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# Occultations

## Peek-a-boo

How do we know if something in our solar system is further away than something else? For example, the distance to the Moon vs. the distance to 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?
One 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 star (which we can assume is fixed in place). What could this tell us about how fast the moon is orbiting? Here is an animation of Saturn’s moon Iapetus drifting in front of a background star, captured on Cassini’s narrow-angle camera.
Consider 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 its 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 its 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)

## Want to join the conversation?

• just wondering how would it be possible for a planet to glow as if it is a star? I mean its not like the moon its reflective dust is it. If so could other stars we see be other planets we have not discoverd yet so can we be able to track other planets in our solar system.
• It is definitely possible for planets to generate their own heat and even glow, but not anywhere near to the level of a star. If you think about it, whenever a volcano erupts, casting out glowing hot lava, that is part of the astronomical body casting off light of its own. Rogue planets larger than Jupiter have been found by their infrared glow.
• why is approximate speed = (diameter in km) km/hour
• It takes the moon 1 hour to pass in front of a star. So if the moon is 1km in diameter, it would have the speed 1km/hour. If we know the correct diameter of the moon, we will get the approximate speed of the moon in this fashion
• Isn't occultation the same as an eclipse?
• yes , it might be
bt there only sun ,moon ,earth plays the role
whereas here other celestial bodies
• Just curious, but at what grade level would one usually learn all this?
• I'm in 8th but I think everyone should be learning these things.
(1 vote)
• Can stars fall on to the planets 🪐?
(1 vote)
• No it can't because gravity holds those stars in that position in the night sky like the equation e=mc2 so the laws of physics tells us no that cant happen
• I am just wondering why does mars look very gray and white when we look close at it I mean very close and what creature lived on mars for years that we did not know about and how do people survive on mars with no air?
(1 vote)
• why did the moon red during the lunar solar Eclipse
(1 vote)
• Other light rays (like blue scatter because of the earth's atmosphere, but not red
(1 vote)
• how is the observation of aristotle in the video in 365BC