This year’s solstice – The longest one yet
*This post has been getting some eyes, five years after I wrote it, because of a story by Joseph Stromberg at Vox. As Ryan Hardy, a graduate student at the University of Colorado-Boulder was nice enough to point out in an email to Joseph, cc’ed to me, my original post (and hence Stromberg’s) oversold the importance of tidal friction in controlling the length of the day. While it is in general true that days, and hence solstices, are getting longer, the specific year-to-year variability can be swamped out by other factors. With his permission, I’ve posted Hardy’s email in full below.
Original text follows.
—
When we hear about the winter solstice being the shortest day of the year, we take it to mean we will have less sunlight hours today than any other day of the year. The tilt of Earth’s rotation around its axis causes the northern hemisphere to be tipped away from the Sun. So rather than rising high in the sky, the Sun skips along the horizon. This means less daylight hours today than yesterday.
Today, however, December 21st, 2009, is longer than yesterday. In fact, this year’s winter solstice has more daylight than any other winter solstice in history.
The simple reason is: the Earth’s spin is slowing down.
Tidal friction is one of the leading causes.
The moon’s gravity pulls not only the oceans towards it, causing tides, but it actually stretches the Earth.
These distortions cause friction which saps energy from the Earth. When the Moon was formed about 4.5 billion years ago, the day was about six hours long! And that wasn’t just six hours of sunshine, that was six hours for a full day/night cycle.
It’s not a speedy decrease, though. We’ve added only around two milliseconds to the length of the day in the last 190 years. Right now we get about nine hours of sunlight at the winter solstice in mid-latitudes. Next year, it will be a tiny little bit more.
So, if you are one of those who want to find philosophical meaning in everything… take solace in the fact that even on the darkest day of the year, it will only get brighter.
*images courtesy of Museum Victoria, and the Department of Physics at the University of Oregon*
—-
Ryan Hardy’s email:
Hi Joseph,
http://hpiers.obspm.fr/eop-pc/index.php?index=lod-1623&lang=en
http://www.nature.com/nature/journal/v499/n7457/full/nature12282.html
Trackbacks
- Večerašnja noć će biti najduža u povijesti Zemlje - Znanost Blog
- Tonight will be the longest night in Earth’s history | INSTRANET
- Long Winter’s Night: Tonight Will Be The Longest In Earth’s History
- 오늘 (북반구의) 밤은 지구 역사상 가장 긴 밤입니다 - NewsPeppermint
- Untitled | 041178
- Vox, The Gift Which Keeps Giving!!! Leftards Voxsplain How Tonight Is The Longest Night In The History Of The World!!!! | suyts space
- http://www.vox.com/2014/12/21/7424371/winter-solstice | The Today Online
- Correction: Tonight will not be the longest night in the history of Earth. | Slantpoint Democrat
- The 2014 winter solstice was not the longest night ever. | Did You Know?
- December 21st, 2014 Was the Longest Night Ever. Except it Wasn't.
Amusing, but not quite true.
The lengthening of the day is such a slow effect that if you measure length of night to that accuracy, precession of the equinoxes, and eccentricity and obliquity changes become important.
The is has 18 36 hours not 12 24 longitude lines around the world 36 of them18 hours am eighteen hours pm