O RBIT OF M ERCURY ASTR 110G Section M09 September 30 th,

P LANETARY O RBITS 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 = (r a – r p )/(r a + r p ) r a is the farthest a planet is from the Sun r p is the closest a planet is from the Sun

M ERCURY ’ S O RBITAL P ARAMETERS WRITE THESE DOWN! You need them for your summary Period: days Semi-major axis: AU Eccentricity:

G REATEST E LONGATION Greatest Eastern Elongation

W HAT 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

E ARTH P OSITIONS Large circle is orbit of Earth Arrows on orbit represent direction Earth is moving around Sun Sun is the center dot

E ARTH P OSITIONS 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°

E ARTH P OSITIONS 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

E ARTH P OSITIONS 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°

E ARTH P OSITIONS This is what all 13 positions should look like All should be labeled with dotted lines between Earth and Sun

M ERCURY P OSITIONS 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°

M ERCURY P OSITIONS 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

M ERCURY P OSITIONS 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

G REATEST E LONGATION Greatest Eastern Elongation

M ERCURY P OSITIONS 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

M ERCURY P OSITIONS 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°

M ERCURY P OSITIONS 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

M ERCURY P OSITIONS 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!)