Question

the law of suggests that the orbit of planets is not circular but .

Explanation:

In astronomy (a subfield of Natural Science, specifically Physics), Kepler's laws of planetary motion are relevant. Kepler's first law (the law of ellipses) states that the orbit of a planet around the Sun is an ellipse, with the Sun at one of the two foci. So the first blank should be "Kepler" (or more precisely "Kepler's first law" but often referred to as the law of ellipses in this context, but the key name is Kepler related) and the second blank is "elliptical" (or "an ellipse").

Answer:

The first blank: Kepler (or Kepler's first law, but commonly "Kepler" in this context)
The second blank: elliptical (or an ellipse)