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Time varies because of turning of the Earth

The sun rises, the sun sets. It certainly looks as if the sun circles the Earth. If, instead, it’s the Earth that’s turning, why don’t we feel the wind in our face?

This was one of the many objections to the proposal that the Earth circles the sun, rather than the other way around. It’s a good question. The key is the realization that the Earth carries its atmosphere with it as it turns so there’s no wind to feel.



In Northern California, Earth carries us around at roughly 725 mph (with nary a breeze). At the equator, the Earth’s 25,000-mile circumference makes its 24-hour rotation at something over 1,000 mph.




There’s a mountain in Maine that’s the first part of the United States to see the sun rise each day. Thinking of this mountain as the prow of a ship, it’s not hard to visualize the world turning beneath the sun, the East Coast leading the nation, forging ahead, with the Midwest then the West Coast following behind. As far as the time of day is concerned, the East is always ahead of the West. The time in Europe, farther east than Maine, is even farther ahead.

Some thousands of years ago, people invented a way to measure circles by dividing them into 360 degrees, and we’ve been measuring circles that way ever since. One of the circles we can measure is the circumference of the Earth – the distance around the Earth’s beltline, around the equator.

The Earth turns through one complete circle – 360 degrees – every day. We can calculate our angular velocity – how many degrees we turn each hour – by simply dividing 360 degrees by 24 hours. The Earth turns 15 degrees per hour.

As a relic of the British Empire, we measure our distance around the world starting at zero degrees, defined as the north-south line going through the astronomical observatory at Greenwich, England. We define time by our distance from that line too.

Nevada County is a third of the way around the world from the Greenwich observatory – very close to 120 degrees, and 20 divided by 15 equals 8. So there’s an eight-hour difference between the time at Greenwich and our own. If we’re a third of the way around the world, the sun should rise on us a third of a day – eight hours – later.

This leads to a simple formula for converting Universal Time (UT, formerly known as Greenwich Mean Time (GMT), aka Zulu Time) to Pacific Standard Time (PST): Simply subtract eight hours from GMT (denoted in 24-hour time) to get PST. If it’s 10:00 UT, it’s 02:00 (2 a.m.) PST; if it’s 23:00 UT, it’s 15:00 (3 p.m.) PST.

One complication: If it’s earlier than 08:00 UT, you start getting into negative numbers. In this case, add 24 hours to UT before subtracting; the answer is Pacific Standard Time, the night before.

Trained as a biologist, Alan Stahler is also an amateur astronomer. He teaches biology and geology at Bitney Springs Charter High School. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).


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