Gravity is not uniform
September 7, 2020 8:35 PM   Subscribe

There are 4 different reasons objects fall faster in different places. Bouguer gravity anomaly maps like these show regional gravitational variation.

This post brought to you by following a link from the Strange Maps Facebook group, which was linked in one of the expandible subsections in the article in this previously
posted by Cozybee (14 comments total) 19 users marked this as a favorite
Neat. I vaguely remember learning about this, but have never seen it explained so concisely, or mapped.
posted by aspersioncast at 9:09 PM on September 7

It's also another simple way to refute most versions of flat earth bullshit.

Most flat earth bullshit relies on the notion that gravity is indistinguishable from acceleration through space, so the whole flat-earth disk is constantly "accelerating" upward. Variations in gravity by altitude and by geographical location are nicely explained by Newtonian spheroidal-Earth gravity and Relativity, but "Accelerating Earth" bullshit can't account for these measurable variations.
posted by tclark at 9:16 PM on September 7 [3 favorites]

Say the Earth was created in 4004 BCE, as I'm sure most flat earthers think. If the disc has been accelerating at one g ever since, it's now traveling at 6,214c. That void it's moving through better be seriously empty.
posted by thatwhichfalls at 9:25 PM on September 7 [5 favorites]

Now I want to know what placse on the earth have the highest and lowest gravity,when you combine all four factors. The Bouguer maps only seem to cover the rock density not the other three.
posted by tavella at 9:26 PM on September 7 [1 favorite]

The variations in gravity mainly show up in deviations in sea level from the expected spherical* shape of the planet (*given that the equator bulges outward from the rotation of the planet).

Along the coasts the ocean flows to where it finds equilibrium so the actual measurable sea level might be slightly too high or low because of rocks deep in the ground under that area being denser or less dense than average and that can come from factors as simple as the mantle deep underground being slightly warmer or cooler than average. I think across the planet, represented by the "lumpy planet pictures", this effect maxes out in the actual surface of the ocean deviating from the expected geometry-based sea level in the range of up to +/- 100 meters or so.

Away from the coasts in the interior of continents this shows up as a slight complication in defining exactly what "sea level" is and subsequently elevation is in places like Illinois. The true answer is to dig a canal from the Gulf of Mexico north and see what the water level does along it, but that is absurd and not economical. That leaves you with doing planet-based geometry off the actual coasts for 1000's of miles and then how do you attempt to incorporate the planetary gravity lumpiness of 10's of meters of elevation deviation or more into that?
posted by Blue Tsunami at 10:14 PM on September 7 [2 favorites]

If bouguers have gravity, why do you have to flick them?
posted by Cardinal Fang at 3:20 AM on September 8 [2 favorites]

I work with equipment that's highly sensitive to gravitational fluctuations, so I'm very familiar with this. On the ship I work on, there are two mooring locations on the same dock that it can tie up to - I've had to scrap tests and restart them because I told our software we were at the wrong end of the dock. Our office gets regular gravitational surveys done. We're looking at micro-g variations, but it's enough to cause problems if it's not accounted for.
posted by backseatpilot at 5:11 AM on September 8 [5 favorites]

Today, I learned that a galileo is a unit measuring gravity.
posted by They sucked his brains out! at 5:13 AM on September 8

I thought this was going to be about (one of) the surveying errors in the supposed-to-be-straight Kentucky-Tennessee border, but it turns out that was due to magnetic errors, not gravitational ones.
posted by Huffy Puffy at 5:48 AM on September 8

"Accelerating Earth" bullshit can't account for these measurable variations.

I am 100% positive that they are ready with some ad hoc explanation to refute the "gravitards".
posted by thelonius at 5:51 AM on September 8 [1 favorite]

I'm pretty sure that the way this family of phenomena was discovered was through the quest to toss a fusion bomb 6000km while making sure it would land within 100m of the spot being aimed at.
posted by Aardvark Cheeselog at 6:26 AM on September 8 [3 favorites]

Say the Earth was created in 4004 BCE, as I'm sure most flat earthers think. If the disc has been accelerating at one g ever since, it's now traveling at 6,214c. That void it's moving through better be seriously empty.

You πŸ‘ can’t πŸ‘ just πŸ‘ add πŸ‘ velocities πŸ‘ when πŸ‘ 1 + u’.v/c2 πŸ‘ is πŸ‘ much πŸ‘ greater πŸ‘ than πŸ‘ 1.
posted by Your Childhood Pet Rock at 8:05 AM on September 8 [1 favorite]

The instruments that measure the gravitational field are called gravimeters.
It's amazing how precise they are.
A Worden gravimeter from the 50's can read to .1 milligal
To put that into perspective , gravity will change approximately .3 milligals per meter of elevation.
You could measure the difference in the earth's gravity from the floor and the top of a desk.

The latest gravimeter can measure to .1 microgal That's 1 part in ten million

These measure relative gravity and would be tied to a gravity benchmark.

Similar to surveyors benchmarks there exists a number of gravity benchmarks put in by the federal government.
Here is one at Devon Ice Cap
(scroll down for a neat photo of the site)

The complete Canadian gravity network is here
posted by yyz at 2:25 PM on September 8 [3 favorites]

There are also mobile gravimeters used in surveying areas for mining exploration and scientific purposes - the variations in mineral density can be used to predict localized geological structures. Mounted on moving vehicles like aircraft or ships, they aren’t as accurate (0.2-1 mgal) as static gravimeters due to vehicle vibrations and position uncertainties that can’t be completely eliminated from the data. However, they provide higher resolution data than satellites, and they can go where static gravimeters can’t, or can at least do it much cheaper.

And then there’s gravity gradiometers, which measure the rate of change in gravity by measuring differences in gravity between two or more points on the instrument, using multiple accelerometers.
posted by cardboard at 7:35 PM on September 8 [2 favorites]

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