In geodesy, a reference ellipsoid is a mathematically defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body. Because of their relative simplicity, reference ellipsoids are used as a preferred surface on which geodetic network computations are performed and point coordinates such as latitude, longitude, and elevation are defined. 

In 1687 Isaac Newton published the Principia in which he included a proof that a rotating self-gravitating fluid body in equilibrium takes the form of an oblate ellipsoid of revolution which he termed an oblate spheroid. Current practice uses the word 'ellipsoid' alone in preference to the full term 'oblate ellipsoid of revolution' or the older term 'oblate spheroid'. In the rare instances (some asteroids and planets) where a more general ellipsoid shape is required as a model the term used is triaxial (or scalene) ellipsoid. A great many ellipsoids have been used with various sizes and centres but modern (post-GPS) ellipsoids are centred at the actual center of mass of the Earth or body being modeled. 

The shape of an (oblate) ellipsoid (of revolution) is determined by the shape parameters of that ellipse which generates the ellipsoid when it is rotated about its minor axis. The semi-major axis of the ellipse, "a", is identified as the equatorial radius of the ellipsoid: the semi-minor axis of the ellipse, "b", is identified with the polar distances (from the centre). These two lengths completely specify the shape of the ellipsoid but in practice geodesy publications classify reference ellipsoids by giving the semi-major axis and the "inverse "flattening, , The flattening, "f", is simply a measure of how much the symmetry axis is compressed relative to the equatorial radius: For the Earth, "f" is around corresponding to a difference of the major and minor semi-axes of approximately . Some precise values are given in the table below and also in Figure of the Earth. For comparison, Earth's Moon is even less elliptical, with a flattening of less than , while Jupiter is visibly oblate at about and one of Saturn's triaxial moons, Telesto, is nearly to . QUESTIONS:

Q1:
1. What is the name of the equatorial radius of the ellipsoid?
2. How is the equatorial radius of the ellipsoid referred to?
3. What is the name for the equatorial radius of the ellipsoid?
Q2:
1. What is the name for the semi-minor axis of an ellipsoid?
2. What does one call the semi-minor axis of the ellipsoid?
3. How is the semi-minor axis of the ellipsoid referred to?
Q3:
1. What is the starting point of the polar distance of an ellipsoid?
2. Where is the polar distance of an ellipsoid measured from?
3. Where does one start when measuring polar distance of an ellipsoid?
Q4:
1. Is it Earth's or Jupiter's moon that has a more elliptical orbit?
2. Is the more elliptical orbit attributed to the moon of Jupiter or of Earth?
3. Is the moon of Jupiter or of Earth the one whose orbit is more elliptical?
Q5:
1. What is meant by a reference ellipsoid?
2. What is the definition of a reference ellipsoid?
3. How can a reference ellipsoid be defined?
Q6:
1. What is a reference ellipsoid a preferred surface for?
2. In what context is a reference ellipsoid considered a preferred surface?
3. When would you consider a reference ellipsoid to be a preferred surface?
Q7:
1. What makes a reference ellipsoid a preferred surface?
2. Why are reference ellipsoids desirable surfaces?
3. What gives reference ellipsoids desirable qualities as surfaces?
Q8:
1. What kinds of computations are done on reference ellipsoids?
2. What computations are reference ellipsoids useful for?
3. What are the computations that can be done upon a reference ellipsoid?
Q9:
1. What coordinates come from reference ellipsoids?
2. What are the coordinates that reference ellipsoids use?
3. Reference ellipsoids are associated with what coordinates?
Q10:
1. Who was the author of the Principia?
2. Who was the Principia written by?
3. What man penned the Principia?
Q11:
1. When did Isaac Newton write the Principia?
2. In what year did Isaac Newton pen the Principia?
3. What was the year when Isaac Newton authored the Principia?
Q12:
1. What term did Isaac Newton deploy in the Principia?
2. What term is used by Isaac Newton within the Principia?
3. What term appears in Isaac Newton's Principia?
Q13:
1. What is the current term for oblate spheroids?
2. What are oblate spheroids called today?
3. What is the present-day nomenclature for oblate spheroids?
Q14:
1. What is the center of current ellipsoids?
2. What currently serves as the center point of ellipsoids?
3. In what location are ellipsoids presently centered?
