Sunrise, Sunset...

Below is a post that I've shamelessly copied from the Mount Holly, NJ (Philadelphia area) National Weather Service office's Facebook page.  I somehow didn't realize that sunrise/sunset times don't exactly coincide with the longest and shortest days of the year....and here's why...


"The sun does not set any earlier than it does now. Below is an explanation as to why it does not coincide with the solstices either on the first days of winter or summer.

If you follow sunrise and sunset times, you’ll notice that on December 11th rounded to the nearest minute, the sun starts setting later in Philadelphia (436 pm) than it does today (435 pm). With the winter solstice still 12 days away how can this occur around the same time every year?

There are two reasons. One is that a solar day (defined as the time it takes to sun to return to your meridian (or longitude) is not the same throughout the year even though our clock days are always 24 hours. It takes the earth 23 hours and 56 minutes to make one complete rotation, but the earth is not stationary, its also revolving. On average it takes an extra four minutes for the sun to return to the same point in the sky because of the combination of rotation and revolution and is defined as a solar day.

This time of year, the earth is nearing its perihelion (closest distance to the sun) which will occur on January 2, 2013. This shows that the earth’s tilt is more important than its distance from the sun in causing our seasons. When the earth is closer to the sun, it is also revolving faster. Thus a solar day is longer than 24 hours. That extra time needed for the sun to return to the same meridian not only “causes” the sun to set later, but will also “causes” later sunrises into early January. The opposite, although not as pronounced, effect occurs around the start of summer. The reduced difference then has to do with reason number two. The sun starts rising later before the summer solstice and the latest sunsets are in very late June.

The second and larger reason is the earth’s tilt itself, called the obliquity of the axis. This one is more difficult to explain and astronomical web sites will give you better explanations than we can. In summary, the earth’s tilt causes the length of solar days to be at their longest around the solstices and shortest around the equinoxes. If you notice, the greatest gains and losses of daylight occur around the time of the equinoxes. Both factors combine to make this effect greater around the winter solstice, but offset each other somewhat around the summer solstice.

A bit of local Philadelphia area trivia as the chart shows: the sun will always set earlier on Thanksgiving Day than it does on Christmas Day even though Christmas is nearly at the winter solstice. Enjoy the later sunsets that are about to start!"