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How do we know if something in our solar system is further away from something else? For example, the distance to the Moon vs. the distance Mars. Maybe our Moon is further away from Mars and only appears closer because it’s much larger. How would you prove this theory wrong? 

Moon/Mars conjunctionOne simple way to do this is to wait for an evening when the Earth, Moon and Mars form a straight line. This will cause the Moon to block out Mars for a short period of time, known as an occultation. Here is a video showing the occultation of mars by the moon:

In 365 BC Aristotle observed an occultation of Mars by the Moon. This is undisputable evidence that Mars must be further away from the Moon during the observation.

Occultation time

What else can we learn from an occultation? It turns out there is more information than meets the eye. Consider when the moon is passing over a bright, yet distant stars which we can consider it fixed in place. What could this tell us about how fast is the moon orbiting? Here is an animation of Saturn’s moon Iapetus drifting in front of a background star, captured on Cassini’s narrow-angle camera.

Moon occulting starConsider a star which passes behind the exact middle of the moon. The time it takes for the occultation to occur represents how long it takes the moon to travel it’s own diameter. If you try this out yourself you will find that it takes approximately one hour. Therefore the moon must be orbiting at approximately 1 moon diameter per hour!

That means if we know the actual diameter of the moon we can figure out it’s approximate orbital speed!

approximate speed = (diameter in km) km/hour

Now a challenge question for you. Could the occultation time help us determine the size of the moon relative to the earth?  (hint: You will need to time the length of a total lunar eclipse)

Image Credit: Jiyang Chen