It Could Be Worse. We Could Have Sidereal Time


2014-10-28 – Lefty’s first walk of the day is now in darkness. So is his last walk. The days are getting shorter. For just short of the next two months the daylight hours will be getting shorter. Then it will start getting longer, but it will take another two months to improve on what we have today. If you like light, that is.

It could be worse. We could be in sidereal time.

Regular solar time gives us a day of about 24 hours. This is more than one rotation of the Earth as you normally think of it. Each day, the Earth progresses in its orbit around the sun. After about 23 hours and 56 minutes—one rotation of the Earth—the sun is in a different position, so to make up the solar day, the Earth has to rotate for four more minutes.

Sidereal time is (more or less) one rotation of the Earth minus those four minutes. If you start measuring at dawn, for example, it would be four minutes into the next day before dawn came again. And this keeps up day after day until there is a whole extra day each year.

The cost of this is that noon could be when the sun is highest in the sky today but the high point will be four minutes later until six months from now noon will be in the middle of the night (and midnight will be the moment when the sun is highest).

You might say: “Who would do this?”

Astronomers do this (unless they are observing the sun). If you are looking at a galaxy far, far away, it will be at the same point in the sky at the same time every day, only if you use sidereal time.

If the whole world used sidereal time, there would be no need for time zones. We could all be on the same time because there would cease to be any correlation between clock time and the daily cycle of daylight and darkness.

Our biological clocks would be very confused. You would simply go to sleep and wake up a tiny bit earlier every day in relation to the light-dark cycle. Half the year you would go to work during daylight. Half the year you would go to work in the dark. But you would be on the same clock as people in London and New Delhi and Singapore.

Lefty wouldn’t care one bit. He has short periods of sleep and short periods of being awake all around the clock. Come to think of it, I do too.

You might say: “Who would do this?”

There is some evidence that people didn’t always sleep in eight-hour stretches in the middle of the night. The ancient ways are preserved in people like me who have insomnia. We didn’t even really keep time in hours and minutes and seconds until the last few hundred years.

Larger units like months and years have historically been kinda loosey goosey. The ancients had years that were nothing like 365 days. The Hebrew calendar is still 354 day. So is the Islamic calendar. This means that, if you want the seasons to coincide with the same months year after boring year, you have to make an adjustment. The Hebrew calendar does this by adding an entire month in seven out of every 19 years.

The Islamic calendar doesn’t make the adjustment, so Ramadan simply drifts through the seasons on the same 19 year cycle. If Ramadan is in summer this year, it will be in winter in nine or 10 years. It seems weird. But not as weird as sidereal time.

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