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Eclipses When?


Activity to illustrate why if the axis of the orbit of Moon always points the same direction in space, eclipses can occur only 2x per year, when the orbits of Earth-around-Sun and Moon-around-Earth intersect (at the nodes).

[This activity is adapted from the NASA Night Sky Network Shadows and Silhouettes astronomy outreach kit, developed by Astronomical Society of the Pacific]

Materials and Construction of Model Parts*
  • One or two sheets of transparency stock (acetate) — to make a clear disk. If using two sheets for stiffness, glue, tape, or staple them together at  a few points.  One sheet should work just fine though and may be preferable.
  • Two polystyrene balls (or styrofoam, in a pinch---not as durable), about 2" and 1/2" for Earth and Moon respectively, and cut them in half. Glue the Earth hemispheres on either face of the disk at the center, so it looks like it's one ball with the disk bisecting it. Do the same for the Moon ball, but put it on the edge of the disk.  
  • Sharpened pencil or chopstick—stick it in the Earth to make it easy to revolve the Moon around it.
  • White sheet of paper or cardstock to cast shadows on.
  • Light source (maglite with top removed?) 

Running the Model

Play with the Earth-Moon disk model and a light source to see how you can make eclipses, and how, if the axis of the orbit of Moon (the chopstick) always points the same direction in space, eclipses can occur only 2x per year, when the orbits of Earth-around-Sun and Moon-around-Earth intersect (at the nodes).

3 people are ideal to run this model:
  1. to hold the Sun,
  2. to walk around the Sun with the model, stopping at various points to spin the Moon in it's orbit, and
  3. to to catch the shadows, holding the white paper so that the model is between the paper and the light, so that the Earth and Moon cast shadows on the paper and everyone can easily see when they're lined up with the Sun----the shadows overlap. 
The #2 person has to remember to always point the orbit axis at the same point in space, but tilted to the floor, and the #3 person has to make sure the paper is catching the shadows.

*there can be various size models for various situations, e.g.,
  • clear CD-ROM, with 2 half-inch dia hemispheres (polystyrene or bead cut in half) for Earth, and a little blob of clay for the Moon. ...As in the original ASP kit.
  • acetate cut to 8" dia disk with two 1" or 2" hemispheres (polystyrene) for Earth and 1/2" ball for the Moon. This could work for pairs or small groups to each have a model to play with.
  • 4 chopsticks stuck at right angles into 2"-3" ball for Earth, with the outer points of the chopsticks attached to an embroidery hoop, for Moon's orbit and a 1/2"-1" ball for the Moon attached to the embroidery hoop or one of the chopsticks.  Good for small or medium size group demo.
  • 4" ball for Earth (polystyrene) with four 1/4" to 1/2" x 18" dowels (or heavy wire) stuck into it at right angles for Earth; a hula hoop for Moon orbit, and 1" ball for the Moon stuck on the hula hoop or one of the dowels.  This would be nearly scale model and ideal for demo in front of large group!

 
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