How tall is a tree?

Added: 2026-05-12

[00:00:00][music] >> If you've ever eaten dates, you might be interested to know that that's the sort of tree they come off. It's a date palm.

[00:00:10]In fact, that's the Canary Island date palm, and it's a beautiful thing. It's a tall tree, gets even taller than that.

[00:00:16]And uh lovely looking, too.

[00:00:18]That raises a question.

[00:00:20]With a tree like that that is so tall, or something like the Sydney Harbour Bridge, or a very tall building, how can you easily estimate exactly what height it is? It's pretty hard to climb up the thing, and you haven't got sophisticated equipment.

[00:00:32]Well, there is one simple method, and it works on shadows. And for that, you need, to begin with, something whose height you already know.

[00:00:41]For convenience, I've got a meter rule here, and what you do is to do this. You stand as close as you can to the object, hold the meter rule, or whatever object it is, straight up in the air, just like the the tree or the building, and you put one marker at its base, and another marker right at the end of the shadow.

[00:01:03]Like that.

[00:01:05]Check that it's upright, and then measure the shadow, which I can do, of course, very conveniently with this meter rule.

[00:01:10]There we are, from there to there, and it comes out to be 80 cm, which is very convenient. Cuz if that's 1 m tall, and it's casting a shadow of 80 cm, the shadow is 4/5 the length of the real object. And now we can turn our attention to the tree and its shadow.

[00:01:30]We really want to start measuring from the middle of the tree. We can't get to that, so we'll start beside it, and put the end of the rule about level with the middle of the tree, about there. And now we'll just measure off the shadow. 1 m, 2, 3, 4, 5, 6, 7, 8, 9, 10. And just while we're going, a couple of other points. This only really works if you've got a flat or a uniformly sloping area. If you don't, if it goes up and down again, you'll get inaccuracies. And also, you really ought to take the two shadow measurements as closely together in time as you can. If you allow a bit of time between them, the sun will have moved, and you'll get inaccuracies that way. Let's go on. 10, 11, 12, 13, 14, 15, and near enough to 16 m.

[00:02:34]Which means if the shadow is 4/5 the height of the tree, or length of the tree, and the shadow is 16 m, the tree must be 20 m. And that's a system that'll work for almost anything.

[00:02:46]I want to know.

[00:02:47]>> [music] >> Curiosity.

[00:02:56]>> [music] >> Curiosity. [singing] Shoo!