1. Orbit of Mercury Yeah protractors!

2. Planetary Orbits - All planets orbit the Sun in an ellipse - Semi-major axis (a) is half of the long axis

3. Planetary Orbits - All planets orbit the Sun in an ellipse - Semi-major axis (a) is half of the long axis - Period (p) is the time it takes to go around the Sun one time - Eccentricity (e) is how squished the orbit is e = 0 is a circle e = 0.5 looks like an oval e = 1 is a very squished oval

4. Mercury's Orbital Parameters Write these down! (You need them for the take home.) - period = 87.9 days - semi-major axis = 0.387 AU - eccentricity = 0.21

5. Inferior planets show phases! Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

6. Inferior planets always appear close to the Sun, so it's easy to measure angular separation (aka elongation angle).

7. Right after sunset every day for one month! Which day had the greatest elongation angle...?

8. “Greatest elongation angle” means largest angular separation

9. Greatest elongations are cool because they tell us exactly where the planet is.

10. We are going to figure out Mercury’s orbit using greatest elongations! First mark Earth’s position for the 13 dates - “Degrees” column - Number the positions - Counter-clockwise! - Angle is at the Sun

11. EARTH POSITIONS Large circle is orbit of Earth Arrows on orbit represent direction Earth is moving around Sun Sun is the center dot

12. EARTH POSITIONS 1. Draw dashed line between Earth (X) and Sun (center dot) 2. Line up bottom edge of protractor on line with center on Sun 3. Measure out calculated angle (last column of Table 7.1) from previous Earth position to get new Earth position 42°

13. EARTH POSITIONS 1. Draw a dotted line between the Sun and your new Earth position 2. Label this position so you know what line this is later

14. EARTH POSITIONS 1. Again, line up the bottom edge of protractor on Earth-Sun line with Sun at center and Earth at 0° 2. Measure out calculated angle and mark it 3. Draw dashed line between Sun and new position 4. Label position 72°

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

16. We are going to figure out Mercury’s orbit using greatest elongations! First mark Earth’s position for the 13 dates - “Degrees” column - Number the positions - Counter-clockwise! - Angle is at the Sun Then mark Mercury’s position for each date - “Elongation Angle” column - Number the positions - EAST : LEFT WEST : RIGHT - Angle is at the Earth - Use correct Earth position

17. MERCURY POSITIONS 1. Line bottom edge of protractor with Earth-Sun line 2. Earth at center of bottom of protractor 3. 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°

18. MERCURY POSITIONS 1. Draw solid line between Earth and mark from measured angle 2. Be sure the line goes past Earth – necessary for the next step 3. Mercury is located somewhere on this line

19. MERCURY POSITIONS 1. Place the protractor so that the solid line goes between the center of the bottom and the 90° tick 2. Keeping that alignment, line up the bottom of the protractor with the Sun 3. Where the protractor bottom intersects the solid line, place a dot

21. MERCURY POSITIONS 1. Place the protractor so that the solid line goes between the center of the bottom and the 90° tick 2. Keeping that alignment, line up the bottom of the protractor with the Sun 3. Where the protractor bottom intersects the solid line, place a dot

22. MERCURY POSITIONS 1. Position #2 has a west angle so angle must be measured to the right of the Sun 2. Line up protractor and measure angle to the left of Sun 3. Draw solid line between Earth position 2 and new angle mark 18.1°

23. MERCURY POSITIONS 1. Line up protractor at 90° on Mercury line 2. Line bottom of protractor on Sun 3. Mark where bottom of protractor crosses Mercury line 4. Label Mercury locations based on Earth position #’s

24. MERCURY POSITIONS 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!)

25. We are going to figure out Mercury’s orbit using greatest elongations! First mark Earth’s position for the 13 dates - “Degrees” column - Number the positions - Counter-clockwise! - Angle is at the Sun Then mark Mercury’s position for each date - “Elongation Angle” - Number the positions - EAST : LEFT WEST : RIGHT - Angle is at the Earth - Use correct Earth position