The Third Spheroid from the Sun

The Earth is round, or so they say. Video games, globes, and graphic art may depict the Earth as a perfect ball shape or sphere, but in reality the Earth is a bit squished. Therefore, we call the Earth a spheroid, rather than a sphere. It is sphere-like, but somewhat elliptical.

To take it one level further, we all know that the surface of the Earth isn’t perfectly uniform. There are mountains and valleys, bumps and dips. Geoid is the term used for a more detailed model of the Earth’s shape. At any point on the globe, the geoid may be higher or lower than the spheroid. Figure A-1 shows an example of the relationships among the sphere, spheroid, and geoid.

Figure A-1 is based on a graphic courtesy of Dylan Prentiss, Department of Geography, University of California, Santa Barbara. Further descriptions are available at the Museum’s Teaching Planet Earth web site,

As you can see, when talking about the shape of the Earth it is very important to know what shape you are referring to. The shape of the Earth is a critical factor when

Illustration of methods for describing the Earth’s shape
Figure A-1. Illustration of methods for describing the Earth’s shape

producing maps because you (usually) want to refer to the most exact position possible. Many maps are intended for some sort of navigational purpose, therefore mapmakers need a consistent ...

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