Michael Maunder

Transit : when planets cross the sun

1: Transits down the Ages.- 1 The Moving Planets.- 2 Transits: Why and When?.- 3 Edmond Halley and the Distance of the Sun.- 4 Transits of Mercury.- 5 Venus: The Transit of 1639.- 6 The Transit of 1761.- 7 Captain Cook and the Transit of 1769.- 8 Venus: The Transit of 1874.- 9 Venus: The Transit of 1882.- 10 The Story of Vulcan.- 11 Other Kinds of Transits.- 12 Future Transits.- 2: Observing Transits.- 13 Your Safety.- 14 Observing the Sun: Full-Aperture Filters.- 15 Projecting the Suns Image.- 16 Observing Transits.- 17 Photographing Transits.- 18 Data Capture and Manipulation.

The sun in eclipse

1 Sun, Moon and Stars.- 2 Daytime Star.- 3 Observing the Sun.- 4 Eclipses - General Principles.- 5 Eclipses in History.- 6 The Development of Eclipse Photography.- 7 Partial Eclipses.- 8 Annular Eclipses - and Eclipse Photography.- 9 Eclipse Chasing.- 10 Total Eclipses.- 11 The 1999 Eclipse.- 12 Eclipse Mishaps and Oddities.- 13 Eclipses of the Future.- 14 Eclipses Elsewhere.

Venus: The Transit of 1639

When Mercury crosses the Sun it is too small to be seen with the naked eye, and so there was no chance of observing a transit before telescopes were invented. Not so with Venus, which is so much larger and closer that there is no difficulty in seeing it against the Sun. It may be that some ancient-world or medioeval observer did in fact see a transit; there is a vague report of a sighting by an Arab star-gazer in 639, but it is very indefinite, and what he saw (if anything) could so easily have been a sunspot. Therefore, the first reliable records go back no further than the 17th century.

Transits:Why and When?

We have seen that if we draw a plan of the Solar System on a flat piece of paper, we are not very far wrong; but the slight inclinations of the orbits of Mercury and Venus make all the difference insofar as transits are concerned. If their orbits were in the same plane as ours, the planet would transit at every inferior conjunction — that is to say every 116 days for Mercury and every 584 days for Venus. In fact this does not happen, and transits are comparatively rare.

The Transit of 1761

There can be little doubt that the moving spirit behind the transit observations of 1761 and 1769 was Edmond Halley, even though he had been dead for almost twenty years before the first transit of the pair took place. He originally planned to use transits of Mercury to help in measuring the length of the astronomical unit, but Venus would be a much more satisfactory target, partly because it is closer and also because it is larger. As long ago as 1716 Halley had delivered a paper to the Royal Society, from which I quote: We therefore recommend again and again, to the curious investigators of the stars to whom, when our lives are over, these observations are entrusted, that they, mindful of our advice, apply themselves to the undertaking of these observations vigorously. And for them we desire and pray for all good luck, especially that they may not be deprived of this coveted spectacle by the unfortunate obscuration of cloudy heavens, and that the immensities of the celestial spheres, compelled to more precise boundaries, may at last yield to their glory and eternal fame.

Data Capture and Manipulation

The previous chapter contains most of the information needed to identify objects in transit. Table 17.1 identifies relative sizes and characteristics. Now that we have some interesting observations “in the bag”, how do we access the data?

Projecting the Sun’s Image

The only really safe way to view the Sun at any time of day with no risk to your eyes, is to project its image.

Other Kinds of Transits

So far we have discussed only transits of bodies across the face of the Sun. But there are other kinds of transits also. Most of the planets have satellites, and those of Jupiter and Saturn, at least, are easily visible with very modest equipment (really keen-sighted people can even glimpse some of the Jovian satellites with the naked eye, and powerful binoculars show them easily). There is a great deal of scope here, and the well-equipped amateur can even carry out some useful work.

Captain Cook and the Transit of 1769

Several important lessons had been learned from the transit of 1761. There was nothing to be done about the “Black Drop”, but at least it had become clear that if a really good value for the astronomical unit were to be obtained it would be necessary to send expeditions all over the globe — as far north and as far south as possible. In the end over 150 sets of measurements were made from over 80 stations, and the overall programme as truly international, as can be seen from Table 7.1.

The Story of Vulcan

Of all the principal planets only Mercury and Venus are closer to the Sun than we are, and so can pass in transit across the solar disk. But could there be any other bodies moving in those torrid regions, which might be tracked in the same way?