 The Earth is big, really big. In this section, we'll cover how we came to know just how big the Earth really is. In so doing, we'll build the first rung in the distance ladder that will take us across our galaxy. We'll begin our journey to the edges of the visible universe very close to home. Here I am in my backyard. This is where we'll begin to build the ladder that will take us to the stars. There's a post. If you want to know how far away it is, the simplest thing to do is to measure the distance with something like a tape measure. If we take a look at this one, it's two meters away. But what do you do if you want to measure something that you can't directly reach with something like a tape measure, like the rock on the other side of the water? For that, we do something called triangulation. Take a point. Measure an angle of 90 degrees. Step off a known amount, say three meters. Measure the angle from the second spot, 50 degrees. And triangulation gives us the distance 3.6 meters to that rock from that spot. That combination of direct measurement and angular measurement and triangulation we can triangulate everything there is on the planet Earth that you can see. Triangulation is mathematical. The only errors that are introduced are with the accuracy of the angle and baseline measurements. Although tape measures and protractors work okay in the backyard, they are rather crude instruments. They don't scale well to distances beyond the backyard. If we use more accurate instruments, triangulation can take us a long way. A protractor can give you an estimated angle to the granularity of a degree. A circle has 360 degrees. For greater accuracy, we break a degree up into 60 arc minutes. And each arc minute is broken up into 60 arc seconds. This gives us 3,600 seconds of arc in every single degree. Here we see a theodolite. It's a surveyor's instrument that measures angles down to a second of arc. Using industrial instruments like these, we can answer questions like, how wide is the Grand Canyon? And how high is Mount Everest? In 1902, an extended survey of the Grand Canyon was conducted. The survey found that the Grand Canyon is 446 kilometers long and up to 29 kilometers wide. In 1856, the Great Triangulation Survey of India established the first published height of Everest at 8,840 meters.