STEM Zone: Measuring the Moon's Motion

Everyone knows that the Moon orbits around the Earth — or so we’ve been told in school. But how do we really know this? Can we see the Moon move? Can we measure it? You may have also been told that the Moon’s motion has something to do with the lunar phases, but how can we understand that if we don’t really even know how the Moon moves through space?

It turns out that the Moon does indeed circle the Earth once every 28 days or so just as you’ve been told in science class, but you don’t have to take your teacher's or textbook's word for this. You can see and measure it yourself!

The best times to measure the Moon’s orbital motion is in the first week or so after the new moon phase.

Measuring the Moon’s motion through the sky is easier than you think. All you need is room for about a 10-foot circle in your backyard, something you can use as markers to stick in the grass and a 5-ft length of string. 

You can use almost anything for markers, such as sticks or small stones. Popsicle sticks with ribbons or strips of construction paper glued on also work well. I like to use irrigation flags, which are available at any home improvement store. Four of the flags should be marked north, south, east and west, one should be left blank, and the rest numbered 1, 2, 3, etc.

Place the blank marker at the center of your circle and tie one end of the string to it (the exact length isn’t important, but larger is better here!) We are going to make a compass rose in your yard by marking off points of the compass (north, south, east and west). Most smart phones have a compass application, if not, there are many free applications available to you – any app that gives your direction in degrees will be fine. Use your compass app to find the directions for north, south, east and west, and then use the string to ensure that each marker is placed at the same distance from the center point.

You now have a 10-foot wide circle, or compass rose, in your yard with all the cardinal directions marked. Leave your compass rose undisturbed until dark.

Any night you can see the Moon, go out after dark and stand in the center of your compass rose. Have a partner help you stretch the string from the center point so that it points to the Moon’s position in the sky. Mark this position at the end of the string with the #1 marker. Wait about 30 minutes, and check the Moon again (you may find it helpful to set a timer on your phone). Measure and mark the Moon’s position as you did before, this time placing the #2 marker in the grass to record the Moon’s position. It is good to keep a notebook record of the times of your observations. If you can measure the Moon in this way for 2-3 hours over a single evening, you will discover some interesting things about the Moon! Leave your markers in place so you can look at them in the daylight tomorrow.

Now that you see your measurements in daylight, you can see that the Moon’s azimuth or compass bearing moved steadily westward all evening. Take the compass application on your phone and measure the direction of each of the markers that you placed and write them down next to the times in your notebook. What do you notice? Does the Moon move at a steady pace? How can you measure this with the data you have?

This measurement made in a single day is really measuring the motion of the Earth as it spins on its axis! Think about it – if the Moon takes 28 days to circle the Earth just once, it wouldn’t move very much at all in just a few hours. The Earth spins much faster than the Moon orbits. Earth takes just 24 hours to spin once, rotating 15 degrees per hour. Check your measurements from the last night – was the Moon moving about 15 degrees across your compass rose each hour?

Measuring the Moon’s orbital motion takes more than just a few hours; instead we measure the Moon at about the same time of night for several nights running. This works best in the week or so after new moon. Pick a time when you can observe every day — just after sunset works best.

Each night, measure and place a marker to indicate the position of the Moon on your compass rose. Four or five nights running will make an excellent observation set. Remember to record the time and azimuth direction for each observation. These daily changes are the result of the Moon’s motion in orbit around the Earth. By observing at the same time each day, we cleverly eliminate the Earth’s spin from our observations, all we see now is the motion of the Moon in orbit!

Once you have a complete data set, what do you notice? What direction is the Moon moving as it orbits the Earth? Did you notice anything about the phase of the Moon as the days went by? Like the Sun and the stars, the Moon rises in the east and sets in the west each day. This daily or diurnal motion of the Moon is caused by the Earth’s rotation in space. It is the same spin that gives us day and night. The 15 degree per hour rate of motion is the same whether you measure the daily motion of the Sun, Moon or the stars – all are simply apparent motion caused by Earth’s daily rotation.

The Moon’s motion in orbit is different. The Moon moves from west to east as it orbits – the opposite direction from the Moon’s nightly path across the sky. The Moon also moves much more slowly in orbit than the Earth spins. Moving just 13 degrees per day, it takes the Moon an entire month to orbit the Earth just once, and you were able to observe and measure it for yourself.

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Dr. Daniel Barth left a career as a research scientist to teach; he has spent more than 30 years teaching astronomy, physics and chemistry at the high school and college level. A successful science fiction writer, Barth is the author of Maurice on the Moon, Doomed Colony of Mars and other works. He is currently assistant professor of STEM Education at the University of Arkansas in Fayetteville, and author of the Astronomy for Educators program.

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