, but Earth’s elliptical orbit skews things a bit.