The Equator is the line that separates the earth into two hemispheres
It is measured around the surface of the earth. The atmosphere has its own measurement excluded from that of the Earth. The Equator is bulged, and the Poles slightly flattened. The figure given is for sea level, and ignores any mountains. The Poles are about 21.4 km closer to the centre of the Earth than the Equator is.
This is known as seismology which is a sub branch of geophysics.
Since the Earth is a Spheroid shape, (not a true sphere) 50% of its surface is illuminated by the Sun, at any given moment.
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The hydrosphere, and it includes all the water on earth.
Each meridian is a half-circle from the north pole to the south pole of the earth.
The circumference of a circle is the distance around it.
Circumference.
Circumference.
At a given distance from the Sun, only at a very specific spead will it go in an exact circle. And that perfect circle will soon be disturbed by gravitation from other planets, comets, etc.
Circumference
the radius is half of the diameter
The distance around an object is called its perimeter. ------------------------------------------------- Or if it is a circle its circumference.
We've not been given a circle.
That's a "circle". The given distance is the circle's radius, and the given point is the circle's center.
A circle of constant longitude passing through a given place on the earth's surface and the terrestrial poles.
Orbit