So you like to sleep late but don’t want to miss the sunrise? This time of year should be your favorite. Sleep on – if you live in the Northern Hemisphere. The latest sunrises of 2015 – for mid-latitudes in the Northern Hemisphere – are happening around now. For example, sunrise time in the central U.S. – say, around Wichita, Kansas – for the next several days will be around 7:45 in the morning. Meanwhile, if you live in the Southern Hemisphere, your latest sunsets are happening around now, assuming you’re at mid-southern latitudes.
Many sky watchers notice this phenomenon, which is part of an unvarying sequence each year. For us in the Northern Hemisphere, the sequence is: earliest sunsets in early December, shortest days at the solstice around December 21, latest sunrises in early January. In the Southern Hemisphere, the sequence is: earliest sunrises in early December, longest day at the December solstice, latest sunsets in early January. This natural order is part of what we can expect, every year, from nature.
The December solstice always brings the shortest day to the Northern Hemisphere and the longest day to the Southern Hemisphere. But, clearly, the latest sunrises don’t coincide with the day of least daylight, and the latest sunsets don’t happen on the day of greatest daylight. Why not?
The main reason is that the Earth’s rotational axis is tilted 23.5o out of vertical to the plane of our orbit around the sun. A secondary reason is that the Earth’s orbit isn’t a perfect circle. Due to our eccentric orbit (that’s an orbit shaped like a squashed circle, with the sun slightly off its center), Earth travels fastest in January and slowest in July. Clock time gets a bit out of sync with sun time – by about the tune of 1/2 minute per day for several weeks around the December solstice.
Because solar noon (midday) comes later by the clock, so do the times of sunrise and sunset. The table below helps to explain:
For Philadelphia, Pennsylvania
|Date||Sunrise||Solar Noon (Midday)||Sunset||Daylight Hours|
|December 7||7:09 a.m.||11:52 a.m.||4:35 p.m.||9 hours 26 minutes|
|December 21||7:19 a.m.||11:59 a.m.||4:39 p.m.||9 hours 20 minutes|
|January 5||7:23 a.m.||12:06 p.m.||4:49 p.m.||9 hours 26 minutes|
The exact date for the latest sunrise or latest sunset varies by latitude. At present, mid-temperate latitudes in the Northern Hemisphere have their latest sunrises, while the Southern Hemisphere’s mid-temperate latitudes are watching their latest sunsets. At latitudes closer to the equator, the latest sunrise or latest sunset has yet to come. Closer to the Arctic and Antarctic Circles, the latest sunrise and latest sunset have already come and gone.
But in either the Northern or Southern Hemisphere, the sequence is always the same:
1) earliest sunset, winter solstice, latest sunrise
2) earliest sunrise, summer solstice, latest sunset
Bottom line: Notice the time of sunrise and sunset at this time of year. If you’re in the Northern Hemisphere, your latest sunrises are happening around now at mid-northern latitudes. If you’re in the Southern Hemisphere, mid-latitudes are watching the year’s latest sunsets. Enjoy them!