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Friday, August 19, 2016

Flat Earth Follies: Why Don't The Days Shift By 12 Hours Between Summer and Winter?

Why Don't The Days Shift By 12 Hours Between Summer and Winter?

Flat Earthers LOVE to post little memes with low information content, for example...

A very good question.  However, the answer is very simple.


Sidereal versus Synodic

One exact rotation of the Earth is called a sidereal day which is a period of 86164.09054 seconds or 23 hours, 56 minutes, 4.09054 seconds.  This is measured as the Sun being in the same relative position as the distant stars.

However, the '24 hour' period we all know and love is called a synodic day or 'mean solar day', this is the average amount of time from Noon to Noon and it is literally just a little bit more than one full rotation of the Earth to make up for our motion around the Sun.  Our orbit around the sun isn't a perfect circle so the actual length of a day varies slightly but on average it is 86400.002 seconds -- this is why Noon is still Noon from Summer to Winter.

And if you look at the stars just before Sunrise you'll notice that they shift by this amount every day, about 236 seconds.

Remember that it was the ancient Sumerians who defined the meaning of '24 hours' (and the division into hours and seconds) based on the Sun being back in the same position as the previous day -- which automatically includes the time shifting due to our motion around the Sun even though they didn't know about it.  Their version wasn't as accurate as ours but we still use their nomenclature because it works for humans.  Noon to Noon is an easy concept to grasp.

Now let's do a little check.

The daily difference between the two is 86400.002-86164.09054 or  approximately 235.91146 seconds per day (average!) so we should expect somewhere right at +1 sidereal day over the year.

Number of days in 1 tropical year is ~365.242188792

235.91146s * 365.242188792 = 23 hours 56 minutes 4.82 seconds !!!

So I'm within 3/4 of a second with my estimate, I feel pretty good about that.

So that's why Noon is still Noon Winter, Spring, Summer, and Fall.

However, even the distant stars aren't fixed - they just appear to move very slowly because of their great distance.  But the very distant stars they take tens of thousands to millions of years to move very much in our view so they are 'fixed enough'.

But with our closer neighboring stars we can observe this more directly, such as with Barnard's star:

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