Geoscience Reference
In-Depth Information
Cone of precession
Lines
magnetic
force
Shadow
1
Diameter
Shadow
2
Curvature
Compass
needle 2
Spin
Rotation
Compass
needle 1
Weight
mass
L
Clock 2
Clock 1
Magnetic
inclination ( l )
Axis
of
rotation
Fig. 1.12 Classic techniques to measure Earth features.
of the radius, r . In symbols g
Gm / r 2 . Earth's radius is
circumference divided by 2
, from Euclid's formula.
Knowing the values of g , G (from a famous experiment by
Cavendish), and r , the value of m is computed as about
6ยท10 21 tons.
1.4.4
Earth's density
Knowing mass from Newton and volume via Eratosthenes
and Euclid to be approximately 1.08
10 12 km 3 , we can
, as about 5,500 kg m 3 .
The fact that this is so much more than that of either
water (1,000 kg m 3 ) or typical crustal rock (granite at
2,750 kg m 3 ) provided the first clue to early geoscientists
that the planet must be very dense internally, most probably
due to a central core of dense metal.
get Earth's mean density,
Eratosthenes of Cyrene
Fig. 1.13 Vintage sketch of Eratosthene's scheme for calculation of
Earth's circumference.
1.4.3
Earth's mass
1.4.5
Latitude
Newton's Law of Gravitation, also known as the inverse-
square law, says that gravity is the product of any body's
mass, m , times a universal constant, g , divided by the square
Chinese astronomers and navigators estimated latitude
by the systematic variation of shadow lengths, from fixed
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