[00:01] to nearby planets like Venus using radar. But the way that people measure this distance for the first time in history is absolutely amazing. The basic idea here is analogous to how when your two eyes are looking at an object based
[00:13] on the angle that each one of them has to turn to see it, your brain can deduce how far away that object is. For some nearby object in the sky, as you sail down to the southern hemisphere, it will appear higher up in that sky relative
[00:26] to, say, the background constellations. The angle of this line of sight changes with position. We call this parallax. Now, the way I'm drawing it here, it real world measurement, you have to keep
[00:38] in mind just how far away everything is. The nearest planet, Venus, when it is at its absolute closest to Earth, it's around 39 million km away, which is over 6,000 times the radius of the Earth. So if this is going to work, your
[00:53] measurements have to be extremely precise and you have to be absolutely looking at the same thing at the same moment. Now at the time clocks were not just say at this specific time make it measurement. Also, you're not guaranteed
[01:07] so forth. But um there are these transits. There's this thing called the transit of Venus. Sometimes Venus travels um along the the sun. So up in watching the transit of Venus, maybe it looks something like this. and far away
[01:21] hemisphere. Due to parallax, Venus would appear higher up. And you essentially want to know exactly how much higher up. Now, this animation is greatly exaggerating the difference. In reality, the two would look much, much more
[01:35] similar, more like this. And remember, there was no photography, so it's not closely compare them. Each observer would not try to directly describe where it was. Instead, they would measure the duration of the transit, how long it
[01:47] takes from the moment that Venus's silhouette first appears on the disc to the moment that it leaves. Because if you compare those two durations, it'll tell you the ratio of lengths for these two lines across the sun's disc. They
[02:00] different. And this in turn lets you deduce the ever so slight change in viewing angle. And then like we discussed that angle deviation can tell you how far away Venus is at that moment in terms of the distance between those
[02:14] observers. That is just really clever to me.