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Class 2 - Parallax

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**1. **Class 2 - Parallax The idea of parallax is nothing new – only its findings are.

**2. **The Theory… ASSUMING:
The Earth orbits the Sun.
We know the distance the Earth is from the Sun.
We can determine the distance to (relatively) near by stars.
How
STELLAR PARALLAX!

**3. **How It’s Done…

**4. **The History of Parallax The actual term parallax comes from the Greek word parallagé meaning alteration.
During Ancient Greek times, the heliocentric theory was a leading competitor for the explanation of the way the heavens worked.
The idea of parallax was hypothesized.
Wasn’t observed – a strike against the heliocentric theory.

**5. **A Possible Heliocentric Theory

**6. **Resurfacing Parallax The idea of parallax would stay on the back burner until the Renaissance.
The idea of a sun-centered solar system had really gained speed around this time.

**7. **Galileo Tries his Hand Galileo found very convincing proof for the heliocentric theory – Jupiter’s Moons!
He was convinced that parallax should be observable as well.
Despite the best
observations he took,
he never detected the
slightest parallax
motion of any star.

**8. **Flash forward 200 years… Friedrick Wilhelm Bessell in 1838 was the “first” (several other astronomers had good evidence) astronomer to detect and measure a stellar parallax.
He measured the parallax of a star 61 Cygni. He measured the parallax to be 0.314 arcseconds.
Today the accepted value of the parallax of 61 Cygni is 0.285 arcseconds – very close to the original calculation done by Bessell.

**9. **Why it Wasn’t Detected Earlier… The logic and theory was right.
The distance to stars wasn’t expected to be so great!

**10. **Flash Forward another 200! We have now launched satellites that orbited the Earth and made precise measurements of the parallax of nearby stars.
On the board
Derivation of parallax formula…
What do you think?
Why not measure stellar parallax of ALL Stars in the Galaxy?

**11. **Your Task… Doing stellar parallax, we use the distance between the Earth and the Sun as a baseline.
You will design an experiment to measure distances using parallax.
You will need:
A Baseline – two separate views at certain distances.
An object placed at a distance away – with a distant background.
A method of estimating the angle between the two baselines.
Go To Town…

**12. **Key Terms and Warnings Baseline: The distance from one viewpoint to the point between the two viewpoints.
i.e. The distance from the Earth to the Sun.
NOT The distance from the Earth in July to the point in January. ? Common error!
Although you measure the angle, the parallax angle is half of that. This is because we are using the 90o triangle set up by the Sun, Earth, and Star.