1.5 space

Suppose you were talking on the phone to your friend who lives in the house beside you. From your windows you are looking at the night sky.

Your friend finds an interesting stellar object and wants you to look at it.

How can you be sure you are both looking at the same thing?

To locate the position of an object in space, two questions must be answered: "How high in the sky is it"? and "In which direction"? This problem can be solved with only two measurements. The first is the compass direction, called the azimuth. With due north as 0º and going clockwise, the azimuth will tell you which direction to point. For example, 180° from 0° would have you pointing due south;

270° would have you pointing west. The second measurement shows how high the object is in the sky.

This is called altitude. The altitude ranges from 0 at the horizon to 90° straight up. With these two measurements, stargazers can pinpoint objects in space. Zenith refers to the highest point directly overhead.

Because they are at such enormous distances from Earth, the stars appear to stay in one place in the sky. Only when viewed over extremely long periods of time can some stars be seen to move very slightly.

When observing planetary motion, however, a person needs to wait only a few days or weeks to see a planet change its position against the background of stars. "Planet" comes from the ancient Greek word for

"wanderer." The movement of these wandering celestial planets mystified early people. Sometimes the planets seemed to speed up over time in their movements across the sky. Other times they appeared to stand still. The path in the sky along which the Sun appears to move is called the ecliptic.

astronomers such as Aristotle and Copernicus tried to explain the motions of the planets which, when viewed from Earth, seemed very complex. Different interpretations of the available information eventually led to new theories being proposed. An example of this was Kepler's suggestion that the planets' paths were