In transit

28th May 2004 at 01:00
Next month, a rare astronomical event, the transit of Venus, will be visible in the UK. Kevin Walsh explains its significance and shows what to look out for


At the Science Museum, London, visitors can see Captain Cook's telescope, an astronomical quadrant (measuring instrument) used in the 1769 transit, and a modern solar telescope in an exhibition at the Antenna Gallery, which also shows you how to view the transit safely. Tel: 0870 8704868

On the day

Magna Science Adventure Centre in Rotherham, South Yorkshire, is holding a free event for schools on the morning of the transit from 9-12. Schools must book. Tel: 01709 720002 At the National Space Centre and the University of Leicester, activities begin at 6am, with several telescopes available. For other centres see

Thy return posterity shall witness;

Years must roll away,

But then at length the splendid sight,

Again shall greet our distant children's eyes

Jeremiah Horrocks (pictured), 1639

On Tuesday June 8, a remarkable celestial event will take place - a rare transit of the planet Venus. "Transit", in this context, refers to the passage of a planet across the visible disc of the Sun. From our vantage point of the Earth, such a thing applies only to Mercury and Venus (the so-called inferior planets, whose orbits lie within the Earth's orbit of the Sun). Last year, on May 7, there was a much-observed and widely reported transit of Mercury, but transits of Venus are much more scarce and hold profound historical significance.

It is difficult to appreciate fully how we know the exact scale of the universe. Activities such as constructing scale models of the Solar System are extremely rewarding, a fine introduction to the idea of there being a huge amount of nothing out there: "space" is not a carelessly chosen word.

Exam board specifications are keen to encourage and excite pupils'

curiosity about phenomena in the universe around them. There are the requirements of the ideas and evidence sections of scientific enquiry to consider too. The reproducibility of scientists' work, the reporting and judging of such work, and the philosophical, moral and social implications of it are also studied from key stage 2 upwards. All KS4 physics specifications include the Solar System and the scale of the Universe, while A-level and Higher specifications include some study of cosmology, including the methods by which we arrive at the ever more familiar astronomical ladder of distances.

But mankind hasn't always been able to look up planetary distances in books or on the internet. In the 17th century, measuring the size of the Universe was as tough as science got. Take one of the greatest scientists of them all, Johannes Kepler. Building on the (literally) revolutionary ideas of Copernicus, Kepler formulated his three laws of planetary motion, which describe how the planets move in elliptical orbits round the Sun. His third law even stated a mathematical relationship between the distance of the planet and the time it takes to complete one solar orbit.

At the time, nobody knew how far away the planets really were, only that Mars was about one and a half times more distant than the Earth is from the Sun, Jupiter about five times further, and so on. This didn't stop Kepler compiling his famous Rudolphine Tables, which provided astronomers, seafarers, and anyone else with an interest in celestial goings-on, with the most accurate calendars and ephemerides then available.

All these achievements were fully appreciated at the time by the young Jeremiah Horrocks, a self-taught astronomer in the village of Much Hoole in Lancashire. Kepler had predicted that Venus would pass between the Earth and the Sun (would be in transit) in 1631, but failed to observe it.

Moreover, he didn't work out that it would happen again in 1639, on November 24 (by the old Julian calendar). Horrocks did, and he observed it as well, the first man to do so, thereby affirming his place in history.

This wasn't his only achievement: Horrocks also determined the motions of the planets Saturn and Jupiter and proved that the orbit of the Moon was elliptical. His work was used by Newton, who acknowledged his greatness, and he is regarded by many as the father of British astrophysics, achievements that seem all the more remarkable given that he died aged just 22.

But why is the forthcoming transit of Venus so significant? Are we using it merely to remember the life of a hitherto little-known English scientist? Not at all. The first decent estimate of the mean Earth-Sun distance, about 93 million miles (the "astronomical unit", or AU), was made in 1672, but it was following measurements of Venusian transits that refinements to this vital cosmic yardstick could be made. Famous expeditions took place in 1761 and 1769 (the latter involving Captain Cook sailing to Tahiti to observe the transit of Venus) and yet more accurate measurements of the transits were made in the Victorian era (1874 and 1882, when photography had emerged as an astronomical aid), enabling the AU to be well and truly nailed.

Indeed, the 1882 transit is the most recent, so no person alive can claim to have witnessed this event before.

Interest in Horrocks grew in Victorian England with that 1882 transit, but while astronomers' interest in transits these days remains deep, their concerns stretch well beyond the solar system. Detecting transits is one way of discovering planets around other stars - "exoplanets"; we have indeed come a long way since the days of Kepler and Horrocks, when many still believed that the Earth was the centre of the universe.

Kevin Walsh is a fellow of the Royal Astronomical Society and teaches physics at Westminster School, London


The transit is a tremendous opportunity to indulge in some real science. It looks set to be the most observed celestial event in history.

Observers in the UK might well groan at this point, with the cloud cover of the total solar eclipse of 1999 still fresh in their minds. I am, however, much more optimistic about this, principally because it will last longer, beginning at 6.13am and ending about six hours later. (We get another chance in 2012, but it will not be visible from the UK.)

There are lots of websites with information and suggestions about pupil-based project work, many of which serve as excellent introductions to solar observing. Projects could include: Photography: no need for camera adapters on telescopes, a simple photograph will be excellent.

* Timing: predictions suggest that the timings between Edinburgh and Plymouth will differ by about 30 seconds, so schools may wish to collaborate to check this.

* Plotting the path: the apparent position of the planetary disc as it crosses the bright circle of the Sun will vary with the observer's latitude. Teachers might wish to consider setting up links with schools in other countries to compare observations.

* Things to watch out for: the "black drop" effect (the distortion of the planetary disc near the edge of the Sun); and the atmosphere, which prevents an observer from seeing a nice, crisp edge, thereby affecting timings.

Also watch out for features on the Sun, such as sunspots and prominences. While these might confuse observations, they are well worth observing in their own right (see websites right).

I would advise observing the Sun for several days before the transit to familiarise yourself with it and the observation technique, which does not involve looking directly at the Sun: even so, extreme care must be taken to ensure safe viewing.


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