PROCEDURE - In the table below you will find the dates of greatest elongations and the degrees east or west of the Sun for Mercury in the years 1995-97. These mark the points on Mercury’s orbit when it was seen edge-on from Earth. After greatest eastern elongation, Mercury would stop moving eastward away from the Sun in the evening sky, and begin retrograding westward toward the Sun, becoming lost in the Sun’s glare usually within a week of GEE. At GWE, Mercury rose highest in the morning sky, then turned and started moving eastward, to become lost behind the Sun in another week or two. If you will carefully locate our position on the Earth’s orbit for a given date, WITH a PENCIL faintly draw in the line connecting Earth and Sun (you will later need to erase it). Now with your protractor centered on the Earth’s position, draw a line from Earth about 3/4 of the way across our orbit. This will mark the direction in which we saw Mercury on that date. Be sure the angle is accurately plotted to the left (for GEE, in evenings) or right (for GWE, in the morning sky). Now go back and carefully erase the original line from the Sun to Earth, so as to avoid clutter. The first example is already done for you for January 19, 1995, when Mercury was 18 degrees east of the Sun. Note the angle is plotted to the LEFT of the Earth Sun line for that date, and lies 18 degrees over. Note that the year doesn’t really matter, since we are back at the same place in our orbit each year. You will now have a partially filled in Earth orbit, with a lot of lines running around its edges, but not through its center. The resulting clear area in the middle is the orbit of Mercury, which you will note is certainly NOT a circle, but an ellipse (Kepler’s First Law). Measure the closest approach of Mercury to the Sun (perihelion), using the A.U. scale at right. Now find aphelion. Their average is the semi-major axis distance. Fill in this data. To find the orbital eccentricity, take the difference between aphelion and perihelion, and divi
