Comments on: The Earth’s lumpy gravity

By: The topographic Earth - SATELITE PHOTOS – SATELITE PHOTOS

The topographic Earth - SATELITE PHOTOS – SATELITE PHOTOS — Tue, 18 Oct 2011 21:13:32 +0000

[...] Staring down an active volcano’s throat – The Earth’s lumpy gravity – Satellite perspective of a volcanic vigour valve – Volcano investigate in [...]

By: Το σχήμα της Γης | Ody Waves

Το σχήμα της Γης | Ody Waves — Sun, 07 Aug 2011 22:44:58 +0000

[...] The Earth’s lumpy gravity – Bad Astronomy [...]

By: ¿Quién dijo que la Tierra era redonda? GOCE demuestra que es… ¡Una patata! | Linkeando

Sat, 23 Apr 2011 01:17:12 +0000

[...] todo lo del GOCE, que se me había quedado en la pila de cosas a comentar, una anotación de Bad Astronomy; José J. Ramos, que trabaja en la el Centro del Observación de la Tierra de la ESA, nos recordó [...]

By: The lumpy 3D Earth | Bad Astronomy | moregoodstuff.info

The lumpy 3D Earth | Bad Astronomy | moregoodstuff.info — Thu, 21 Apr 2011 13:50:06 +0000

[...] month, scientists using the GOCE spacecraft released a model of the Earth’s geoid: essentially, a shape telling you which way is down. If the Earth were a perfectly smooth sphere of [...]

By: TJ Czeck

TJ Czeck — Sat, 09 Apr 2011 05:15:02 +0000

@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.

By: icemith

icemith — Wed, 06 Apr 2011 18:29:15 +0000

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?

By: Earth’s Gravity Map Unveiled « ICU Online Blog Central

Earth’s Gravity Map Unveiled « ICU Online Blog Central — Tue, 05 Apr 2011 16:12:44 +0000

[...] gravity distorts the shape of the oceans, it’s hard to translate from one system to the other. Discover Magazine’s Phil Plait says GOCE fixes the [...]

By: Potato Earth | Richer Ramblings

Potato Earth | Richer Ramblings — Mon, 04 Apr 2011 21:18:15 +0000

[...] gravity in the same way. Although, apparently, we don’t, as this Potato Earth (BBC and Bad Astronomy) shows! Yellow shows areas of strong gravity, whereas blue indicates areas of weak gravity. This [...]

By: Jasini

Jasini — Mon, 04 Apr 2011 15:08:30 +0000

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

By: Sion

Sion — Mon, 04 Apr 2011 09:58:26 +0000

@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.

By: Sion

Sion — Mon, 04 Apr 2011 08:52:20 +0000

@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…

By: HIRAM113

HIRAM113 — Mon, 04 Apr 2011 07:57:12 +0000

@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.

By: Messier Tidy Upper

Messier Tidy Upper — Sun, 03 Apr 2011 05:59:56 +0000

@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!

By: Gravity in 3D! «

Gravity in 3D! « — Fri, 01 Apr 2011 22:13:30 +0000

[...] The Earth’s lumpy gravity (blogs.discovermagazine.com) [...]

By: Roger

Roger — Fri, 01 Apr 2011 22:11:59 +0000

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

By: Joseph G

Joseph G — Fri, 01 Apr 2011 19:20:16 +0000

@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.

By: Joseph G

Joseph G — Fri, 01 Apr 2011 19:01:32 +0000

@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

@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!

By: Solius

Solius — Fri, 01 Apr 2011 18:11:23 +0000

Roger, cut and paste this into your browser:

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

By: Roger

Roger — Fri, 01 Apr 2011 17:56:16 +0000

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.

By: TJ Czeck

TJ Czeck — Fri, 01 Apr 2011 17:27:47 +0000

@(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.

By: DrivethruScientist

DrivethruScientist — Fri, 01 Apr 2011 16:38:14 +0000

@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 …

By: naveen

naveen — Fri, 01 Apr 2011 15:35:38 +0000

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

By: Sion

Sion — Fri, 01 Apr 2011 15:20:48 +0000

@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.

By: Gus Snarp

Gus Snarp — Fri, 01 Apr 2011 14:14:29 +0000

@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.

By: Solius

Solius — Fri, 01 Apr 2011 13:17:42 +0000

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.