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Build an understanding of solar eclipses with a model that’s correctly scaled in both size and distance.
Make a Model Earth
Make a Model Moon
Make a Model Sun
Make a Model Earth–Moon–Sun System
(Note that Steps 9, 10, and 11 can be completed ahead of time.)
Bring your hole-punched Earth, clay moon (on its toothpick), ruler, and calculator to your scale- model location, and stand at the 0-meter line. This is the position of the earth with respect to the Sun. The hole-punched Earth and paper Sun are now correctly scaled in both size and distance.
When you look toward the model Sun, what do you notice? Most people are surprised by how small the Sun looks when its size and distance are scaled correctly. But what about the moon?
The distance between the earth and moon is about 30 Earth diameters. If your hole-punched Earth is 7 millimeters in diameter, then the clay moon will be 21 centimeters away.
Participants should work in pairs to complete this part of the model, with one person (“the measurer”) helping with measurements, and the other (“the eclipser”) using their clay moon to eclipse the Sun. Using a ruler, have the measurer help the eclipser by holding the moon model at the correct distance from the eclipser’s face. The eclipser should close one eye and imagine that their open eye is an observer on Earth. Then, without changing the moon’s distance from their face, the eclipser should line up their open eye with the moon and Sun. (Click to enlarge photo below.)
For the person in the role of the eclipser: What do you notice? You should see that the moon completely blocks the Sun, appearing to be the same size as the Sun from your Earth point of view. This is a scaled model of a total solar eclipse.
Try making slight adjustments to the position of the clay moon. What happens when the moon is slightly higher or lower in the sky? What happens when the moon is slightly farther away from the earth? What happens when the moon is off to the side?
Partners should change roles and repeat the creation of this model.
Most astronomical scale models do not scale both sizes and distances simultaneously. The distances are so large that it’s usually impossible to do.
The Sun is much, much larger than the moon, and also much farther away, both of which are apparent here. In this scaled model, from our point of view on the hole-punched Earth, the tiny clay moon will perfectly cover the far-away Sun, resulting in a total solar eclipse (click to enlarge photo below), but only when the moon is exactly in the correct position.
When the moon is in between the earth and the Sun, the moon is in its new-moon phase. Solar eclipses can only happen when the moon is in this position.
The moon’s orbit is actually tilted 5 degrees with respect to the ecliptic, the apparent path of the Sun across the sky (called “ecliptic” because this is where eclipses can happen when crossed by the moon). The moon’s tilted orbit is why we don’t have solar eclipses during every new moon: The new moon is usually too high or too low to block out the Sun.
As it orbits, the moon crosses Earth’s ecliptic twice a month. These are the two orbit locations where eclipses can occur. One is called the ascending node; the other is the descending node. The moon must be at one of these nodes, and in its new-moon phase, in order for a solar eclipse to occur.
For those who want to explore more mathematics, here are some ways to think about setting up this scale model on your own.
First, some information about the earth and the Sun:
Earth’s Diameter: 12,756 km
Sun’s Diameter: 1,392,000 km
Earth–Sun Distance: 149,600,000 km
There are two ways to find out how big the Sun would be if the earth were the size of a hole-punched circle: 1) Set up a proportion to determine the scaled diameter of the Sun, or 2) Find the scaling factor and use it to determine the scaled diameter of the Sun.
Here are the numbers needed to make a correctly-scaled moon out of clay:
Moon’s Diameter: 3,475 km
Earth–Moon Distance: 384,000 km
For more, see the Earth and Moon Science Snack to investigate the sizes and distances between the earth and the moon.
Most eclipse models do not show these celestial objects to scale. This scale model shows the correct positions as well as the sizes and distances of the earth, moon, and Sun. This model also helps to show why eclipses don’t happen every month during a new moon: If the new moon is just a bit too high or a bit too low, there is no solar eclipse.
It’s important for learners to understand that the position of the hole-punched Earth is where they are standing (at the 0-meter mark, some distance away from the Sun model). Designated “measurers” can help make sure designated “eclipsers” maintain the correct Earth-moon distance, closing one eye and using the other eye to line up the clay moon with the paper Sun.
This Snack works best if learners are familiar with the relative motions of the earth, moon, and Sun (for example, that the earth orbits the Sun, and the moon orbits the earth).
You can introduce the basic concepts of solar eclipses with the Solar Eclipses Science Snack, which requires no special materials and is a good entry point for younger learners.
To emphasize mathematical thinking and computational skills, older students can use proportional reasoning to determine the scaled sizes of their Sun and moon models, and find the correct scaled distances between the earth, moon, and Sun. See the Going Further section, above, for more information to help get started on these calculations. Older students can also make their Sun and moon models based on their own calculations, and find the correct scaled distances between the earth, moon, and Sun.
The material contained in this document is based upon work supported by a National Aeronautics and Space Administration (NASA) grant or cooperative agreement. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA.
Create a scale model of the earth-moon system using different-sized spheres.
So close, yet so far away.
Measure the height of an object indirectly using your hand and a ratio.
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Attribution: Exploratorium Teacher Institute