5 ms·
Height Modernization: Recalibrating where the U.S. physically sits on the planet
- Rexxar 6y agoI would have expected that the recalibration have big impact in middle of continent and no impact near coasts but strangely it's not the case. How coastline altitude could have been incorrectly calibrated before ?
- throwaway2048 6y agoOceans do not have the same height everywhere, there can be up to about 2M of difference. https://en.wikipedia.org/wiki/Ocean_surface_topography https://en.wikipedia.org/wiki/Ocean_surface_topography
- arethuza 6y agoThis caused problems when a bridge was being built between Germany (which uses the North Sea) and Switzerland (which uses the Mediterranean) - someone made a mistake including the known height difference between the two seas: https://www.science20.com/news_articles/what_happens_bridge_when_one_side_uses_mediterranean_sea_level_and_another_north_sea-121600 https://www.science20.com/news_articles/what_happens_bridge_...
- aasasd 6y agoNo sweat, the CIA factbook will just define the U.S. elevation as including its aerospace. Like it does for area.
- deleted 6y ago[deleted]
- ARandomerDude 6y ago> That’s because height is only height compared to a reference point — and geodesists...are redefining the reference point, or vertical datum, from which height is derived. So it isn't getting shorter. We're changing our reference point. Those are very different things!
- Causality1 6y agoWe really need a "flag for misleading headline" button.
- wutbrodo 6y agoI think it's just called the "flag" button.
- Causality1 6y agoNah. There are plenty of interesting articles that just happen to have clickbait headlines. I wouldn't flag someone just for that.
- flattone 6y agobut clickbait titles are the antithesis of this website
- nichos 6y agoI thought the link to the new York times was enough to indicate it's a crap article :D
- dang 6y agoPlease don't be biliously predictable in HN comments. Obviously there are plenty of good (and bad) articles at the NYT.
- dang 6y agoIf anyone can suggest a better title, we can change it. "Better" means: more accurate and neutral, and preferably using representative language from the article itself (https://hn.algolia.com/?dateRange=all&page=5&prefix=true&sort=byDate&type=comment&query=representative%20title%20by:dang https://hn.algolia.com/?dateRange=all&page=5&prefix=true&sor...). Edit: I've taken a crack at it.
- istjohn 6y agoReading this, I wonder if the melting of the ice caps causes a measurable slow down in the Earth's rotation and an increase in the length of a day by moving mass farther from the Earth's axis of rotation.
- throwaway2048 6y agoNot sure why this is downvoted, this is absolutely the case, but the effect is small https://climate.nasa.gov/faq/30/if-all-of-earths-ice-melts-and-flows-into-the-ocean-what-would-happen-to-the-planets-rotation/ https://climate.nasa.gov/faq/30/if-all-of-earths-ice-melts-a...
- foota 6y agoI'm surprised by the magnitude of the effect. I wouldn't have expected things to move around enough for the length of a day to vary by a millisecond?
- kd5bjo 6y agoIt doesn’t mention what sort of frequency those variations happen at. If the variation is caused primarily by plate tectonics, it might take a year or more to swing between the two extremes.
- quietbritishjim 6y agoExcellent link, thanks! Here are some key points for convenience (although the article's so short that this is practically half of it): > the rate of rotation varies by up to a millisecond per day [regardless of ice melting] > Melting land ice ... will change the Earth’s rotation only if the meltwater flows into the oceans > For example, if the Greenland ice sheet were to completely melt and the meltwater were to completely flow into the oceans, then ... the length of the day [would become] longer ... by about two milliseconds > Melting sea ice, such as the Arctic ice cap, does not change sea level because the ice displaces its volume and, hence, does not change the Earth’s rotation.
- DiogenesKynikos 6y ago
- supernova87a 6y agoIf you ever want to be bored out of your mind (at whatever level of detail that occurs for you), just ask someone in this field about the definitions of geoid, ellipsoid, etc. etc. and you will have hours (well, maybe minutes) of fun.
- globular-toast 6y agoMy PhD supervisor told a funny story about this to illustrate the phenomenon of learning more, but seemingly knowing less. First you learn that the Earth is "round". Then you learn about shapes and that the Earth is actually a sphere. Then you learn more shapes and that the Earth isn't actually a sphere but is slightly squashed at the poles: it's spheroid. Then you learn that it is actually more squashed at the top than at the bottom. It is in fact a geoid. So you look up geoid in the dictionary and find that geoid means "Earth shaped".
- hug 6y agoThis reminded me of The Relativity of Wrong. https://chem.tufts.edu/answersinscience/relativityofwrong.htm https://chem.tufts.edu/answersinscience/relativityofwrong.ht...
- MertsA 6y agoAnd for added fun if you look up some of the reference points maintained by NOAA they'll list a position, a date, and a velocity. Due to the relative movement between different parts of the Earth even "fixed" points will be moving so you need to calculate it based off of the velocity and length of time since the position was surveyed. Geodesy is weird.
- neonate 6y agohttps://archive.is/539Hv https://archive.is/539Hv
- mdturnerphys 6y agoIt's unfortunate and potentially confusing to readers that they've used the Washington Monument and the Flatiron Building as illustrations in the article and suggested that their heights might change. The reference point for measuring a structure's height is its base, not the geoid. Unless the definition of a foot changes the height of these structures will not change.
- taejo 6y agoCuriously, the definition of a foot is changing. Or rather, one of the two definitions of a foot (the U.S. survey foot) is being deprecated in favor of the other (the international foot). The two differ by about two parts per million.
- kd5bjo 6y agoMy understanding was that an inch is exactly 2.54cm, and a foot is exactly 12 inches ever since the metrification of US customary units. Is that not correct?
- capnrefsmmat 6y agoThe foot was defined in terms of meters only in 1959, and the definition was slightly different than prior use. (The US and the Commonwealth previously used slightly different feet.) The survey foot is the definition prior to the definition in terms of meters, and is still widely used in surveying. https://en.wikipedia.org/wiki/Foot_(unit)#Survey_foot https://en.wikipedia.org/wiki/Foot_(unit)#Survey_foot
- taejo 6y agoNo, the original metric definition of US units in 1894 [0] made 1 yard = 3600/3937 metre (i.e. 1 inch = 100/3937 metre). The US survey foot is 12 of those inches. The 1 inch = 25.4 mm definition was adopted by the British Standards Institution in 1930 and by the American Standards Association in 1933 (with the name "industrial inch"); in 1959 the US, UK, Canada, Australia, New Zealand and South Africa signed a treaty agreeing on that definition [1] (and also a definition of the pound). The international foot is twelve of those inches, but the US allowed (but recommended against) the continued use of the old foot in surveying. [0] https://usma.org/laws-and-bills/mendenhall-order https://usma.org/laws-and-bills/mendenhall-order [1] https://en.wikipedia.org/wiki/International_yard_and_pound https://en.wikipedia.org/wiki/International_yard_and_pound
- deleted 6y ago[deleted]