Shape of Lunar Orbit

Kepler first law implies that the Moon's orbit is an ellipse with the Earth at one focus. The distance from from the Earth to the Moon varies by about 13% as the Moon travels in its orbit around us. This variation can be measured with a telescope; we will make a series of measurements and combine them to study the Moon's orbit.

The most useful laws of nature can be applied in many different situations. Kepler's three laws, invented to describe the motion of planets around the Sun, are very useful. With minor modifications, they also describe the Moon's motion about the Earth, the orbits of Jupiter's satellites, and even the orbital motions of binary stars. The Moon provides a natural laboratory for orbital motion; we can use it to make a simple test of Kepler's first law.

As originally stated, Kepler's three laws of planetary motion are:

  1. A planet travels around the Sun in an elliptical orbit with the Sun at one focus.
  2. A straight line drawn from the planet to the Sun sweeps out equal areas in equal times.
  3. The square of a planet's orbital period, divided by the cube of its average distance from the Sun, is the same for all planets.
In effect, the first law describes the shape of an orbit, the second says how a planet's speed varies at each point on its orbit, and the third law provides a way to compare different orbits.

These same three laws can also describe the Moon's orbital motion around the Earth: just substitute Earth for Sun and Moon for planet. (Of course, the Earth has only one Moon, but we could use the third law to compare the Moon's orbit with the orbit of the Space Station or any other artificial satellite.)


On average, the Moon is about 384,400 km (almost a quarter million miles) from the Earth. But the actual distance varies; sometimes the Moon is closer, and other times it is farther away. This variation is due to the Moon's elliptical orbit. As Kepler's first law implies, all orbits are ellipses, but most planets (and large satellites) have orbits which differ only slightly from circles. In contrast, the Moon's orbit is definitely not a circle. The Moon's distance varies from 7% less than average (at perigee, when the Moon is closest to the Earth) to 6% more than the average (at apogee, when the Moon is farthest from the Earth).

The Moon's orbit about the Earth lies in a plane which is tilted by about 5.15° with respect to the plane of the Earth's orbit about the Sun. (If this tilt was zero, we would have total solar and lunar eclipses every month!) The additional gravity of the Sun creates several complications. For one thing, the Moon's orbital plane slowly swivels around while keeping its tilt of 5.15°, so the Moon's path across the stars changes slightly from month to month. For another, there are subtle changes in the shape of the Moon's orbit over the course of a year. But we don't need to worry about these complications; the main thing is the change in the Moon's distance from Earth.

This variation in distance produces several effects which we can observe here on Earth. For example, the Moon travels faster across the sky at perigee, and slower at apogee. The Moon also appears to nod back and forth a bit as it travels around its orbit. But the most dramatic effect is the change in the Moon's apparent diameter: when the Moon is closer, it looks bigger, and when the Moon is farther away, it looks smaller. We will use this effect to determine the change in the Moon's distance.


To measure the Moon's apparent diameter, use a 25 mm eyepiece equipped with a measuring scale. Looking through this eyepiece, you can see the scale, which is something like a ruler, projected across the Moon's image. The basic idea is to point the telescope at the Moon, align it so the scale goes right across the Moon at its widest point, and measure the Moon's diameter in the units on the scale.

Measurement of Moon's apparent diameter on 02/20/03 06:55 (16:55 UT). At this time, the image of the Moon's disk was 5.8 mm + 5.7 mm = 11.5 mm in diameter.

The photograph above shows how the measurement works. Notice that this scale, unlike a ruler, has its zero point in the middle. So to determine the diameter of the Moon's image, you measure from the midpoint to each side of the Moon's disk, and add these two values to get the total. The scale is calibrated in milimeters, so your result should be expressed in milimeters. Also, notice that the eyepiece has been rotated so the scale crosses the disk of the Moon at widest point. If the scale had been vertical instead of horizontal, the measured diameter would have been much less than the true value. It's always possible to turn the scale so you measure the Moon's true diameter, regardless of the Moon's phase; for example, a crescent Moon should be measured from `horn' to `horn'.

The most efficient procedure is to use the Earth's rotation to slowly move the scale across the face of the Moon. First, rotate the eyepiece in the holder until the scale is parallel with the widest part of the image (if the eyepiece doesn't rotate easily, loosen the screw holding it in place). Second, point the telescope a little to the West of the Moon - you can easily tell which is West since that's the direction the Moon appears to move as a result of the Earth's rotation. Try to place the dividing line somewhere in the middle of the Moon's disk, but don't worry about centering it exactly. Third, wait while the Moon's image drifts past the scale, and make a measurement when the widest part of the image falls on top of the scale. Record the distances from the dividing line to the two sides of the Moon's disk separately; then add them and record the total.

Repeat these steps three times, making three sets of measurements! This includes the initial step of rotating the eyepiece in the holder. Repeated measurements yield better accuracy; they also give you a fighting chance of spotting any errors you may have made.

Weather permitting, we will make measurements each time the Moon is visible in March and April (3/11/03, 3/18/03, 4/08/03, 4/15/03).


The three measurements you've made each night give you three independent (and probably different) values for the total diameter of the Moon's image. Don't worry if these values differ by 0.1 or 0.2 mm or so; that's normal measurement uncertainty. But if one value is wildly different from the other two, you probably made a mistake while making that measurement. If you think one of your values is wrong, make a note of it but don't use it in your analysis.

For each night, average all the values you think are reliable; the result is your best measurement of the diameter of the Moon's image that night. Call that average value d. Now to calculate the Moon's distance, use this equation:

D =  F

Here F is the focal length of the telescope's main mirror, which is F = 1200 mm. Because d and F both have units of milimeters, D is a pure number. In fact, D is the Moon's distance in units of the Moon's actual diameter.

An example may help clarify this point. In the photograph above, the Moon's image is d = 11.5 mm across (note that there's no need to make three measurements of the photograph and average them; a single measurement will do). Using this value in the equation, we get D = 104.3 for the Moon's distance, in units of the Moon's diameter, on the morning of Feb. 20, 2003. To express the Moon's distance in units of, say, kilometers, you can multiply D by the Moon's actual diameter in kilometers (3,476 km); the result is about 363,000 km, which is a reasonable distance for the Moon when it's near perigee. But for this assignment, the Moon's diameter provides a perfectly good yardstick, so there's no need to go through the final step of expressing the distance in kilometers.

Once you've calculated D for each night, you should make a graph showing how the Moon's distance varies with time. Unfortunately, the handful of data points you'll have won't look like a smooth curve; there's too much time between measurements, and your graph won't include the half of each month when the Moon rises late at night. So during March and April we will take more photographs like the example shown above, and put them on the class web site; you'll find the link below. (If you don't have convenient access to the Web, we can give you a print-out of the photographs.) You can read the diameter d of the Moon's image directly from the photographs, and use the equation to calculate D for those nights. With these additional numbers, your graph should show a smooth variation in the Moon's distance with time.

To actually plot the Moon's orbit as an ellipse we need more information. It's not enough to know how far away the Moon is; we also need to know the direction from the Earth to the Moon. One way to get this information is to measure the angle between the Moon and the Sun. This is a fairly easy daytime project; it requires a number of measurements, but each one only takes a couple of minutes. If you'd like to do this as an extra-credit project, please let us know.




Make the observations described above, and write a report on your work. This report should include, in order,

  1. the general purpose of the observations,
  2. a description of the observing site and equipment you used,
  3. a summary of your observations, and
  4. the conclusions you have reached.

In more detail, here are several things you should be sure to do in your lab report:

This report is due in class on April 22.

Joshua E. Barnes (

Last modified: March 11, 2003