Kepler Explorer.

Every closed orbit under gravity is an ellipse with the Sun at one focus, not the center. The perihelion (closest point) and aphelion (farthest point) sit on the major axis, on opposite sides of the Sun.

A line from the Sun to the orbiting body sweeps out equal areas in equal time intervals, anywhere on the orbit. Because the swept region near perihelion is short and wide, the body must move fastest there; near aphelion the region is long and thin, so it moves slowest.

The orbital period depends only on the semi-major axis, not on eccentricity: doubling the axis multiplies the period by about 2.83, not 2. Two orbits with the same size but different shapes always share the same period.