<P> The same two factors influence the direction of the effective gravity (as determined by a plumb line or as the perpendicular to the surface of water in a container). Anywhere on Earth away from the Equator or poles, effective gravity points not exactly toward the centre of the Earth, but rather perpendicular to the surface of the geoid, which, due to the flattened shape of the Earth, is somewhat toward the opposite pole . About half of the deflection is due to centrifugal force, and half because the extra mass around the Equator causes a change in the direction of the true gravitational force relative to what it would be on a spherical Earth . </P> <P> Gravity decreases with altitude as one rises above the Earth's surface because greater altitude means greater distance from the Earth's centre . All other things being equal, an increase in altitude from sea level to 9,000 metres (30,000 ft) causes a weight decrease of about 0.29% . (An additional factor affecting apparent weight is the decrease in air density at altitude, which lessens an object's buoyancy . This would increase a person's apparent weight at an altitude of 9,000 metres by about 0.08%) </P> <P> It is a common misconception that astronauts in orbit are weightless because they have flown high enough to escape the Earth's gravity . In fact, at an altitude of 400 kilometres (250 mi), equivalent to a typical orbit of the Space Shuttle, gravity is still nearly 90% as strong as at the Earth's surface . Weightlessness actually occurs because orbiting objects are in free - fall . </P> <P> The effect of ground elevation depends on the density of the ground (see Slab correction section). A person flying at 30 000 ft above sea level over mountains will feel more gravity than someone at the same elevation but over the sea . However, a person standing on the earth's surface feels less gravity when the elevation is higher . </P>

Where on earth surface is gravity the strongest