Orbital eccentricity measures how far an orbit departs from a perfect circle, running from zero for a circle to just under one for a highly stretched ellipse. Earth's is about 0.017, nearly circular, while comets on long journeys from the outer solar system can approach one. Eccentricity governs how much a body's distance from its star varies over a year, and therefore how severely it is heated and tidally flexed. Slow cycles in Earth's eccentricity, part of the Milankovitch pattern, help pace the ice ages.