March 27, 2008

Shortest Day Doesn't Have Earliest Sunset

Stupid Question ™
Dec. 17, 1998
By John Ruch
© 1998

Q: Why doesn’t the shortest day of the year—Dec. 21—have the earliest sunset, which came on Dec. 8?

A: Dec. 21 is the winter solstice, the date when the Sun reaches its southernmost position, and therefore lights the Northern Hemisphere for the shortest period of time.

Dec. 21 truly has the earliest sunset—and latest sunrise—of the year. The reason we think the earliest sunset happens on Dec. 8 is that the clocks we use don’t agree with the Sun about what time it is.

Things were simpler back in sundial days, when the Local Apparent Solar Time (LAST) based the time directly on the Sun’s position in the sky. When the Sun was highest, that was noon.

The shortest day had both the fewest hours of daylight before noon (meaning the latest sunrise) and the fewest after noon (earliest sunset). So far, so good.

However, irregularities in the movements of the Sun and Earth give us a lot of days that are slightly longer or shorter than 24 hours. Dec. 23, for example, is 24 hours and 29.8 seconds long.

The invention of good mechanical clocks led to the standardization of a day’s length into Local Mean Solar Time (LMST, headquartered at Greenwich, England as Greenwich Mean Time). It’s based on the average length of a solar day—24 hours.

While this average is useful, it’s only an average. “It’s based on solar motion, but is not equal to it,” said Tom Burns of Delaware, Ohio’s Perkins Observatory.

In fact, “clock” time differs from true solar time by up to 16 minutes.
But those imperfections aren’t enough to throw things off over in Greenwich, where Dec. 21 does have the earliest sunset and latest sunrise.
So why are the winter solstice and the earliest sunset 13 days apart here in Columbus, Ohio? The answer is big, wide standard time zones.

Time zones make life convenient—it’s easiest to pretend Chicago is exactly one hour behind us instead of some odd number of minutes—but the cost is even more deviation from solar time.

On its most accurate day of the year, in the solar sense, a clock in Columbus thinks it’s noon 32 minutes before the Sun peaks.

By our clocks, Columbus’ earliest sunset falls around Dec. 8 and our latest sunrise around Jan. 5. But that’s just because all the shifting we’ve done adds up.

It’s still a fact that Dec. 21 has the year’s shortest span of daylight (nine hours 19 minutes). If our noon matched the Sun’s noon, Dec. 21 would have both the latest sunrise and earliest sunset. Instead, our clocks tell us that Dec. 21 has the least amount of time between sunrise and sunset (about 7:50 a.m. to 5:09 p.m.).

“It helps to illustrate a really important point,” said Burns. “There ain’t no such thing as real time. Time is how you measure it.”

“Every time you look at a watch you’re living a lie.”

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