When I heard that a 300-meter asteroid will approach Earth at a distance of only 18,000 miles in 2029, I figured it was time to break out the old slide rule. Wouldn't an object that big and close be able to eclipse the Moon, if it should pass in front of it?

You're referring to asteroid 99942 Apophis. The simplest way to get its size in degrees is to divide its physical diameter by its distance (in the same units) and multiply by 57.3. Since 300 meters is 0.300 km or 0.186 mile, we have 0.186 / 18,000 = 0.0000103. Multiplying by 57.3 gives 0.00059°, which is awfully tiny. Even in a telescope it will look like a star as it goes by.

If the answer had come out 0.59° instead, then Apophis would be just large enough to hide the Moon completely. (I love slide rules, too, and have quite a collection of them, but their big shortcoming is not being able to keep track of the decimal point!)

According to the latest calculations posted on the University of Pisa's Near Earth Objects Dynamic Site (http://newton.dm.unipi.it), Apophis will pass 19,700 miles from Earth's surface at 21:46 Universal Time on Friday, April 13, 2029.

— Roger W. Sinnot

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Apophis Asteroid Impacts Comets & Asteroids