Some Facts about the Earth and Its Orbit around the
The Earth's radius is 6370 kilometers (about 3980 miles). The sun's radius
is about 100 times greater.
The earth revolves (orbits) around the sun once per year. The orbit is slightly
elliptical in shape, with the sun at one of the foci of the ellipse. The orbit
defines a plane containing the sun.
The average distance from Earth to the sun is about 149 million kilometers
(about 93 million miles).
The distance from Earth to the sun varies from about 147 million km to
about 152 million km (that is, from about 92 million to 95 million miles,
or about +/- 1.4% from the mean distance).
The earth is closest to the sun (perihelion) during January and
farthest (aphelion) during July.
The earth rotates once per day (24 hours). The rotation defines an axis
of rotation, the North Pole, the South Pole, and the
equator. [Actually the earth rotates slightly more than once per
solar day, because the earth moves a little bit in its orbit around the sun
each day. As a result, any particular place to face the sun most directly
again, the earth must rotate a little bit more than 360 degrees.]
[Do you know how latitude and longitude are defined? How
about a latitude circle?] During the course of a day, any particular
location will move in a circle coinciding with its latitude circle.
The local time of day (defined in terms of the position of the sun in the
sky) is the same everywhere along a longitude line.
At any moment the sun illuminates half of the earth. The boundary between
the sunlit and unlit halves of the earth is called the circle of illumination
or the terminator.