Mass shifts around inside the planet, altering those gravitational anomalies. Mountains and valleys emerge and disappear due to plate tectonics. Occasionally meteors crater the surface. And the gravitational pull of the moon and sun not only cause ocean and atmospheric tides but earth tides as well.
In addition, the changing weight of the oceans and atmosphere can cause deformations of the crust "on the order of a centimeter or so," notes geophysicist Richard Gross at the Jet Propulsion Laboratory in Pasadena, Calif. Moreover, to even out Earth's imbalanced distribution of mass and stabilize its spin, "the entire surface of the Earth will rotate and try to redistribute mass along the equator, a process called true polar wander," Meert says. To keep track of Earth's shape, scientists now position thousands of Global Positioning System receivers on the ground that can detect changes in their elevation of a few millimeters, Gross says.
Another method, dubbed satellite laser ranging, fires visible-wavelength lasers from a few dozen ground stations at satellites. Any changes detected in their orbits correspond to gravitational anomalies and thus mass distributions inside the planet. Still another technique, very long baseline interferometry, has radio telescopes on the ground listen to extragalactic radio waves to detect changes in the positions of the ground stations. It may not take much technology to understand that Earth is not perfectly round, but it takes quite a bit of effort and equipment to determine its true shape.
To measure and specify coordinates on a 3D surface, one first must define the geometry of the surface itself. Geospatial Activities. The Earth is Not a Sphere! It's an Ellipsoid Key points: The Earth is not a perfect sphere. Category: Earth Science Published: December 20, The earth is not a sphere. The earth is an oblate spheroid with a diameter at the equator that is 43 km larger than at the poles, according to the National Oceanic and Atmospheric Administration.
This bulging at the equator is caused by the spinning motion of the earth, much like how spinning pizza dough in the air causes it to flatten due to centrifugal force. This is what gives rise the bulge. I will provide a mathematical description when mathjax is added. Show 12 more comments. Gaialogist Gaialogist 1 1 silver badge 4 4 bronze badges. In very good approximation, due to the small value of Earth flattening, km is at the same time 1 the arithmetical mean radius, 2 the authalic or equal area radius and 3 the volumetric or equal volume radius.
Update: Why is this the right picture? David Hammen David Hammen This example sounds weird to me Something else: the gravitational force can be normal to the surface even when it deviates from the geoid and not normal even when it does not deviate. Since gravitational acceleration is the gradient of the gravitational potential, the gravitational acceleration vector is necessarily normal to the geoid.
It's in the math. Here's a relevant answer at math. Ridge push and slab pull, for example. Consider a surface close and parallel to the geoid but not superimposed: it may have a normal gravity; consider a surface crossing the geoid: on the crossing line the two surfaces match but the gravity is not normal to the Earth's surface.
I just think it's daring to make the link between tangential components of the gravity vector and earthquakes. Maybe this point of view could even mistakenly reverse causes and consequences the impact of tectonic structures on gravity anomalies, not the contrary Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name.
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