296 The Poetry of Physics and The Physics of Poetry
The Earth
The planet Earth is a nearly spherical body with a radius of 6,378 km,
which is slightly flattened at the poles and bulged at the equator. Its mass
is equal to 6 × 10^24 kg, and its density is 5.4 grams per cubic centimeter
or 5.4 times the density of water. The Earth orbits the Sun once every
365 ¼ days or once a year. The extra 1/4 day gives rise to our leap year.
The Earth rotates on its axis every 24 hours. The axis of rotation is
inclined 23° to the plane of the planetary orbit. The tilting of the axis
accounts for the four seasons of the non-equatorial zones of the Earth.
When the Earth is tilted such that the northern hemisphere is closer to the
Sun it is summer in the North and winter in the South. At the opposite
end of the orbit the northern hemisphere is farther from the Sun and the
seasons are reversed, winter in the North and summer in the South. In
passing between these two extremes the Earth passes through a point
where its inclination is perpendicular to the line between the Earth and
the Sun. The period corresponds to the transitional seasons, spring and
fall.
The Earth possesses an atmosphere consisting primarily of nitrogen
and oxygen. Its surface is 71% ocean and 30% land. The surface of the
Earth both below the ocean and on the continents displays considerable
structure. The interior of the Earth consists of iron-nickel core
surrounded by a rocky mantle upon which the crust of the Earth sits. The
continental crust is 32 km thick but the crust under the oceans is
considerably thinner.
The Earth possesses its own magnetic field, which is related to its
rotation as evidenced by the proximity of the geomagnetic North Pole
and the geographical North Pole. The Earth’s magnetic field has trapped
charged particles, which form radiation belts high above the Earth’s
atmosphere. The Earth has one satellite, the Moon.
The Moon
The Moon, the second brightest object in the sky, orbits the Earth only
384,000 kilometers away completing a revolution once every 27⅓ days.
The Moon always shows the same face to the Earth and, hence, rotates
on its axis once every 27.33 days. The Moon is a nearly spherical object
with a radius of approximately 1700 kilometers, a little more than 1/4 of
the Earth’s radius. The Moon’s actual shape is egg-like with the longer