Solar Eclipse in 3D

Tom Walsh
This is a simulation to help with understanding how a solar eclipse occurs. The whole simulation is from the earth's perspective. From our perspective here on earth, the moon and the sun both seem to orbit earth, primarily due to the rotation of the earth. But even when you subtract out the motion due to earth's rotation, both the sun and the moon seem to circle the earth over time. The sun takes a year to make its complete "apparent" orbit around earth. The moon takes about 27.3 days to make its orbit around earth. If the plane of the moon's orbit were the same as the plane of the sun's apparent motion around the earth, we would expect a solar eclipse about once a month. However, due to the tilt of the moon's plane, solar eclipses can only happen at certain points in the orbit. This simulation is not specific to our earth-moon-sun system. You can use the sliders to adjust the ratio of the moon's period of orbit to the sun's period of apparent orbit, the animation speed, and the tilt of the moon's plane relative to the sun-earth plane. You can use the button to start and stop the animation, or to step slowly forward or backward in time (while the simulation is paused). The button's at the bottom allow you to view the system from various perspectives. The "Sun View" gives a magnified 3D view looking from the sun toward the earth.