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This year’s solstice – The longest one yet

December 21, 2009

*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,

I’m a graduate student at the University of Colorado who specializes in geodesy.  I want to point out that the claim that the Earth’s rotation period today is the longest ever is verifiably inaccurate.  Earth’s rotation period has been measured precisely with various geodetic and astronomical techniques for the past century, and has been further extended back in recorded history using records of solar eclipses.  Since 1962, the longest day on record (April 12th, 1972) was about 4.4 ± 0.7 milliseconds longer than the 86400 s atomic seconds defining the day.  On the record of annual average lengths of day, extending back to the 17th century, the longest day occurs sometime in 1912, with an excess length of about 3.9 milliseconds.  Today, the excess length of day is less than 1 millisecond, which is why we’ve only had three leap seconds since 1999.

While your description of tidal friction slowing Earth’s rotation is accurate, it’s important to realize it isn’t the only factor in Earth’s rotation.  You describe these effects as “minor” in the article, and on geologic timescales, this is mostly true, but on human timescales, other forces do matter.  The primary competitor with tidal friction is glacial isostatic adjustment, or the redistribution of mass in response to the loss of ice sheets from the last ice age.  This has the effect of appreciably slowing the rate of increase in Earth’s length of day: the rate of change in the Moon’s orbit suggests that the length of day should be increasing at 2.3 milliseconds per century, but the observational record has 1.7 milliseconds per century.  On decadal timescales, fluctuations in fluid circulation surrounding Earth’s core have an effect on the rotation of Earth’s crust relative to the rest of the planet.
The takeaway is that while the length of day averaged over the past century is indeed the longest in the planet’s history, it is plainly inaccurate to say that any particular day in the past 40 years is the longest.
Ryan Hardy
Graduate Student
Department of Aerospace Engineering Sciences
University of Colorado Boulder

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