H H-R Diagram - A plot of two separate characteristics of stars which reveals, among other things, the different sizes of stars horizon - most simply, the horizon is the circle around us that separates the sky from the ground.  But a strict definition of the astronomical horizon is the great circle that is ninety degrees away from the zenith overhead.  Anyone who lives in a hilly or mountainous region never actually sees the horizon. hour angle - the distance that an object is away from the observer’s celestial meridian, expressed in units of time.  For example, the Sun has an hour angle of zero hours when it crosses the meridian at noon each day. Hubble Constant - Now referred to as the Hubble Parameter, it is a measure of the expansion of the universe.  Edwin Hubble first established that galaxies that are farther away from us are moving away faster, as if it all began with a single event 13.7 billion years ago called the Big Bang.  Astronomers have now fixed its value at about 70 km/sec per megaparsec.
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Glossary
Carpe Caelum