Our Moving Earth
After Kepler and Galileo, the scientific community embraced the notion that the earth does, indeed, revolve around a stationary sun. It rotates on its axis to produce the day/night cycle as well. Galileo even offered logical reasons why we don't "feel" this motion. Despite the overwhelming evidence, some skeptics remained, insisting that the earth remains stationary in space. But even the last of the doubters had to yield as more evidence trickled in. Classic examples came in 1675, 1729, 1835, 1838, 1851, and 1887. Your mission is to discover TWO of these observations/experiments that show that the earth does move through space. Give a brief overview (in your own words) what was observed and why it demonstrated that the earth moves. Don't forget to list your sources.
Find any two of these and you get credit.
Measurement of the speed of light (1675) by Ole Romer
Romer noticed that the orbital periods of the moons around Jupiter varied over time. He reasoned that it was because light had a finite speed and that the variations in these periods resulted from our changing distance with Jupiter. He determined the speed of light from this. However, the entire explanation could only occur if the earth was moving in the first place.
Aberration of Starlight (1729) - James Bradley
This one is tricky. Bradley notice that he had to tilt his telescope a bit different to view the same object then he did 6 months earlier. He reasoned that because light had a finite speed, the change in tilt was due to the motion of the earth as it moved through space. He got the idea by observing the direction a flag waved on a sailboat. Despite a slight steady wind from one direction, the flag always flew toward the back side of the boat (stern) no matter which direction the boat was sailing. He wondered if a similar effect would be observed on a moving earth. Perhaps an analogy would make this clearer. Imagine it is raining and the wind is not blowing so the drops fall straight down. However, if you are moving (in a car, for example) the rain appears to be falling at an angle. If you change direction (like the earth does in 6 months), the rain now looks like it is falling from a different direction. That is what stellar aberration is ... except it applies to light, not rain.
Coriolis Effect (1835) - Gaspard-Gustave Coriolis
Coriolis described a phenomena observed when an object is moving on (and viewed from) a rotating object. It has since been named the Coriolis effect. This effect has been observed on our rotating earth (although explained much earlier and observed much later than described by Coriolis). Basically, objects moving in the northern hemisphere appear to deflect right. However, this is an illusion since as viewed from space, the object is actually moving in a straight line. This can be observed in the path of airplanes, air currents, long range artillery shells, etc.
Stellar Parallax (1838) - Friedrich Bessel
As the earth moves in space, there is an parallax with nearby stars. It wasn't until 1838 that this small phenomena was actually observed.
Foucault Pendulum (1851) - Léon Foucault
Foucault noticed that a pendulum would continually change its plane of oscillation over time. He reasoned that the pendulum was always swinging along the same plane (in space) ... it was the rotating earth underneath that was changing.
The Annual Doppler Shift (1887) - Vogel and Scheiner
The Doppler Effect (which we cover in unit 3) does work but you have to be careful. All stars move relative to the sun and produce some kind of Doppler shift. We move around the sun as well. Added together, we observe a Doppler shift relative to the stars. From just one observation of one star we have no idea if the Doppler shift is coming from the earth's motion, the star's motion or both. The trick here is that as we move around the sun, the Doppler value changes in time. For example, in 6 months we will be going in the opposite direction relative to the sun. This will (in most all cases) change the value we see. (The exception are for stars which are perpendicular to the ecliptic since the earth's speed produced no Doppler shift at all). So the trick here is to notice a changing Doppler shift throughout the course of a year for nearly all stars ... not just one star in one observation.
For simplicity, imagine a distant star is stationary with respect to the sun (and lies on the ecliptic). At one time in the year the earth is moving directly toward the star and we notice a Doppler blue shift. However, in 6 months the earth is moving directly away from the same star and we notice a Doppler red shift. This can only be explained if the earth moving. We cover the Doppler Effect in the next unit.