Kepler's 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 orbital
motion of planets about 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.

Kepler's three laws of planetary motion are:

- A planet travels around the Sun in an
**elliptical orbit with the Sun at one focus**. - A straight line drawn from the planet to the Sun sweeps out
**equal areas in equal times**. - The quantity
*P*^{2}/*a*^{3}, where*P*is a planet's orbital period and*a*is its average distance from the Sun,**is the same for all planets**.

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 other artificial satellite.)

Kepler's first law says that planets have elliptical orbits. As a
result, the distance between a planet and the Sun changes rhythmically
as the planet moves in its orbit. In many cases, this rhythmic change
is rather subtle; for example, the Earth's distance from the Sun
varies between 98.3% and 101.7% of its average value. (By the way,
the Sun is closest in January, and furthest in July, so this change
**doesn't** explain the seasons!) In contrast, the ellipticity of
the Moon's orbit is fairly dramatic; the Moon's distance from the
Earth varies between 92.7% and 105.8% of its average value of
384,400 km.

This variation in distance produces several effects which we can
observe here on Earth. For example, when the Moon is closest to the
Earth (* perigee*), it moves faster, while when it is
furthest from the Earth (

To measure the Moon's apparent diameter, we 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, superimposed on 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.

Fig. 1. 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. |

Fig. 1 shows how the measurement is made. 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 millimeters, so
your result should be expressed in millimeters. 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 to span the Moon's true
diameter, no matter what the Moon's phase; for example, the diameter
of a crescent Moon is 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 at least 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 until the end of November (9/30, 10/07, 10/28, 11/04, 11/11, 11/25).

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 **very** different from the other two, you probably made
some kind of mistake while taking that measurement. You should drop
any obviously incorrect measurements before going on to analyze your
observations.

For example, suppose you made three measurements, and found total diameters of 11.0 mm, 11.1 mm, and 11.2 mm. These values are all pretty close to one another, and you can average them to get 11.1 mm. On the other hand, suppose you found diameters of 10.1 mm, 11.0 mm, and 11.2 mm; while two of these values are reasonably close together, the other is very different. In this case, it's likely that the 10.1 mm value is incorrect, while the others are reliable and can be averaged to get 11.1 mm.

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
d |
. |

An example may help make this clear. In Fig. 1, the Moon's
image is *d* = 11.5 mm across. Using this value
in the equation, we get *D* = 104.3 for the Moon's
distance, in units of the Moon's diameter. 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 we will take more photographs like the one in Fig. 1,
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 (note that there's no
need to make three measurements of the photograph and average them; a
single measurement will do). Then, use the equation above 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.

**Lunar Photographs**Use these to supplement your own measurements when graphing the Moon's distance over time.

**Blank Graph for Moon's Distance:**GIF file or Postscript.Use this chart to make a graph of

*D*over time.**Inconstant Moon:**The Moon at Perigee and ApogeeWeb page describing the variation in the Moon's apparent size as a result of its elliptical orbit. Created by John Walker.

**Lunar Perigee and Apogee Calculator**JavaScript program to calculate dates of lunar perigee and apogee. Created by John Walker.

**Diameter of the Moon**high res. (3.0 Mbyte mpeg); low res. (0.8 Mbyte mpeg).Animation showing the Moon as seen from the Earth from 08/26/03, 14:00 to 12/31/03, 08:00 (08/27/03, 00:00 UT to 12/31/03, 18:00 UT). Note the variation in the Moon's apparent diameter. The ``wobbling'' motion, known as libration, is an indirect consequence of Kepler's second law. Generated using NASA's

*Solar System Simulator*.

- During a solar eclipse, the Moon comes between you and the Sun. In some total eclipses, the Moon completely blocks the Sun's light. In others it does not, even when the Moon is exactly in front of the Sun. Why?
- Why does the Moon appear to move faster across the sky at
perigee, and slower at apogee? (Note: a complete answer to this
question also involves Kepler's
**second**law.) - Suppose the Moon's orbit was an ellipse with the Earth at the
**center**, rather than at one focus. How many times per month would the Moon approach and recede from the Earth?

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

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

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

- For each night, list all the individual measurements from the midpoint to both sides of the Moon, as well as their totals. You should organize this data in a table. Put an `x' next to any total values you consider incorrect due to mistakes during measurement.
- Make a second table listing the date and the average values of
*d*computed for each night you observed. Also list the dates and values you measured from the on-line photographs. Finally, add a column to this table listing the*D*values you computed. - Plot your
*D*values using the graph provided with this handout, and include this graph in your report. Try to draw a smooth curve connecting the points you plotted, but don't hide the points with the curve!

**This report is due in class on December 2.**

Joshua E. Barnes (barnes@ifa.hawaii.edu)

Last modified: November 29, 2003

`http://www.ifa.hawaii.edu/~barnes/ASTR110L_F03/moonorbit.html`