Happy solstice, everyone. The December solstice will come tomorrow at 17:11 Universal Time. At this instant, it’ll be at 11:11 a.m. for the Central U.S., just after sunrise for the Hawaiian Islands and New Zealand, noon for eastern North America, sunset in Africa and midnight in Asia. We in the Northern Hemisphere will have our shortest day and longest night of the year. And yet – if you consider the word day in another light – the longest days of the year come each year in December for the entire globe.
Day and night sides of Earth at December 2013 solstice
We’re talking about day not as the period of daylight – but as the interval from one solar noon – or midday – to the next.
In December, the days are about one-half minute longer than the average 24 hours.
Keep in mind that the clocks on our walls don’t measure the true length of a solar day. To measure the time from one solar noon to the next, you need a sundial. It can tell you the precise moment of local solar noon – when the sun reaches its highest point for the day.
So it’s December now, and that means the days are about one-half minute longer than the average 24 hours – for the entire globe. Half a minute longer doesn’t sound like much, but the difference adds up. Some two weeks ago, when the mid-northern latitudes were having their earliest sunsets, solar noon came seven minutes earlier by the clock that it does today. Two weeks from now, when mid-northern latitudes will wake up to their latest sunrises, solar noon will come about seven minutes later by the clock than on the December solstice.
It might sound confusing. But it’s an effect that many people notice. It causes the earliest sunsets of the year for the Northern Hemisphere to happen in early December, for example – not on tomorrow’s December solstice – despite the fact that the solstice brings our shortest day.
Likewise, the latest sunrises don’t happen on the December solstice. They happen in early January.
Days are always longer than 24 hours around the solstices (and less than 24 hours around the the equinoxes). However, we’re much closer to the sun on the December solstice than the June solstice, so the Earth at present moves a little faster than average in its orbit – which means we travel a little farther than average each day. As a result, Earth has to turn maximally on its axis each day for the sun to return to its noontime position.
So while winter is the shortest season in the Northern Hemisphere, ‘tis the season of bountifully long days.
Feature photo at top: Analemma on globe shows sun’s declination and difference (in minutes) between time as measured by the clock and sun. Image credit: Wikimedia Commons