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Counting Craters
The object of this exercise is to discover how often gigantic meteorites hit the Earth.
These large impacts are big enough to kill animals and people-even dinosaurs-and
cause huge tidal waves and earthquakes. We begin by looking at the moon.
Step 1:
Look at the attached picture of the moon, page 4.
Step 2:
Cut out the little circle at the bottom of the page.
Step 3:
Tape the circle onto the end of your pencil or finger.
Step 4:
Count the number of craters in the picture that the circle will fit inside of.
Step 5:
Write this number in the box.
The moon and the Earth are different sizes, so the number of large craters on the
Earth will be different from the number of large craters on the moon. For example,
think about throwing a ball at a target. When a target gets bigger it quickly gets
much easier to hit.
Look at the comparison of the Earth and the moon on page 5. To compare their sizes
we need to count the number of squares in the Earth's circle. The moon's circle
is done for you.
Step 1:
Color or make an "x" in each whole square. This means that the whole square must
be completely inside the circle. Count these squares as you mark them.
Step 2:
Write the number of squares in this box.
Step 3:
Choose a different color or mark with an "o" the remaining partial squares, make
sure you count each square as you mark it.
Step 4:
Divide this number by 2.
Step 5:
Write the number in this box.
Step 7:
Add the number of "whole squares" to the number of "partial squares".
Step 8:
Write this new number in the box.
This gives us the size of the Earth. Now we will take the size of the Earth and
divide it by the size of the moon.
 
Write this answer in this box.
We call this calculation "relative size" because it shows the relationship of the
Earth's size to the moon's size. In other words, it tells you how many moons fit
inside one Earth.
Now we will make a second calculation, the number of large craters on the Earth, based
on the cratering of the moon.
Think about this for a minute. If there are that many craters on the Earth where
are they? How many have you seen or heard about? The reason that the craters are
no longer visible is that they have been filled in by dirt and rock and eroded over
the years. Some craters have been erased by the motions of the Earth's crust, such as plate
tectonics. If we dig down far enough we can find some of the craters that have occurred
in the past, but many have been permanently erased.
We will now make one last calculation and we will find out what we wanted to know:
how often do giant meteorites hit the Earth? We take our second result, and a new
piece of data to get our final result. The new piece of data is the age of the Earth
(4,500,000,000 years), which is actually someone else's data:
This is our final calculation and it gives us our answer, how often meteorites, large
enough to kill animals or people hit the Earth. Throughout the history of the Earth,
there have been many great extinctions, the most famous was that of the dinosaurs,
and that was thought to be caused by a giant impact!!
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