This is a quote from Edward Duensing’s Talking to Fireflies, Shrinking the Moon: Nature Activities for All Ages
It’s not only a nature study, it a terrific application of math skills.
“This little trick of measuring a tree with a stick is called the thumb jump. It requires no special skills; in fact, as long as you know the height of the person you are with, you can easily accomplish this feat.
Have your companion stand up against the tree you want to measure while you find a stick.
Next, step back far enough so you can easily see the top and bottom of the object to be measured and hold the stick upright at arm’s length.
Sight over the top of the stick and line it up with your friend’s head.
Then, holding the stick steady, move your thumb down the side of the stick until it lines up with that person’s feet.
Now comes the thumb jumping. Holding your thumb snug against the stick, move your arm upward until your thumb is lined up with the head of the person standing against the tree.
Note the spot where the top of the stick now stops on the trunk of the tree and “jump” your thumb up to that place.
Continue jumping your thumb until you reach the top of the tree you are measuring and then multiply by the number of times you jumped the stick up the tree by the height of your companion. The result is the height of the tree.
… The thumb jump can be used on anything vertical …cliffs …
The width of a stream is another natural feature worth knowing how to measure - especially if it is one you are considering jumping across. How can you tell if you can make the leap from one bank to the other? Simple.
Stand facing the stream and look at the spot where you want to land on the far bank.
Then, put your chin down to your chest and raise your hand up so its inside edge covers both eyebrows in something like a salute.
Next, rotate the hand you have on your eyebrows downward until its lower edge lines up with the spot across the stream.
Now, keeping your hand and head in the same position, rotate your edge of your hand. The distance between that object and yourself is equal to the width of the stream. If you can leap that far, you have a good chance of making it across the stream without getting wet. This technique is quite accurate and will help keep your socks dry.”