Observing the Stars and Planets

Abstract

ORIGINS Throughout their existence, humans have used the apparent positions of celestial objects to define the day, month, year, and seasons, as well as for navigation. T he Sun, the Moon, and the stars rise every day in the eastern half of the sky and set in the western half. If you leave your camera on a tripod with the lens open for a few minutes or hours in a dark place at night, you will photograph the “star trails” – the trails across the photograph left by the individual stars. In this chapter, we will discuss the phases of the Moon and planets, and how to fi nd stars and planets in the sky. Stars twinkle; planets don’t twinkle as much (■ Fig. 4–1 ). We will also discuss the motions of the Sun, Moon, and planets, as well as of the stars in the sky. THE PHASES OF THE MOON AND PLANETS From the simple observation that the apparent shapes of the Moon and planets change, we can draw conclusions that are important for our understanding of the mechanics of the Solar System. In this section, we shall see how the positions of the Sun, Earth, and other Solar-System objects determine the appearance of these objects. The phases of moons or planets are the shapes of the sunlit areas as seen from a given vantage point. h e fact that the Moon goes through a set of such phases approximately once every month is perhaps the most familiar astronomical observation, aside from the day/night cycle and the fact that the stars come out at night (■ Fig. 4–2c , 4–3 ). In fact, the name “month” comes from the word “moon.” h e actual period of the phases, the interval between a particular phase of the Moon and its next repetition, is approximately 29 1 / 2 Earth days. h is type of period that depends on the orientation with respect to a third body (in this case, the Sun) is known as a synodic period .