Astronomical explanations on The Sky at Night

In The Sky at Night we use down-to-Earth demonstrations to clarify some all-too-common misconceptions.

There are some aspects of astronomy that defeat the human mind. Can you really appreciate a distance of a million miles, let alone a lightyear? What happened before the Big Bang, or was there no ‘before’? If the Universe has a boundary, what lies outside it? To say ‘nothing’ is meaningless. If the Universe comes to an end, what can happen after that? Well, these are some of the questions I can’t answer in plain English. Neither could Einstein, in either English or German – I know, because I asked him.

In The Sky at Night we cannot dodge these concepts, and it would be cowardly to try. On the other hand we can, I hope, use down-to-Earth demonstrations to clarify some all-too-common misconceptions. Take the Moon; how large does it look in the sky – the size of a dinner plate, perhaps, or a pound coin? Years ago I decided to find out what most people believed, so on a clear evening with a full Moon I went down to Selsey beach, where some people were enjoying the warmth and peace. I selected stones and pebbles of various sizes and asked the holidaymakers to select a stone that would just cover the Moon when held at arm’s length. Almost everyone was helpful and intrigued, though there were a few courting couples who were not too pleased in being disturbed (one teenager went so far as to tell me what I could do with my stone, and the Moon as well if I felt so inclined).

The results were fascinating. I’m not going to tell you the answer; try the experiment for yourself at the next full or gibbous Moon. I think you would be very surprised. Artists who depict the Moon as dinner plate size are very wide of the mark. (Incidentally, this experiment could also apply to the Sun, which looks virtually the same size as the Moon. Never try it. It would mean staring at the Sun, and almost inevitable damage to your eyesight. As I have said time and time again, there is only one golden rule about looking straight at the Sun without absolutely foolproof protection: DON’T.)

A good way to demonstrate how far away the nearest star lies is to draw a line one inch long. Put the Sun on one end of the line, and the Earth on the other. Where do I then put Proxima Centauri? On the other side of the room? No – four miles away!

Optical illusion

One experiment tried on the programme, around 40 years ago, was extremely successful. I had been talking about constellations, and stressing that they were no more than line-of-sight effects. Consider the seven stars of the Plough in Ursa Major, which are not all at the same distance from us. (I used the Cambridge values, the best then available. Since then, the distances and absolute magnitudes have been revised by the Hipparcos astrometric satellite, but it has to be admitted that some of the Hipparcos results are decidedly suspect.) The Cambridge values for the distances of the seven stars, in lightyears are: Alioth 62, Dubhe 75, Alkaid 108, Mizar 59, Merak 62, Phad 75 and Megrez 65.

To demonstrate the point, I first set white balls on the studio floor, at the correct relative distances from the camera. With the camera on the same plane, the result was the familiar Plough pattern, but moving the camera out of alignment showed that the stars were not really close together. In reality, five of the seven are moving at about the same speed and in the same direction, forming what is termed a moving cluster; the remaining two, Dubhe and Alkaid, are moving in the opposite direction, so that in 50,000 years’ time Ursa Major will look very different. We feature Ursa Major again in this month’s episode of The Sky at Night (see page 85 for details).

Everyday objects are often used. In November 2010, we employed an avocado to illustrate how a comet tumbles through space. The month before, we dealt with ‘light echoes’, and since this is none too easy to explain we set up another demonstration. In 1272, the Danish astronomer Tycho Brahe observed a brilliant star in the constellation of Cassiopeia, where no star had been seen before. It became bright enough to be seen with the naked eye, and remained visible for months; it faded slowly, and after some months was lost (there were no telescopes in 1572). We’ve long known what it was: a supernova, the collapse and subsequent explosion of a very massive star. We wanted more information about it, and the chance came in 2008, when Japanese astronomers using the Subaru telescope on Mauna Kea picked up on its ‘light echo’ on nebulosity near it.

Light rays and roses

To explain light echoes on the programme, we went into the garden. I represented the Earth, with Chris Lintott as Tycho’s star. Paul Abel and Pete Lawrence were light rays sent out when the star exploded. Paul came straight toward me, unobstructed by nebular material. Pete started off at an angle, and collided with nebulosity (actually a rose bush) and only then did he resume his journey in my direction. He was the ‘echo’ and reached me later than Paul, because he’d had further to travel. What all this means, therefore, is that the Japanese observers on Mauna Kea can actually study the spectrum of Tycho’s Star at the maximum of 1572.

The demonstration involved parallax. The first astronomer to use this method was Friedrich Bessel, in 1838. To show what parallax means, shut one eye, hold up a finger and align it with an object some way away (such as a picture on the wall). Now use the other eye. There will be no alignment, because you are observing from a different direction; your eyes are not in the same place. If you know the length of the baseline (the distance between your eyes and the angular shift (the parallax), you can work out the distance between your finger and your face. Bessel selected a star, 61 Cygni, which he believed to be close. As a baseline he used the diameter of the Earth’s orbit. Observing when the Earth was on opposite sides of its orbit, he measured the parallax of 61 Cygni and found its distance: 11 lightyears.

I was helped in my demonstration of parallax by the boys of Holmewood House Prep School. We went to the cricket field, and a dozen boys acted as background stars, with Charles Copeland representing 61 Cygni. As we moved the camera around, Charles showed very obvious parallax. It was an enjoyable afternoon, and in the nets I was able to have some bowling practice. As usual, my long, leaping run, windmill action and quick leg-breaks caused much comment!