5 ms·
Explain it like I'm five version : a plumb bob at the Greenwich observatory doesn't point at the center of mass of the earth. This is because of different dens
by dzdt 11y ago
Explain it like I'm five version : a plumb bob at the Greenwich observatory doesn't point at the center of mass of the earth. This is because of different densities of rocks below the surface, and of mountains on the surface, and so on. Prior to the satellite era, the meridian plane was defined to be parallel to local surface gravity at Greenwich (so the plumb bob string lies in the meridian plane) and perpendicular to the rotation of the earth as determined by observations of stars. When satellites started to be used, the new meridian plane was kept parallel to the original but moved over to go through the center of mass of the earth. The center of mass of the earth is a nice reference point for satellites orbiting the planet. The new system chose to keep noon (the time when the sun is directly overhead, as measured by local gravity direction) at Greenwich fixed instead of keeping the location of the meridian line fixed. That was because lots of people depended on the Greenwich Mean Time standard and not so many people depended on the exact longitudinal coordinates of any location, which prior to GPS had been very hard to measure anyway.
- kokey 11y agoThank you for that. I think this is very complex to explain without a lot of diagrams.
- Htsthbjig 11y agoThanks. You are not explaining it like we are five, but as if we were humans. The explanation in the article is a non explanation, "look at me how smart I am that could obfuscate knowledge, so I look smarter".
- animefan 11y agoRe your second sentence, I think it's unfair to attack the author in this way. By my reading, the author didn't attempt took describe the discrepancy and just included the abstract of a paper that does. We should thank the author for their best effort. This touches me because I've often been in the situation where my explanation was inadequate, and I was accused of trying to sound smart. We shouldn't be too eager to take people down a notch.
- ubernostrum 11y agoThanks. You are not explaining it like we are five, but as if we were humans. Actually, Reddit's ELI5 subreddit describes it as: LI5 means friendly, simplified and layman-accessible explanations. Not responses aimed at literal five year olds (which can be patronizing). So it was a comment which would be appropriate for ELI5.
- iamthepieman 11y agoEven before my eyes glazed over I understood exactly 0° 0' 0.000" of that last paragraph. So thanks for both simplifying and expanding it.
- kwelstr 11y agoSo, should we all move 100m to the east?
- dx211 11y agoI'm pretty sure some of us need move east, some need to move west, some need to start digging and others need to start building towers. Me, I need to dig into the ground for about 90 meters at a 43 degree angle to the north. But it's a small price to pay if it keeps my watch running.