- 2800 - TIME - why 24 hours in a day? We count the days it takes for the Earth to make one complete orbit around the Sun and that is 365 days. Well, not exactly 365 days. Actually it takes 365.24219 days to complete the trip around the Sun. And, it turns out the Earth’s rotation on its axis is not the same 24 hours every day either.
----------------------- 2800 - TIME - why 24 hours in a day?
- Most of us are comfortable with our 24 hours in a day and 365 days in a year. We simply measure the time lapse in one complete rotation of Earth on its axis and say that is 24 hours.
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- Since 1820 the second has been defined as 1/86,400 of a Solar Day. (24h*60m*60s).
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- And, we count the days it takes for the Earth to make one complete orbit around the Sun and that is 365 days.
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- Well, not exactly 365 days. Actually it takes 365.24219 days to complete the trip around the Sun. And, it turns out the Earth’s rotation on its axis is not the same 24 hours every day either.
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- For most of human history we did not have clocks accurate enough to record the changes in Earth’s spin. But, over the last 50 years, atomic clocks have accuracies that can record the differences. Now, we have this problem that the atomic clocks are “more accurate” and we are getting our clocks out of step with the Earth’s rotation that is “less accurate“.
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- The Earth’s rotation rate is affected by changes in motion of the Earth’s core, the effects of ocean tides and weather, and changes in the shape of Earth due to plate tectonics and ice sheets.
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- Most of these changes are expected to naturally slow Earth’s rotation by a few microseconds each day. A microsecond is 10^-6 seconds = 0.000001 seconds. If our rotation slows down our days are getting longer and we need to add seconds to our clocks.
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- Just like we add a day with a leap year every 4 years to compensate for the orbit around the Sun taking 365.24219 days, we need to add a leap second to our day to account for the Earth’s rotation slowing down.
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- We started adding leap seconds to our days on June 30, 1962. The Universal Time clocks all ticked ……… 23h59m59s ……… 23h59m60s …….. 00h00m00s. The 23h59m60s was the leap second that was added to try and keep the Universal Time and the atomic clocks in sync.
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- To date we have added 23 leap seconds since 1972. The last one was in January, 2006. The average is to add a leap second every 18 months, on June 30, or on December 31.
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- However, the leap second added prior to 2006 was in 1999. We went through a period of 7 years with out adding a leap second. Because the Earth’s rotation is not perfectly predictable we do not know more that 6 months in advance if we should add a leap second or not. What’s going on?
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- We think it is global warming of the oceans and the atmosphere. The warmer currents are thinner and causing less drag on the Earth’s rotation to slow it down. The iron core’s rotation is increasing inside the Earth at the same time to compensate and conserve the total angular momentum.
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- With our rotation speeding up our days are getting shorter. We may need to start subtracting with a negative leap second. Now science is saying the average day is only lengthening 11 millionths of a second per decade. ( 0.000011 seconds every 10 years.) If that keeps up we will need to advance our clocks 1/1000 of a second in 2100.
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- Aren’t you glad someone is keeping track of this. Someone “else” I mean.
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- This all might not sound like much but it can drive computer programmers nuts. How to keep all these computers in sync with the “right” time? Computers are likely to abandon Universal Time altogether and standardize on GPS time.
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- The Global Positioning System is all held together with a network of atomic clocks and it is easy to access their time with a GPS receiver. Today if you are on GPS time and you want to know Universal Time just add 19 seconds.
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- It is time for me to have another cup of coffee.
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- August 25, 2020 830 2800
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--------------------- Tuesday, August 25, 2020 -------------------------
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