---
title: 'Measuring the Distance to Venus Without Radar'
source: 'https://youtube.com/watch?v=aw-8BhCilj0'
video_id: 'aw-8BhCilj0'
date: 2026-08-10
duration_sec: 141
---

# Measuring the Distance to Venus Without Radar

> Source: [Measuring the Distance to Venus Without Radar](https://youtube.com/watch?v=aw-8BhCilj0)

## Summary

This video explains the historical method of measuring the distance to Venus using the transit of Venus and the principle of parallax, a technique that predates radar. It details how observers in different hemispheres timed the transit to deduce the planet's distance from Earth.

### Key Points

- **Parallax principle** [00:01] — The distance to an object can be deduced from the angle of line of sight, similar to how human eyes gauge distance. For Venus, observers in different hemispheres see it at different heights relative to background stars.
- **Challenge of distance** [00:38] — Venus at its closest is about 39 million km away, over 6,000 times Earth's radius. Measurements must be extremely precise and simultaneous, which was difficult with 18th-century clocks.
- **Transit of Venus** [01:07] — During a transit, Venus crosses the sun's disc. Observers in different hemispheres see it slightly higher or lower due to parallax, but the difference is tiny and hard to measure directly without photography.
- **Measuring duration** [01:35] — Instead of describing position, observers timed the duration of the transit—from first contact to exit. Comparing durations from different locations reveals the ratio of chord lengths across the sun, which indicates the slight change in viewing angle.
- **Deducing distance** [02:00] — The angle deviation, combined with the known distance between observers, allows calculation of Venus's distance at that moment. This clever method was used before radar.

### Conclusion

The transit of Venus method, using parallax and precise timing, was a brilliant pre-radar technique to measure astronomical distances, showcasing human ingenuity in overcoming technological limitations.

## Transcript

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
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
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
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
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
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
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
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
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
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
observers. That is just really clever to me.