topography (such as the presence of mountains) and geology (the density of rocks in the
vicinity) also influence the gravitational field to a small extent.
Other forces acting on an object may augment or oppose the earth's actual gravitational
field, causing variations in the apparent force of gravity (see also Apparent weight.) One
example is the fictitious centrifugal force caused by the earth's rotation, which imparts an
upwards force opposing gravity and diminishing its apparent effect. This effect is
stronger at lower latitudes (i.e. nearer the equator), reducing to zero at the poles. Another
example is buoyancy: even in air, objects experience a small supporting force which
reduces the apparent strength of gravity. Finally, the gravitational effects of the Moon
and the Sun (also the cause of the tides) also have a small effect on apparent gravity,
depending on their relative positions; typical variations are 2 µm/s² (0.2 mGal) over the
course of a day.
In combination, the equatorial bulge and the effects of centrifugal force mean that sea-
level gravitational acceleration increases from about 9.780 m/s² at the equator to about
9.832 m/s² at the poles, so an object will weigh about 0.5% more at the poles than at the
equator [1]. See acceleration due to gravity for further information.
Gravity also decreases with altitude (since greater altitude means greater distance from
the earth's centre). All other things being equal, an increase in altitude from sea level to
the top of Mount Everest (8,850 metres) causes a weight decrease of about 0.28%. 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 250 miles (roughly
the height that the space shuttle flies) gravity is still nearly 90% as strong as at the earth's
surface, and weightlessness actually occurs because orbiting objects are in free-fall.
If the earth was of perfectly uniform composition then, during a descent to the centre of
the earth, gravity would decrease linearly with distance, reaching zero at the centre. In
reality, the gravitational field peaks within the Earth at the core-mantle boundary where it
has a value of 10.7 m/s².
Comparative gravities of various cities around the world
The table below shows gravitational acceleration or various cities around the world.
[1]
