This morning’s shower brought me an interesting series of thoughts that I thought you might appreciate, and it all started with the simple question of “How do you set The Atomic Clock?”
My first thought was that at some point you have to find some other clock and precisely sync up with it. Then again, they may have said “fuck it” and just had Bob press a button when some other clock flipped over, but then I wondered “How do we know what time it is in the first place?”
Well, our calendar is based on the birth of Jesus, and we claim we know what day that was, and what town that was in. I’m ignoring that the details in the Bible point to a date that’s nowhere near Dec. 25th. Anyway… The first moment of the first day would, I suspect, be associated with the moment at which dawn broke in Jerusalem that morning. Or maybe some more esoteric moment that day. Regardless, let’s stick with dawn for the sake of attempting to figure out what time it is.
So, dawn, in Jerusalem, on day 1. Great we can work forwards from there. But, wait… how do we know how long a year is?
A year is the time it takes for the Earth to travel around the Sun, but how do we know when we are at the same place in the rotation we were exactly one year ago? You could try and position yourself relative to other heavenly bodies “When star X is directly behind the Sun.” But our orbit’s not circular, it’s elliptical. Actually, that’s not true either “The Earth travels around the sun in an elliptical orbit that varies in shape, with time from nearly circular to distinctly elliptical.” Then there’s the fact that every galaxy in the universe is shooting out away from the center of it.
If you ignore the orbital complication and say “ok, we can compensate for orbital weirdness”, how do you verify it with observation? Well you’d look up on… wait a minute. How the fuck do know where the same up is? In order to calculate our point in the Earths solar orbit we have to first be able to look up at the same point on different days. Which means we have to calculate exactly how long a day is, but “The tilt of the earth changes cyclically between 21 3⁄4 degrees and 24 1⁄4 degrees. The period of cyclicity is 42,000 years.”. On top of that we’re going around the sun.
So, even if the Earth wasn’t wobbling, and even if I knew exactly how long it took to spin 360° when I looked up the sun would be in a different place because we’ve moved around it. The stars would be in a different place because we’ve moved around the sun, and even if we hadn’t done that, the contents of the universe is still shifting outwards.
Back to figuring out how long a day is. So, we look up. We find something in the universe that we’re not rotating around to mark as a point of reference. We can compensate for the universe’s expansion by observing its Redshift (i think). So, we look up but once 23 hours have passed and we’re ready to start looking straight up again to observe the exact instant we’re lined up with that distant star (or whatever) the planet has wobbled on its own axis and now “up” will be pointed in a different direction than it was exactly one day ago. So, now we have to calculate the the wobble of the planet…
At that point I needed to get out of the shower. But I do know that the wobble’s going to be affected by the fact that we’re on a large ball of rock that’s got a molten core, which means liquid sloshing around in the center (for some definition of “sloshing”). I’ve no clue how one would determine that we even have a liquid center. Maybe by the fact that we’re wobbling?
In order to calculate the correct time you must: * Figure out the speed at which the universe is expanding so that we can use a distant star as a reference point to observationally confirm all the other things we need to figure out. * Figure out the Earth’s eccentric wobble. No clue how to do that. My shower ended. Once you’ve got that you can then figure out how long a day is. * Now store away what you’ve learned about our wobbling rock with a liquid center. We’ll need that. * Once you know how long a day is, and what direction you’re pointing after a given span of time, you can then start observing our orbit by marking the location of the sun relative to you and some distant star. So, now you can figure out how long it takes you to get around the sun. Unfortunately that’s going to change every single year as our orbital path changes. * So observe those changes for enough years to calculate how our orbit changes. Then factor in the effects of a wobbling mass on an orbit.
Once you’ve done all that you can then work backwards to calculate out where the hell the sun was over Jerusalem on December 25th 2013 years ago, and then you can work forwards to figure out what time it is.
You’re still going to have the problem of actually syncing the clock with the math though.
P.S. How was your shower?
P.P.S. Those links? Missouri Department of Natural Resources, Division of Energy. “Global Climate Change: Effect of the Earth’s Orbit”
P.P.P.S. I’m not a Physicist. I just play one on the Internet.