Introduction to Galilean Astronomy

Introduction to Galilean Astronomy

INTRODUCTION. Galileo lived a long time ago, and had discovered something during the dark nights as he looked at the heavens through a new-fangled device called a telescope. When he put his observations into words, his conclusions were so radical and so against the common thinking of his day that he put his life in jeopardy. People obviously were not very tolerant in those days! While everyone else was saying that the earth was the center of the universe, Galileo saw a few things with his telescope that just didn't fit with what people thought. He said that the earth was NOT the center of the universe - and that meant that not everything revolved around humanity. This was a severe blow against human pride, and other people didn't want to hear it! These next exercises dealing with the moon, Venus, Mars, Jupiter and Saturn are basic to Galileo's radical thinking. If you have done the previous planetary exercises of determining the orbit of the moon relative to the orbit of the earth, and investigating the orbital configurations of the other visible planets, you should be rather amazed at what can be observed with the unaided eye. As we push on, your eyes will need a little help - except for the case of observing our moon. It would be good if you had some binoculars or a small telescope of at least 8-power.

BUT FIRST! You should do a little observation INSIDE a building! Place a bare lightbulb in the middle of an otherwise dark room. YOU stand about 6 feet away from the bulb ("sun") and pretend that your head is the earth. Have a friend hold a white soccerball, and pretend that that ball is a planet. Slowly have the friend make two circles around the lightbulb. The first time the friend's circle should be 10 feet or more away from the light bulb, and the second time the friend's circle should be about 3 feet around the lightbulb. Your friend will move and then stop at each quarter the way around the bulb. You should sketch in your notes what you see of the ball. Do you see a crescent or mostly a round lit circle?

What you should find is then when the friend's orbit is outside of your own, you will see the sun-side of the planet as mostly or completely lit. But when the planet is closer to the sun than your are, you will see various phases from fully lit through crescents to next to nothing, and then back to fully lit when the planet is between you and the sun (a planetary eclipse of the sun*). Thus, if you look at the real planets, those which are sometimes seen as crescents must be "interior," and those which never show crescents must be "exterior" to our earth's orbit.

* Because the sun is actually so huge compared to any of the "interior" planets, the planet's eclipse of the sun hardly dims the sun at all. In fact, all you would see is a small dark circle slowly passing across the sun's brilliant disk. This is called a "transit" (→) Since you know that Mercury and Venus are interior planets, there are occasional opportunities for seeing their transits across the solar face IF you know how to indirectly see the sun by using a pin-hole projector.