Galilean Project Three: Venus

Galilean Project Three: Venus

INTRODUCTION. In this Galilean exercise, you yourself will see Galileo's third rebuttal to the popular knowledge which said that all the planets and stars revolved around the earth. (The first two dealt with Jupiter.)

Does Venus Show Phases (Crescents) at Anytime?

Even with a very low power telescope or binoculars (that's all Galileo had!) you have seen that Jupiter was more than a pin-prick of light - it was discernably a very small disk. (Hint: the sun is the only star that you will see that is a disk and not just a point of light. (Even with the most powerful land-based telescopes, the next nearest star appears to be just a point of light.) Well-, Venus (and Mars and Saturn, too) can be seen as something more than a point of light.

THE question is: When you look at Venus, do you EVER see a crescent? What does that mean? (No, it is not correct to say that that fact means its orbit is completely inside that of earths. Afterall, the moon shows crescents. So, again: what does a crescent mean?)

ANOTHER question is: if you ever see Venus as a more or less complete circle, what does that mean? (And remember that you do occasionally see the moon as a full circle - "full moon".) Go back to your light bulb in the center of the room, and have your friend hold the soccer ball as if it were Venus. When do you see a "full" Venus, and when do you see crescents?

A parallel question is: Do you ever see Venus in the far side of the night sky? By that is meant, is it in the half of the sky that is furthest from the sun, or is it in the closer half of the sky nearer the sun? Answer these same questions for the moon and Jupiter also - for comparisons, of course - just to check out your thinking!

In conclusion, you should be able to prove that Venus is not revolving around the earth, but instead is revolving around the sun in an orbit that is either "interior" or "exterior" to earth's orbit. Furthermore, you should also be able to determine if Venus's orbit is close to being on the same plane as that of earth's orbit. For this you will want to make a set up similar to one shown for determining the shape of the solar system.