The frontispiece shows three views of the paths of comets:
- The Aristotelian theory that comets consist of vapors beneath the Moon (left);
- Kepler’s theory that comets move in straight lines (right); and
- Hevelius’ view that they originate in the outer regions and descend in a parabolic trajectory passing by the Sun (center).
From his observation of the comets of 1652, 1661, 1672, 1677 and 1680, Hevelius concluded that comets pass by the Sun in a parabolic path.