1. ASTR 110G Section M09 September 30th, 2010
Orbit of Mercury
ASTR 110G Section M09
September 30th, 2010

2. Planetary Orbits All planets orbit the Sun on an ellipse
Semi-major axis: a, half of longer axis length
Period: P, the time it takes a planet to go around the Sun once
Eccentricity: e, measure of how “stretched” the orbit is from a circle
e = 0: orbit is a circle
0 < e < 1: orbit is an ellipse with 1 being very stretched and 0 being circular
e = (ra – rp)/(ra + rp)
ra is the farthest a planet is from the Sun
rp is the closest a planet is from the Sun

3. Mercury’s Orbital Parameters
WRITE THESE DOWN!
You need them for your summary
Period: days
Semi-major axis: AU
Eccentricity:

4. Greatest Elongation Greatest Eastern Elongation

5. What you have to do
Work in pairs (or three people if you have an odd number in your group) to create Mercury’s orbit
Do calculations, then find all Earth positions, then all Mercury positions. LABEL EVERYTHING!
Each person should find some of the Earth and Mercury positions – 2 people: 6-7 positions each, 3 people: 4-5 positions each
Check with me to be sure your first couple points are correct
Once done with that, compare orbits to rest of group and pick best orbit. Ask me if you are unsure
Work on the rest of the lab in groups using the best orbit and attach it to group lab

6. Earth Positions Large circle is orbit of Earth
Arrows on orbit represent direction Earth is moving around Sun
Sun is the center dot
Earth Positions

7. Draw dashed line between Earth (X) and Sun (center dot)
Line up bottom edge of protractor on line with center on Sun
Measure out calculated angle (last column of Table 7.1) from previous Earth position to get new Earth position
Earth Positions
42°

8. Draw a dotted line between the Sun and your new Earth position
Label this position so you know what line this is later
Earth Positions

9. Again, line up the bottom edge of protractor on Earth-Sun line with Sun at center and Earth at 0°
Measure out calculated angle and mark it
Draw dashed line between Sun and new position
Label position
Earth Positions
72°

10. Earth Positions This is what all 13 positions should look like
All should be labeled with dotted lines between Earth and Sun
Earth Positions

11. Line bottom edge of protractor with Earth- Sun line
Earth at center of bottom of protractor
Measure out given angle (column 3 in Table 7.1) – position #1 has east angle
East angle: left of the Sun
West angle: right of the Sun
27.2°
Mercury Positions

12. Draw solid line between Earth and mark from measured angle
Be sure the line goes past Earth – necessary for the next step
Mercury is located somewhere on this line
Mercury Positions

13. Place the protractor so that the solid line goes between the center of the bottom and the 90° tick
Keeping that alignment, line up the bottom of the protractor with the Sun
Where the protractor bottom intersects the solid line, place a dot
Mercury Positions

14. Greatest Elongation Greatest Eastern Elongation

15. Place the protractor so that the solid line goes between the center of the bottom and the 90° tick
Keeping that alignment, line up the bottom of the protractor with the Sun
Where the protractor bottom intersects the solid line, place a dot
Mercury Positions

16. Position #2 has a west angle so angle must be measured to the right of the Sun
Line up protractor and measure angle to the left of Sun
Draw solid line between Earth position 2 and new angle mark
Mercury Positions
18.1°

17. Mercury Positions Line up protractor at 90° on Mercury line
Line bottom of protractor on Sun
Mark where bottom of protractor crosses Mercury line
Label Mercury locations based on Earth position #’s
Mercury Positions

18. This is what all 13 Mercury positions should look like
Each position should be labeled same number as corresponding Earth position (needed for later question in lab!)
Mercury Positions