Orbital Mechanics


Making Kids Dizzy

Description: This is an activity that I do at a very informal level with scout troops and other groups who come to the observatory. It is a basic lesson to teach them about some simple orbital mechanics, ie the motions of the Earth and Moon and then the phases of the moon. The children actively participate because they are the objects. The remainder of the audience becomes the background starry sky.
Audience: all ages
Materials: None at the simplest level, but you can get fancy by using flashlights, using balls of different sizes.

  • Have kids and parents form a circle.
  • Ask for a volunteer. This child is the 'Sun.' Ask them 'What does the sun do in the solar system?' With younger kids, they may not know, older kids and the parents should be able to give answers like "Provides heat/light/energy", "Without it there's no life on Earth", ... A few might say it rotates at which point you can inform them that the rotation rate is very slow (about 3-4 weeks depending on the latitude, but that's another lesson!). You may have to prod by asking 'What would happen if there was no sun?' Once you've gotten a couple of responses, you can repeat that the sun is the primary source of energy, etc, that it does rotate albeit slowly and it is moving through the galaxy; HOWEVER, for this exercise, the sun just sits shining brightly at the center of the Solar System.
  • Ask for a second volunteer. This child is the 'Earth.' Ask them 'What does the Earth do?' Pretty much all the kids will be able to say that the Earth goes around the sun. They may say rotate rather than revolve, but we'll correct that later.
  • Have 'Earth' walk around 'Sun' a few times (make sure they go counterclockwise, which most do naturally -- which is interesting...). Ask 'How long does it take for Earth to revolve around the sun?' Hopefully you'll get the right answer... if they say 1 year ask 'What is that equal to?' and vice versa. Ask what else does the Earth do? Now hopefully we'll elicit that the Earth rotates on it's axis. Have 'Earth' demonstrate by rotating (counterclockwise) in place just a few times. 'How long does it take to rotate once?' If they say 1 day try to get them to also say 24 hours.
  • Now guide 'Earth' to rotate and revolve around 'Sun' while reiterating the rotation and revolution rates. I usually emphasize the words rotate and revolve as I'm saying the values. I also usually joke with the rest of the audience about hoping that 'Earth' is not prone to upchucking. Be sure to stop 'Earth' or they will attempt to keep rotating and revolving (and may very well upchuck)!
  • Depending on the number of people in your circle and the number of kids, I will either continue with the same two or thank them and get 2 new volunteers for the 'Sun' and 'Earth.' If I do new volunteers, I will very quickly review the basics.
  • Ask for a third volunteer. This child is the 'Moon.' 'What does the moon do?' Now things get interesting! Most kids will be able to say that the moon goes around the Earth, but when you ask 'How long does it take?' I will get mixed reactions. Sometimes, if the kids have recently taken a science class that discussed some astronomy, they might say 1 month, but I usually have to prod. When you finally get them to say month, note that the word month derives from the word moon. Now if you ask them 'How long is a month?' again, you'll likely get mixed reactions. Although there is considerable more detail that can be explored here, an average month is about 29 days.
  • Have 'Moon' go around 'Earth' but keep 'Earth' still for the moment. Usually, they will just naturally walk around thereby presenting different sides. Now you say, 'Remember, the Earth revolved around the su

Why didn't it save the other half of my page?? Crap, I had finished typing it all up and hit save, and this is all it saved?? I spent 2.5 hours typing it in...and it was perfect!

okay where were we...
  • Have 'Moon' go around 'Earth' but keep 'Earth' still for the moment. Usually, they will just naturally walk around thereby presenting different sides. Now you say, 'Remember, the Earth revolved around the sun and rotates on its axis. And the Moon revolves around the Earth. What else does the moon do?' Ask 'Earth' 'How many sides do you see?' with an answer along the lines of 'all sides.' Ask the whole group, 'But do we see all sides of the moon when we see it in the sky?' Hopefully you'll get no, but they may be hesitant. Have 'Moon' walk sideways around 'Earth' and then ask Earth 'Do you see all sides?' Answer, 'No.' Ask the starry circle 'Do you see all sides?' Answer, 'Yes.' Now ask, 'So how long does it take for the moon to rotate once on it's axis?' Again, depending on the age group, some may get it quicker, but the answer is a month! While you continue to have 'Moon' orbit 'Earth,' you can summarize that the moon's rotation period is about equal to it's revolution (ie it's "day" is roughly equal to it's "year" which is about a month long).
  • Now for the dizzy part, have 'Moon' orbit 'Earth' while 'Earth' slowly rotates and revolves around 'Sun!'

That takes care of the basic motions, now we move onto eclipses and phases. (I had it so well written the first time and now I'm trying to write from memory and probably leaving something out.)
  • Again, if you have enough students, you can switch and get three new volunteers to be 'Sun,' 'Earth' and 'Moon.'
  • Have 'Earth' face 'Sun' and ask 'What time is it?' Responses should be 'daytime' or 'noon.' Turn 'Earth 1/4 counterclockwise so that 'Sun' is on the right. This is sunset. Another quarter turn, midnight. Another quarter turn, sunrise.
  • Now have 'Moon' stand between 'Earth' and 'Sun' and ask 'Earth' 'what happened to 'Sun?'' The answer should be obvious 'The sun disappeared!' Now have 'Moon' walk around 'Earth' a couple of times and ask 'But if the moon goes around the earth every month, why don't we see the Sun disappear every month?' You'll probably get silence. 'Well, it turns out the moon's path around the earth is not level, it has a little tilt, so sometimes the moon actually passes underneath the sun in the sky and sometimes above. Only when the orbit is lined up just right does the moon go in front of the sun. What do we call one of these events?' Answer: Solar Eclipse.
  • Now move 'Moon' so that 'Earth' is in between 'Sun' and 'Moon' and ask 'What is happening now?' They should be able to guess that this is a Lunar Eclipse, but you'll want to reiterate that we don't have an eclipse every month because of the tilt. In fact, if there is a lunar eclipse, there is usually a solar eclipse 2 weeks before or after because of the alignment of the orbit.
  • Explaining this aspect has been a bit tough because it is hard for the kids to visualize the tilt of the orbit, although the parents have glimmers of understanding. One idea is to maybe use a hula hoop to represent the moon's orbit... 'Moon' holds the hula hoop tilted but the orientation has to stay fixed relative to the starry sky.
  • Now that we understand that the moon's orbit is slightly tilted so that we don't have solar and lunar eclipses every month, we'll move onto the phases of the moon.
  • Have 'Moon' stand between 'Sun' and 'Earth' and ask 'What half of the moon do we see?', 'But what half is lit by the sun?' 'This called New Moon. Solar eclipses occur at New Moon.'
  • Have 'Moon' move 1/8 ccw. 'What half of the moon do we see?', 'But what half is lit by the sun?' Here, I usually use my hands like chopping on top of 'Moon's' head to show which side is lit and which half we see. 'This is a crescent.'
  • Another 1/8.... first quarter
  • Another 1/8.... gibbous
  • Another 1/8.... full
  • Another 1/8.... gibbous
  • Another 1/8.... last quarter
  • Another 1/8.... crescent
  • Run through at least one more time. Then 'What time are these phases visible?' We are now tying several concepts together.
  • Have 'Moon' go to new moon position. Now have 'Earth' rotate in place and figure out when 'Moon' is visible. Answer: all day.
  • Do this for each of the phases.
    • New Moon: rises with the sun, up all day
    • crescent: rises and sets just after the sun
    • 1st quarter: rises at noon, high at sunset, sets at midnight
    • gibbous: rises in the afternoon,
    • full moon: rises at sunset and sets at sunrise
    • gibbous: rises later in the evening, up through the night, sets shortly after sunrise
    • 3rd quarter: rises at midnight, high in the sky at sunrise, sets at noon
    • crescent: rises shortly before sunrise, is ahead of the sun during the day and then sets before the sun
    • back to new
  • Whew!
  • Note that from new to full, about 2 weeks, the moon is visible in the evenings and we say that it is waxing. And from full back to new, the moon is visible in the morning sky and is waning.
  • Depending on time and interest/age level, I will sometimes continue to describe why we say that some constellations are winter constellations etc. This brings in the circle of people. But I won't detail it here since that digresses from our topic.

  • Have different size balls (to scale if possible) for the kids to hold.
  • Have a flashlight or lamp.
  • Layout the "dance steps" to help guide the kids in their motions. They often tend to bunch up and 'Earth' effectively is grazing 'Sun!' The footprints or stepping stones can be calculated to get a roughly scaled idea of the motions.
  • Use a hula hoop to represent the tilt of the moon's orbit.

Evaluation: Although I do have a form for the group leaders to fill out, they often do so in a hurry before they leave. If I let them take it home or try to email it to them, I often don't get a response back. And since I am in a very informal setting, my main criteria is that the kids have fun while they are learning and stay interested.