Rowan Introduction to Astronomy
Lab 6 / Plotting the Orbit of the Moon
Name: ________________________________________________ Score: __________________________
Objectives After completing this lab, the student will be able to
use a series of lunar photographs to make a scale drawing of the Moon’s orbit.
be able to confirm the elliptical nature of the Moon’s orbit by determining the major axis, minor axis, and
eccentricity of the orbit.
compass (get cheap ones in dollar stores, Wal-Mart)
The Moon’s Orbit As applied to the Moon’s orbit, Kepler’s first law would state that the Moon’s orbit is an ellipse
with the Earth at one focus. If the Moon’s orbit is an ellipse, its distance from the Earth should change during one
complete orbit. This means the Moon should appear larger at perigee (closest approach to Earth) than it does at
apogee (farthest from Earth). Figure 1 shows a series of lunar photographs. These were obtained as the Moon passed
several different positions in its orbit. It is possible to use these photographs to determine a few orbital properties of
the Moon. Fill in Table columns in this order: 6, 7, 3, 4, 5 and before plotting your data points.
1. Measure the diameter, d, of each lunar image shown in millimeters. Be careful not to simply measure the
illuminated portion. Make all measurements vertically through the center of each image. Estimate each value to 1⁄2
millimeter (0.5 mm). Record your measurements in column 6 in Table 1. (Note: These values should range between
40 mm and 60 mm.)
2. From each diameter measurement, the relative distance to the Moon can be determined. This distance will be
scaled down to fit on a piece of graph paper. Calculate the scale distance, D, (in mm) to the Moon using the relation
D = 4000/d, where d is the diameter recorded in column 6 of Table 1, and 4000 is a scale factor. Record your values
of D, to its nearest tenth of a millimeter, in the last column of Table 1 (column 7). Round to 1 decimal place.
3. Calculate the average scale distance, D
, to the Moon and record your answer in the space provided at the
bottom of column 7.
4. Calculate the difference in days from one image to the next by subtracting the prior day count from the next day