Kepler's First Law
Orbits are ellipses
Teaching this? Open projector mode
Johannes Kepler discovered that planets do not move in perfect circles. His First Law: a bound orbit in the ideal two-body model is an ellipse, with the central body sitting at one focus (not the centre). A circle is just the special case of an ellipse with zero stretch.
An ellipse is described by two numbers. The semi-major axis (a) is half the long width — it sets the orbit's overall size. The eccentricity (e) measures how squashed it is: e = 0 is a circle, and as e climbs toward 1 the ellipse grows long and thin. The nearest point to the focus is periapsis; the farthest is apoapsis.
Check yourself
In Kepler's First Law, where is the Sun located?
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