Chapter 2: The Sky

Discussion How is a statement in a political campaign speech different from a statement in a scientific discussion? What is the difference between our solar system, our galaxy, and the universe? Why is it difficult to detect planets orbiting other stars? What does the size of the star image in a photograph tell you?

Knowledge check What is the object to the right and which of the following objects would be found in it? Stars Planets Galaxy clusters Filaments Spiral arms

Knowledge check In the figure to the right, how can you tell that Mercury follows an elliptical orbit?

Constellations In ancient times, constellations only referred to the brightest stars … … that appeared to form groups, representing mythological figures, much like finding shapes in clouds.

Constellations Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

Constellations The stars of a constellation only appear to be close to one another. Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us.

Celestial Sphere When we look at the sky, we see stars but have no actual clue as to how far away they are. Therefore it is as if they were all on a sphere out a long distance from us. This conceptual device is known as the celestial sphere. Distances between objects then are measured in angles since all objects appear to be at the same distance. This is an example of the use of a model.