The Earth's lumpy gravity

By Phil Plait | March 31, 2011 9:30 am

[UPDATE (April 5, 2011): It turns out some of the descriptions I used below to describe a geoid were not accurate. I refer you to this page at the University of Oklahoma for a good description. I've made some changes below to hopefully ease any confusion.]

Most people think of the Earth as being a sphere. For most purposes that’s close enough, but it’s actually a spheroid, something close to but not precisely a perfect sphere. It bulges in the middle (as so many of us do) due to its spin, the Moon’s gravity warps it, the continents and oceans distort the shape. And the surface gravity changes with all this too; it’s different on top of the highest mountain, for example, compared to its strength in Death Valley.

So if you could map out the average shape of the Earth’s gravity, a shape where the gravity is the same no matter where you stood on it, what would it look like?

So if you could map out the Earth’s gravity — essentially, a diagram showing you the direction of "down" — what would it look like?

It would look like this:

That is a (somewhat exaggerated for easy viewing) map of the Earth’s geoid, produced by the European Space Agency’s GOCE satellite. A good way to think of the geoid is the shape a global ocean would take if it were governed only by gravity, and not currents or tides or anything else. If the Earth’s gravity were a little stronger in one place, water would flow toward it, and if it were weaker water would flow away. In the end, the surface of this global ocean would feel the same gravity everywhere, shaping itself to the geoid. If the Earth’s surface were an actual geoid, then the direction of "down" would point perpendicularly toward the geoid surface (or, in the same vein, if you had a carpenter’s level, the level would be, um, level if it sat parallel to the geoid). It’s the ultimate "sea level".

This may seem esoteric, but this knowledge is actually important. For one thing, it gives a standard reference for topographic maps. For another, it will help scientists better understand ocean currents, circulation, sea wave heights, and other tricky measurements. There are many uses for such a geoid map.

GOCE — for Gravity field and steady-state Ocean Circulation Explorer — has been orbiting the Earth since 2009, using extremely sensitive accelerometers (like what your smart phone uses to measure movement, but a whole lot more precise and expensive) to detect the whispery tug of gravity on the spacecraft. If it passes over an area with slightly higher gravity, the spacecraft accelerates a bit, and the detectors measure that. Over time a map of Earth’s gravity is built up, resulting in the measurement of the geoid.

The map itself is pretty cool. Since this is the shape of a global ocean, you can see the lump over the north pole means gravity is a bit stronger there, and the same is true over southeast Asia and other land masses. That may sound counterintuitive, but think of it this way: where the gravity is stronger, the water will flow toward that place, piling up, and the geoid sticks up more. The weakest gravity is in the Indian Ocean, where the map is deep blue. A global ocean would flow away from there, leaving a depression. [UPDATE: Sorry about the confusion here, but this is where I blew it: the strength of gravity is not necessarily the same on the geoid everywhere; it's just where the gravity potential is the same. The two are related but not the same thing. Again, think of the geoid as the surface where the force you feel down from gravity is always perpendicular to the surface.]

GOCE is actually a pretty cool spacecraft. It had to fly in low orbit to get the best measurements, but at an altitude of just 280 kilometers (170 miles, much lower than most satellites) there is a trace of air. That air drags on the satellite, lowering its altitude. GOCE has a long, sleek shape to help minimize drag, but more importantly it uses an extremely low-thrust ion engine which is precisely calibrated to counteract the atmospheric drag. Without the engine, GOCE would lose about 200 meters of altitude a day, and quickly re-enter.

I find this whole project pretty interesting. Gravity is a very weak force, and measuring it with such precision is an amazing achievement. GOCE has mapped the geoid to a vertical accuracy of 1 – 2 centimeters — about an inch! I don’t think the GOCE geoid will replace globes across the world in classrooms — it’s a bit off-putting, I’ll admit, and distressingly lumpy — but I do hope that people get an idea that while the Earth is pretty close to a perfect sphere, nobody, and no planet, is actually perfect.

Image credit: ESA/HPF/DLR


Related posts:

- Ten Things You Don’t Know About The Earth
- The cloudy, warming Earth
- A glint from Earth
- How far away is the horizon?

CATEGORIZED UNDER: Cool stuff, Pretty pictures, Science
MORE ABOUT: Earth, ESA, geoid, GOCE, gravity

Comments (78)

  1. Just wait. Someone will take this and use it to claim that the LHC caused a black hole to form in the Indian Ocean, sucking the Earth into it there and deforming the rest.

  2. Gus Snarp

    The ad I’m getting on this in Google Reader is for RNA purification. I have no idea what that is, but why does Google’s system insist on bombarding skeptical blogs with ads for utter bunk?

  3. Andrew

    Interesting that there is extra gravity over the Andes and Indonesia, but not the Himalayas, and that deep low to the south of India. Do we know why? Should we expect material to pile up in areas that are above average so they “sink”, and “rebound” for areas that are below the average?

  4. Ian Regan
  5. DanO

    Say I weigh 200 pounds at the north pole, what would I weigh just off the southern tip of India?

  6. DrivethruScientist

    @gus

    RNA purification is a legitimate technique when doing molecular biology. Basically, if you have an RNA product that you’ve synthesized, you can use invitrogen’s line of products to purify it so that only the RNA remains. You can then use that RNA for many different applications.

  7. So would one feel any different in those areas of different gravity strength, or are the differences so minuscule you wouldn’t notice?

    What about indigenous people who’ve lived in an area for thousands of years such that their bodies are used to the gravity level there? If they suddenly moved to another area of the Earth with a big difference in gravity would they notice?

    Inquiring minds want to know.

  8. The Beer

    I wonder if they can use this info to help model the ‘event’ when the Moon was created. Even with tectonic plates moving… something that drastic may leave a global scar of some sorts. Maybe the impact was in the Indian Ocean??

    Just a thought.

  9. Jason Dick

    Interesting. However, I don’t see any way that the satellite can measure the differences in acceleration, because the accelerometers would be accelerated along with the rest of the craft and left unchanged.

    My naive suspicion would be that they actually use the tidal effect of gravity (the difference in gravity between the part of the craft closest to the Earth and the part furthest away). [i]Maybe[/i] they can make use of the fact that there still is some atmosphere at that level, but that would strike me as a very error-prone way of performing the measurement.

    Edit: Yeah, here it says that the satellite uses a gradiometer with three pairs of accelerometers:
    http://www.esa.int/SPECIALS/GOCE/SEMZQ2VHJCF_0.html

    So it looks like it measures the tidal force, not its own acceleration.

  10. Jeff Schwarz

    I think “Lumpy Gravity” will be the name of my Frank Zappa & The Mothers cover band.

  11. Caleb

    It bulges in the middle, sixteen thousand light years thick,
    But out by us, it’s just three thousand light years wide.

    (yeah not talking about earth, but it reminded me of the song.)

  12. Charlie

    So is gravity “less” in North America than Europe, standing let’s say on the beach at sea level? And if so, are we talking 1% or much much less?

  13. GOCE is a nice satellite to observe actually. Because of the reflective solar panels on the sides and the fact that its attitude is carefully controlled, it can produce flares that are visible by the naked eye. They happen when the satellite culminates and you are looking exactly perpendicular on it’s direction of movement.

    The GOCE geoid is certainly not the first satellite based geoid, but it certainly is the most accurate (previous ones were accurate to a few decimeters).

  14. Gus Snarp

    @DrivethruScientist – Ah, I see. I wasn’t about to click the link to learn more, so I’m glad it’s not some kind of health scam. I suppose their target audience knows what it is, but that ad has a picture of what looks like a roll of lifesavers with “RNA Purification” on the label. You can understand my confusion.

  15. chris

    I first learned about this from reading Arthur C. Clarke’s ‘The Fountains of Paradise’.
    The space elevator that is constructed in the book connects to the Earth on a mountain top in Sri Lanka (Clarke moved the island south in the book so it is on the equator, but I’ll let him off with that, he’s Arthur C. bloomin’ Clarke!) precicely because of the gravity in the Indian Ocean region.

  16. GrogInOhio

    “It bulges in the middle (as so many of us do) “… I resemble that remark!

  17. This is too cool. Thanks Phil!

  18. CB

    @7 Jason Dick:

    Um, accelerometers measure the magnitude of the acceleration. Yes they would accelerate along with the space craft. And that is what they would measure. Accelerometers don’t measure a difference in velocity between the accelerometer and whatever it is attached to. As the link you found says: “each accelerometer can detect accelerations to within 1 part in 10 000 000 000 000 of Earth’s surface gravity. ”

    However you are correct that they are using a gradiometer, because they don’t want just the magnitude of gravity at any given location, they also want the derivative.

  19. DrFlimmer

    @ #7 jody and #12 Charlie

    The “average” acceleration on earth is 9,81 m/s^2 (also called “1g”). The differences affect at most the second decimal place, therefore, yes, the difference is of the order of 1%.

  20. John Baxter

    Good stuff, especially when supplemented by the BBC page by @Ian (which give the scaling–rather small).

    I bulge in the middle, and sadly it’s not because I’m spinning.

  21. WJM

    Almost looks like a baseball. A very colourful baseball.

  22. J. R. Braden

    @#2 Gus: If DrivethruScientist is correct (and why wouldn’t he be?), its not bunk, but I’ve noticed a lot of bunk ads on my skeptic blogs on Google Reader too. It might be annoying, but at least if they’re coming to us, not as many of the ads will be seen by people who might fall for them. Plus it gives us something to laugh at. Skepchick had an ad this morning for an instructional course on Reiki healing or some such nonsense. It looked like it was just a $40 CD-ROM. I lol’ed.

  23. Dan

    It’s probably worth pointing out that the Geoid varies in height relative to the spheroid by only about 200 metres (it’s from about -105 to +85 metres).

    You would not feel any heavier or lighter at different points on the spheroid surface, any more than you would if you changed your height above sea level by 100 metres while maintaining your latitude and longitude. The variation if the force of gravity is too small to feel it.

    And of course if you moved to a different geographical location while remaining at the same height relative to the Geoid, your weight would not change at all.

  24. Awesome map! I am curious, why is it that gravity is not uniform over the entire planet? I always thought it’d be heaviest around the equator; or would that be centrifugal force?

  25. DrFlimmer

    @ 22 J.R. Braden

    That’s why I like the Forefox addition “AdBlock Plus”, which basically removes every ad banner.

  26. @The Beer:

    Doesn’t it? It looks to me like a hole was punched through it, displacing heavier material around it. The “low” gravity zones are almost on the opposite sides of each other. I’m an arm-chair scientist, but that looks like a clear indicator of a massive impact.

  27. JT

    If you’re interested in the GOCE material, check out the GRACE mission, which pioneered this field (http://www.csr.utexas.edu/grace/). Scientists were even able to use GRACE data to see the gravitational effect of the Sumatran earthquake by comparing data from beforehand and afterwards.

  28. Gus Snarp

    I know the geoid is lumpy, but I’ve always been under the impression that it’s a little less lumpy than that. Anyone know if this image is stretched in any way to highlight the differences?

  29. DavidB

    I would have thought this would closely follow landmass features, but it does not. Parts of the polar regions are blue, parts are red. Parts of the equatorial regions are blue, parts are red. Continents are not consistently red or blue, neither are ocean basins and mountain ranges. Why are the Himilayas blue and the Mariana Trench red? Totally counterintuitive! Apparently this is far more driven by mantle features we can’t see than by surface features.

  30. BJN

    Gravity varies by .5% between the equator and the poles, and by .o1% by geology. Obviously the lumpy model has a dramatically enhanced scale.

    http://curious.astro.cornell.edu/question.php?number=465

  31. @CB (#18): A simple accelerometer at the center of a spaceship would measure exactly zero, which is why Jason asked. The trick is the incredible sensitivity: the relative size of the tide (i.e. s*(dg/dx)/g where x=coordinate, s=size of spaceship) is about 2e-6 in the vertical direction, which is clearly visible with a device that sees changes of 1e-13. This still leaves seven orders of magnitude to resolve the irregularities in the field.

  32. kafantaris

    With the varying gravitational forces tugging at the earth as it rotates in space, we may have not yet seen the last great earthquake of our lifetime. Indeed, considering that the earth is surrounded by a highly viscous mantle – and not too deep from the crust either — it is a miracle earthquakes aren’t more frequent. This might change, however, as our globe heats up, if the melting polar ice is any indication.
    The Goce Satellite has certainly given us food for thought — and in brilliant colors too.

  33. CB

    *BRAAAAP* <- the sound my brain made.

  34. Rob

    @26 Gus Snarp

    The first sentence under the video, “That is a (somewhat exaggerated for easy viewing) map of the Earth’s geoid…”

  35. Gus Snarp

    @Rob – That’s what I get for skimming. I knew it had to be.

  36. DrBB

    “more importantly it uses an extremely low-thrust ion engine which is precisely calibrated to counteract the atmospheric drag. Without the engine, GOCE would lose about 200 meters of altitude a day, and quickly re-enter.”

    Kinda reminds me of one of a perennial favorite among Original Star Trek physics errors. Orbiting a planet, engines conk out, “We’ll spiral in if we canna get ‘em back on line cap’n!” Um, no, actually you won’t, not in any big hurry anyway, unless you’re down in the top of the atmos. where you’ve got no business being.

    Also, @10. Jeff Schwarz wins the thread.

    Also also, and totallyO/T, I dunno nuttin about RNA purification (@2) but what really bugs me is the ad for high powered laser pointers I keep seeing here on BA. I’m not sure what legitimate purposes anyone wants those for, but there’s been a marked increase in “hobbyists” using these things for illuminating aircraft cockpits (read: blinding pilots). Had a cretin busted for doing that to one of the helicopters escorting an LNG tanker in Boston Harbor earlier this year. Imagine the fun of a helicopter crashing into one of those, essentially right in the middle of a populous city. Wouldn’t that be funny. Yukkity yuk.

  37. DavidB

    @DrBB -
    Those high powered laser pointers are a must have for group star watches. Its the only way to show your fellow starwatchers where in the sky you are looking so they can point their telescopes there too.

  38. DrBB – That was “The Naked Time,” and it was like the second or third episode aired, so I cut them some slack (it was also my favorite, being only 14 at the time, although I already understood the gaffe).

    Also, shining any sort of laser deivce (even the milliwatt variety) into an airplane cockpit, railroad engine cab or ship’s bridge is a federal felony, for exactly the reasons you mention.

    - Jack

  39. The military has been mapping the Earth’s gravity field since the ’60s. I first saw this when I worked for Lockheed on the Trident missile’s Mk4 Reentry Body (that’s “warhead” for you civilians). If you want to drop one of these things with 100 yard/meter accuracy after a ballistic flight of over 6,000 miles/10,000 Km (more for land based missiles) you have to know how gravity works every inch/cm of the way.

    This map is much finer resolution, but they had it mapped to the unitary milligal (1/1,000 g) by the late ’70s when I was there.

    - Jack

  40. Robert

    @18, 31 I was wonderring as well how an accelerometer would measure anything. The fact that that you can measure the tidal effect of inhomogeneties in the earth gravitational field is quite incredible.

  41. Thank you for this, Phil. The next time I go for my physical, and my Dr. says I am too fat and need to lose weight, I will say, “I’m not fat! It’s lumpy gravity!”

  42. DrBB

    @39 Jack Hagerty: “That was “The Naked Time,” and it was like the second or third episode aired, so I cut them some slack (it was also my favorite, being only 14 at the time, although I already understood the gaffe).”

    Yes–the episode that gave us one of the great Trek quotations of all time: “Captain, Ah can’t change the laws of phuzuks!”, to which I have resorted on many an occasion.

    @38 Point taken on laser pointers for star parties, but it still makes me uncomfortable knowing there are a lot more people buying these gadgets than accounted for by star party demand and incidents are very much on the rise. In the US, 311 in 2005 vs 2,836 in 2010 (7.8 incidents each night–yikes!–and nearly double the number for 2009). Glad the idiot from Medford got jugged for a couple of years for his little exploit. But enough of that O/T subject. I like star parties!

  43. Darren

    @3 Andrew
    “Interesting that there is extra gravity over the Andes and Indonesia, but not the Himalayas, and that deep low to the south of India. Do we know why?”

    Continental crust is less dense than marine crust. So just because the Himalayas are tall, they also displace denser mantle below, so they don’t contribute much more to gravitational pull.

    The Andes and Indonesia are strong subduction zones. Besides the crust on the surface, there’s another layer of (dense oceanic) crust below adding to the pull.

  44. TJ Czeck

    Just want to say: First time poster, loooong time reader.

    Apologies if someone asked this already, but I only skimmed the comments.

    We know that the equator bulges out due to the centripetal force of earth’s rotation. Which means that there is more matter (read: mass) along the equatorial line than compared to a line running from the north pole to the south pole. Since mass and gravity are directly related, this should mean that the highest gravity readings are along the equator. But from the video, it clearly shows the highest gravity follows a line over Africa and the western Pacific. I think Darren (#44) hit on the answer, but I would think that there would be a more noticeable “belly” around the equator.

    Thoughts?

  45. Messier Tidy Upper

    So does that lack of completely round shape mean our Earth’s NOT really a planet by the IAU definition strictly applied? ;-)

  46. Kaleberg

    I was involved with a group that did a video of gravimetric data like this back in the late 80s. I worked with the graphics group back when you had to write your own displacement and texture mapping code, in the snow, uphill both ways. We too had exaggerated the vertical dimension, so we had a pretty lumpy earth, though not quite as lumpy as that one. I was showing the video at a conference when someone asked, “Does the earth really look like that from space?” To my amazement, I responded, “Oh yes, the earth is in much worse shape than you think.” (Wow, that was a long time ago. I think I still have the U-matic cartridge with the video on it in the garage.)

  47. Joseph G

    I know I sound like a little kid but holy eff a sleek spaceship with an ion engine continuously knifing around the earth through the upper atmosphere while scanning our gravity field holycrapthatsAWESOME! :D

    @#44 Darren: I was wondering about that. Fascinating that the actual composition of the earth’s crust can be traced out by minute changes in the acceleration of a satellite.

  48. Joseph G

    @45 TJ: My only thought is that this bulge must be so small as to be swamped by the huge density variations in the earth’s crust – which, to be fair, are substantial. The combination of liquid water oceans, an oxidizing atmosphere, volcanic activity and active plate tectonics means that the earth’s crust has quite a bit of mineralogical variety to it – much more then a rocky body made of similar materials but lacking these processes (such as the moon, for instance).

  49. Phil: “the surface of this global ocean would feel the same gravity everywhere, shaping itself to the geoid.”

    GOCE’s video represents an exaggerated view of the geoid, a gravitational equipotential surface. The potential being constant does *not* mean that gravity (the gradient vector field of the potential, with a minus sign) has constant magnitude everywhere on the geoid.

  50. Nigel Depledge

    Joseph G (48) said:

    I know I sound like a little kid but holy eff a sleek spaceship with an ion engine continuously knifing around the earth through the upper atmosphere while scanning our gravity field holycrapthatsAWESOME!

    Agreed.

    It’s just a shame they didn’t equip it with Twin Ion Engines . . .

  51. As an interesting aside. If the Earth was shrunken down to the size of a billiard ball, it would be smooth enough to be accepted under standards regulations, but not round enough!

  52. Sion

    I work with this stuff for a living, so maybe I can clear it up a bit. We measure gravity anomalies using a unit called the Galileo (or Gal).
    1 Gal is equal to an acceleration of 0.01 metres per second squared or, if you prefer, 1 centimetre per second squared.
    In the field these anomalies are tiny, typically tens of milliGals, that is, 0.00001 metres per second squared. You will never feel these with your body no matter where you go on Earth.
    I’m looking at magma migration under volcanoes and those variations are measured in microGals, 0.00000001 metres per second squared.
    I’d be surprised if that lumpy Earth model had a range of much more than 100 to 200 milliGals.

    If that lumpy Earth is doing your head in, try this: If you dropped a ball anywhere on that lumpy surface it wouldn’t roll away from you. Now your heads spinning properly!

  53. kafantaris @32 wrote:


    With the varying gravitational forces tugging at the earth as it rotates in space, we may have not yet seen the last great earthquake of our lifetime.

    Most assuredly, we have not… there are about 20 quakes/years greater than 7.0 and about 1/year greater than 8.0


    Indeed, considering that the earth is surrounded by a highly viscous mantle – and not too deep from the crust either — it is a miracle earthquakes aren’t more frequent.

    Define “highly viscous”. The mantle is mostly solid with some partial melting. And, quakes are very frequent… about 15000 a year above 4.0


    This might change, however, as our globe heats up, if the melting polar ice is any indication.

    Isostatic rebound from the melted glaciers will cause quakes in those regions covered by glaciers, but surface temperature has nothing to do with tectonics. Tectonics is driven by radioactive decay.

  54. Gus Snarp

    @MTU – Well, when it says the image is slightly exaggerated, the word “slightly” is an exaggeration too. It’s pretty exaggerated. By any definition of roundness that would make the Earth fail to be a planet, there’d be damn few planets, I think.

  55. Sion

    @32 kafantaris,
    There are very few earthquakes in the mantle as it is so hot. Rock there tend to slowly deform in a ductile manner, rather than break. Most earthquakes occur in the cooler, more brittle crust.

  56. naveen

    um why is sri lanka black? does have the highest gravity?

  57. DrivethruScientist

    @Gus

    I understand :) Discovery does let a few slip by now and then, but the majority of advertisements are ok. It’s odd to see an invitrogen ad on a website like this though …

  58. TJ Czeck

    @(49) Joseph G & @(44) Darren

    Thanks for the cross-discipline info. I guess this is just one of those situations that one needs to take a deep breath, accept the data, and try not to hurt yourself redoing your preconceived ideas.

  59. Roger

    I read through some of the comments and if this was answered already, I apologize. Does this have any indication of how thick the crust is? In areas where gravity is higher (stronger?), is the crust thicker in those reagions? Thanks.

  60. Roger, cut and paste this into your browser:

    earthquake.usgs.gov/research/structure/crust/index.php

  61. Joseph G

    @Gus Snarp: That RNA Purification product caught my eye, too. Funny that it’s a totally legit thing for lab work, but the actual ad makes it look like one of those woo products like magnetic bracelets.
    The Lifersavers candies don’t help – they make it look like some kind of pill that you swallow :P

    @53 Sion: That’s interesting! Do you do geological work?

    Shoot, now I’m wondering, to give a layperson a sense of scale, is there anything we could compare, say, a milliGal to? For instance, a cube of steel X meters across might represent a mass that produces X milligals (or microGals, I guess) of gravity?

    @59 TJ: Yeah, just when we were getting used to the idea of living on a spheroid, now they have to go complicate things! :)

  62. Joseph G

    @60 Roger: It does and it doesn’t :)
    For instance, you can see how the north American continent is fairly low on that gravity map. Even though the continental rock there is just as thick as the continental rock in other “higher” locations on that map, its composition is different – in general, you have a thick granite basement layer covered by lots and lots of sedimentary rocks from back when much of the continent was underwater. Both granite and sedimentary rocks such as limestone are not as dense as, for instance, basalt, which also forms the majority of the oceanic crust.

  63. Roger

    @Solius, thanks for the link
    @Joseph, thanks for the info…..makes sense like that.

  64. @55. Gus Snarp :

    @MTU – Well, when it says the image is slightly exaggerated, the word “slightly” is an exaggeration too. It’s pretty exaggerated. By any definition of roundness that would make the Earth fail to be a planet, there’d be damn few planets, I think.

    Very true – Jupiter and Saturn are noticeably flattened by their rotation and thus well out of the round and, I suspect, similar applies to other planets as well – and, yeah, I know the scale has been exaggerated by a *lot*. Did you miss my ;-) emoticon there did you? ;-)

    I strongly disgaree with the IAU “planet” definition & their Pluto-bashing decision – but in this particular case I was just joking – admittedly at their expense a little! 8)

  65. HIRAM113

    @40 Jack Haggerty is right on! Our military “discovered ” the geoid in the earliest Pre-NASA days of our satellite launches. They saw small anomalies in the orbits of our and Russian satellites, which they couldn’t account for. Some of Einstein’s work had suggested such phenomena. Telemetry data from two early circum-polar orbiters were used to create a crude digital gravity map of the Earth to assist in plotting potential trajectories for satellite and missile launches, especially ICBM’s. This map, called “the geoid” was classified “Top Secret”
    by the Air Force, until the British & French got into the satellite game. I worked with it at the Tech Lab at Patrick AFB in the mid-60′s, where all the telemetry data from NASA and military launches was processed.

  66. Sion

    @62 Joseph G: I did, now I’m an old student once more. I’ve not come across a layperson scale example before, but I could try and rough something out. Hang on…

  67. Sion

    @62 Joseph G: Okay, allowing for Monday morning errors, I reckon that an astronaut, weighing a svelte 100 kg, standing on the surface of a steel sphere 8944 m in diameter (~5.5 miles) and with a steel density of 8 g/cm3, will feel a pull of 1 Gal.

  68. Jasini

    I’m assuming a “to scale” version of a model that size would be, to all appearances, a perfect sphere?

  69. icemith

    Nice mind expanding fare Phil. Where do you get ‘em?

    Now I have been propelled to ask a couple of questions.

    1. At the very center of the Earth, is there a gravitational effect? If so, what would it be in terms of Gals? (But I think it would be zero!)

    2. During the Earth’s 26000 years Polar precession, (have I got that term correct?), where the Earth “wobbles” in its orbit, and the North Pole, (and obviously the South Pole too ), varies over a quite large area of the surface, does that alter the “lumpiness” of that surface. I feel it must, at least, be a factor in tectonic plate movements, and consequently earthquakes.

    Any thoughts?

  70. TJ Czeck

    @73 icemith: 1) At the center of the earth you would be weightless because the gravitational forces around you would cancel each other out. Given my very limited understanding of the scaling of the geoid, the locations of the real earth and the geoid earth would be in the same spot.

    2) Precession refers to where the spin axis point in the sky. In other words, the actual axis of the earth does not change, just the direction it points. You may be thinking of the magnetic north/south poles which do move, but have no influence on gravity (no one has been able to tie E&M and gravity equations together yet). Plate tectonics would influence the gravity geoid, but that would be due to mass distribution. (see posts 44 & 49).

    Hope that helps.

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