Download Ellipses, Parallax, and Retrograde Motion – Study Guide

Astronomy
2011
Name __________KEY____________
Ellipses, Parallax, and Retrograde Motion – Study Guide
Parallax
1. The farther an object is from the observer, the _smaller__ the parallax.
2. T or F Parallax is an apparent motion.
3. The smaller the baseline of the observer, the __smaller_ the parallax.
4. T or F
It is not possible to measure parallax with your unaided eye.
5. T or F
If a star has a parallax angle of 0.2, it is 5 parsecs distant.
6. T or F One parsec equals 3.26 light years.
7. The largest baseline astronomers use today is about equal to ________ .
a. the diameter of Earth
b. Earth—Moon distance
c. 2 AU’s
Ellipses
8. The greater the eccentricity, the more _oval (or flattened)_ the ellipse.
9. Kepler’s 1st Law: All planets orbit in _ellipses__(shape).
10. Kepler’s 2nd Law: The closer a planet is to the sun, the _faster_ it moves.
11. Kepler’s 3rd Law: The closer a planet is to the sun, the _shorter__ its year is.
12. The Sun is at one __focus__ of the elliptical orbit.
Retrograde Motion
13. T or F All planets as observed from Earth will exhibit retrograde motion at some time.
14. T or F There are only two inferior planets in our Solar System.
15. T or F Retrograde motion is an apparent motion.
16. T or F Mars is brightest in our night sky when it is seen during retrograde cycle.
17. T or F Only superior planets can be in opposition with Earth.
18. T or F Any two celestial objects may appear in conjunction.
19. T or F The closer a planet is to the Sun, the faster it moves in its orbit.
20. T or F The more distant a planet, the longer it will appear in retrograde motion.
21. T or F All celestial objects will appear to move Eastward over time.
22. T or F During retrograde motion, an object appears to move westward over time.
23. T or F An example of opposition is when Mars appears directly overhead at midnight.
24. T or F When two objects are in conjunction, they are not actually very close together.
25. T or F Mercury and Venus do not ever appear very far away from the Sun.
Key Concepts/Possible Essay Questions
1. Why did the ancient astronomers not see parallax? What did they conclude?
Distances to the stars were TOO far; they concluded that Earth does NOT move.
2. How can you determine the latitude of Anchorage (or any place on Earth) using just your
hands and a clear night sky?
The altitude of Polaris (the North Star) is equal to the latitude of any where on Earth.
You can determine the altitude of Polaris using the angular measure equivalents of your
Hand (fist = 10 degrees, hand span = 20 degrees, pinky = 1 degree, etc)
3. Discuss the difference between what we see (apparent) and what is real (actual) with
relation to retrograde motion or parallax.
Apparent means what we see when we make our observations. Actual means what really
happens as if observed from outside the solar system. For example, stars do NOT really
shift in the sky as Earth orbits Sun (parallax) OR planets do NOT really change course in
their orbit around the Sun (retrograde motion).
4. Draw or describe a shape with an eccentricity of 0 (zero), 1 (one), 0.2 and 0.9. Include
where the foci (or center) are.
A perfect circle has an eccentricity of zero; a line has an eccentricity of 1 (one).
This is eccentricity of 0.2:
This is eccentricity of 0.9:
5. If the Earth still had a 23.5 tilt, but orbited the sun with an eccentricity of 0.8, how would
the length of our seasons change?
We would have VERY short and warm winters, and LONG, cool summers.
6. What causes retrograde motion? Do all planets appear to have retrograde motion?
Planets move at different speeds in their orbits (Kepler’s 2nd Law)
7. How is parallax like retrograde motion? Hint: see question 3 above.
Both are apparent motions, just appear to change or move in a certain way.
Make a system (sun-earth-planet) diagram that shows:
A. A planet at opposition with Earth.
Sun – Earth – Mars in one line
B. A planet in conjunction with Jupiter.
Earth – Mars – Jupiter in one line
C. A planet’s path during prograde and retrograde motion (label each portion, include E and W)
(prograde: moving West to East) Retro grade: first East, then West, the East again)
(retrograde: moving East to West), making an S or a loop.
Draw a picture that defines:
A. Rotation
Spinning on axis
B. Revolution
Going around the Sun
C. retrograde motion
Makes a loop to W before
continuing an EASTward direction
D. parallax
Closer objects appear to shift more against more
distant objects
BONUS QUESTION(S) Only one will appear on the test.
1. What does eccentricity of Earth look like (from internet site)?
Almost a perfect circle
2. Which planets are visible in our early morning sky (around 6AM) this month? Jupiter
and Venus
3. Who explained retrograde motion by saying planets orbit in epicycles? Ptolemy
4. How many light years is a parsec? 3.26 Light Years