The imaginary circle around the celestial sphere that separates that sphere into a northern and southern half. It is analogous, of course, to the equator of Earth, but out there on the sky.

celestial equator

The celestial meridian is an arch across the sky which begins on the horizon at the north point, passes through the zenith, then proceeds down to the horizon at the south point. This vertical circle separates the sky into an eastern half and a western half. All celestial bodies must cross the meridian once each day. When the Sun crosses, we call that ‘noon’

celestial meridian

The distinctive red layer of the Sun which is a transition between the bright photosphere that we see and the faint, extended corona. Fountains of gas called spicules are found in the chromosphere.

chromosphere

(B-V) The numerical difference between an object’s brightness in the blue part of the spectrum, B, and its brightness in yellow, V (for visual). The color index of stars is often used on the horizontal axis of H-R diagrams.

color index

A relatively small, cold, icy, dirty object orbiting the Sun. Comets spend the vast majority of their time far from the Sun in a region known as the Oort Cloud. It’s when they get gravitationally bumped into a different orbit that takes them very close to the Sun when the fun begins!

comet

An alignment of two astronomical bodies so that they are seen to be in the same general direction in our sky. Planets are occasionally in conjunction with each other or the Sun.

conjunction

A grouping a stars forming some mythological figure or other object of our imagination. There are eighty-eight officially recognized constellations in the sky. Hey, that’s the same as the number of keys on a piano!

constellation

A concave indentation on the surface of a planet or moon usually caused by the impact of a meteoroid.

crater