From The Royal Institution.

In his fourth lecture, RV Jones investigates measurement on very large scales. How are waves used to measure the distance between stars or how fast celestial objects are moving in relation to us?

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This was recorded on 4 Dec 1981.

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This year marks 200 years of the Royal Institution Christmas Lectures — a world famous series showcasing science, curiosity, and mind-blowing demos, and started by the legendary Michael Faraday himself.

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From the 1981 programme notes:
The classic way of measuring large distances was by triangulation, where the properties of similar triangles could be applied to observations of the difference in directions to a distant point from two accessible points at the end of an already measured baseline to estimate the distance to the far away point. This is still the method by which our distances to the nearer stars are found.

More recently radar, in which we send out waves of known velocity to be reflected from the distant object, and measure the time for the echo to return, can give more accurate results than triangulation, and by its means we can reach out as far as the nearer planets.

If at the distant object something happens which sends out two kinds of wave which travel with different velocities, then we can determine the distance to the object by measuring the time between the two kinds of wave arriving at our observing point. Thus if we can see the flash of a lightning stroke and measure the delay before we hear the thunder, we can find how far away the lightning stroke must have been. In the same way we can estimate the distance of a far away earthquake from the time interval between the arrivals of the longitudinal and transverse waves which reach us through the earth.

Even with only one kind of wave available we can locate the position of a gun which we cannot see if we listen for its report at three different points and note the differences of times at which observers at these points hear it. This method, which uses the properties of the hyperbolic curve, can also be used with light or radio waves, and is of widespread use in navigation.

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About the 1981 CHRISTMAS LECTURES
The Christmas Lectures at the Royal Institution started in 1826, and there have now been more than one hundred and fifty in the series; and yet none has previously been on the theme of this year’s lectures, which is measurement. Perhaps this is because the measurement is so much part of human life that we tend to take it for granted; but if we are to understand how our modern world, with all its achievements and its dangers, has evolved, then we need to know what measurement is, the principles by which measurements can be made, and why their applications have been of so much importance in the advance of science and in the development of technology. And we also need not be carried away by the spectacular successes of measurements, such as awakening mankind to the huge store of energy in the atomic nucleus, or the microchip, or the control of space probes as far away as Saturn; for despite Plato’s proud claim that ‘man is the measure of all things’ there are many qualities in life that are very difficult, if not impossible, to measure such as love or courage, and we should therefore be cautious about extending measurement beyond its proper domain. But within that domain, measurement is one of the most challenging, fascinating and rewarding of human activities: this is what the lectures are intended to show.

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