Astronomy Chapter 2

Front 1

horizon

Back 1

in common language, the horizon is the line where the sky meets the ground. Astronomers mean the line you would see if you held your eyes perfectly level and turned all the way around. A line to the horizon always makes a right angle with a line to the zenith

Front 2

altitude

Back 2

in common language, altitudes is the height of an object, such as an airplane, above the ground. Astronomers use the word to refer to the angle formed between an imaginary line from an observer to an object and a second line from the observer to the point on the horizon directly below the object

Front 3

revolve

Back 3

in common language, the words revolve and rotate are sometimes used synonymously to describe something that spins. Astronomers distinguish between the two terms, using rotate to mean that an object spins around an axis through its center, and revolve to mean that one object orbits another. Earth rotates about its axis (causing our day) and revolves around the sun (causing our year).

Front 4

day

Back 4

in common language, this word means both the time during which the sun is up in the sky and the length of time it takes to rotate once (from midnight to midday). the context of the sentence tells us which meaning is intended. Unfortunately, astronomers use this word in both senses as well.

Front 5

focus

Back 5

in common language, this word is used in several ways, to indicate directed attention, or the place where light is concentrated by a lens. In this mathematical context, it refers to a special point within an ellipse. An ellipse has two of these special points, and the sum of the distance from these points to any point on the ellipse is constant

Front 6

period

Back 6

in common language, the word period can mean how long a thing lasts. Astronomers use this word only when talking about repeating intervals, such as the time it takes for an object to orbit once