▪ eccentricity - The roundness of an ellipse, usually referring to the shape of an orbit. Nearly circular ellipses have eccentricities of zero, whereas a very elongated ellipses have eccentricities approaching the value of 1.0 The eccentricity of Comet Halley is 0.967 for excample. ▪ ecliptic - The apparent annual path of the Sun. This is the plane defined by the Earth’s orbit. Most of the other objects in the solar system orbit in essentially the same plane, so as we observe from Earth, we find those planets near the ecliptic. ▪ eclipse - A blocking of either the Sun, (solar eclipse), or the Moon, (lunar eclipse). Both of these two types of eclipses happen somewhere every six months or so. It is decidedly more difficult to arrange to see a solar eclipse since you need to be in the somewhat small shadow of the Moon. Lunar eclipses are much easier to see since of all the night side of Earth can see any given lunar eclipse. (weather permitting, of course). ▪ epicycle - The circle used in the geocentric universe of ancient Greece to explain the occasional backward motion we see in the wanderings of a planet. The heliocentric system has no need of these silly circles.