A Transit of Venus

(This column was first published in the June 6, 2004 issue of The Buffalo Sunday News.)

There are two planets in our solar system whose elliptical orbits are closer to the sun than the orbit of our Earth. Those planets are Venus and Mercury. For that reason those two planets occasionally pass between the Earth and the sun. When this happens, it is called a transit. (Recall that "trans" is a root meaning "across".) Tuesday morning, June 8, 2004, a Transit of Venus will occur.

From Buffalo only the latter part of this rare event will be visible. If clouds do not intervene, the transit may be seen from sunrise at 5:37 until 7:25 a.m. when the small black shadow moves completely off the face of the Sun, having first touched the edge at 7:06.

All this will take place while the sun is within 17 degrees of the horizon so I advise watching from a high open area. The Buffalo Astronomical Association will sponsor a viewing from the Buffalo Museum of Science rooftop.

A transit is like a solar eclipse which occurs when the moon passes between Earth and the sun, but the moon is close to the Earth so it covers most or all of the solar disk. Venus is much farther away so it crosses the face of the Sun as a small dot. You must look closely to see it and to differentiate it from sunspots.

Here I must enter an important warning:




Instead, if you plan to view the sun on your own, use one of the following means to project it onto a neutral screen for observation.

1. Use a binocular lens to focus sunlight on a piece of paper.

2. Carefully punch a small round hole in one end of a shoebox (with its lid removed) and hold the box so that the sunlight focuses on the inside of the other end.



A lidless shoebox makes a simplified camera obscura for observing the sun.


If the paths of Venus and the Earth were in the same plane, these transits would occur quite often. However, neither plane coincides with ours. The orbit of Venus inclines at an angle of about 3.4 degrees to that of Earth. Transits occur only when the two planets are on the same side of the sun along the line where these two orbital planes intersect. This is further complicated by the fact that Venus makes thirteen revolutions around the sun in the same time it takes Earth to make eight.



The angle between the orbital planes of Venus (V) and Earth (E) is about 3.4. The sun is at S, a focus of both near-circular ellipses. Transits occur only when the Earth is at d and Venus is at c, or the Earth is at a and Venus is at b.


Because of this, transits of Venus are very rare. The most recent occurred on December 6, 1882, before any of us were born. Four transits occur in a regular cycle of 243 years with irregular intervals of 121 1/2, 8, 105 1/2 and 8 years. Because this June's transit follows that longest gap, another will happen in eight years on June 6, 2012. After that the gap will again be over a hundred years.

Some History

Early in the 17th Century astronomer Johannes Kepler not only discovered the basic properties of the elliptical planetary paths but he also published a set of tables based on his theory and associated observations. These Rudolphine Tables, named after one of Kepler's sponsors, included a prediction of a Transit of Venus on December 7, 1631. Unfortunately, the tables were off by a few hours and this transit was not observed.

Kepler having died, a 21-year old Englishman, Jeremiah Horrocks, reinterpreted his predecessor's results and, finding a transit date Kepler did not discover, observed Venus' path across the sun on December 4, 1639. This was the first recorded Transit of Venus. Sadly, this largely self-trained astronomical prodigy died within a year of his accomplishment.

In 1716 and well before the next pair of Venus transits were to occur, the famous astronomer and friend of Isaac Newton, Edmund Halley, published a seminal paper in the English Philosophical Transactions entitled "A new Method of determining the Parallax of the Sun, or his Distance from the Earth." Halley's "method" involved extremely careful observations of the forthcoming transits.

It is difficult for us to understand today how the remarkable science of Kepler and Newton still left a fundamental problem open. They showed the relative distance relationships among the sun, planets and moons of the solar system, but they were unable to determine a unit in order to calculate the actual distances among these objects.

What was needed was the mean distance between the Earth and the sun. This distance is so important that it is designated the Astronomical Unit (AU). Once established, all the other measures may be calculated quite simply by using Kepler's proportional relations. For example, astronomers following Kepler knew that Venus was about 0.7 AU from the Sun; they only needed the value of AU to determine how many miles that represented.

It was exactly this distance that Halley suggested could be calculated by timing the passage of Venus across the Sun.

In order to gain some insight into Halley's proposal, consider how wildly inaccurate were estimates of the Astronomical Unit until his time. Today we know this unit down to a fraction of a meter. To the nearest mile it is 92,955,807 or about 93 million miles.

The estimated value for the A.U. until the end of the 16th century was based on Aristarchus' 270 BC value of only 5 million miles. Astronomers like Ptolemy, Copernicus, Brahe and Gilbert continued to use this wildly inaccurate estimate. Kepler then raised the value to 14 million miles and Riccioli in 1660 raised it again to 28 million miles. Finally Cassini in 1672 came close with an estimate of 87 million miles, but few accepted his figure.

Halley's paper set off a firestorm of activity. Countries competed to establish sighting stations around the world. Even with the Seven Years War going on, the English, French, Germans, Swedes, Danes, Italians, Russians and even the then British Colonies in North America sent out expeditions to observe the 1761 and 1769 transits in remote places like India, Siberia, St. Helena, Jakarta, Capetown, Tahiti and Newfoundland.

The roster of observers is equally extraordinary. It included the famous explorer, Captain James Cook; William Harrison, the son of John Harrison, who was carrying one of his father's clocks to solve the longitude problem; and two reluctant participants, Charles Mason and Jeremiah Dixon, who were later to survey the Pennsylvania-Maryland boundary.

But surely the most remarkable expedition was that of a Frenchman known as Le Gentil who took two volumes to recount all of his adventures. Enroute in 1760 to allow plenty of time to prepare for the 1761 transit, he learned that the site in India where he planned to observe had been captured by the British. Then he was delayed by a hurricane, suffered dysentery and so was only able to observe the transit from shipboard. Undeterred, he stayed on for eight years in the Indian Ocean preparing to observe the 1769 transit only to have the Sun covered by local clouds. Shipwrecked twice on his return voyage to France, by the time he finally reached home in 1771 he found that he had been presumed dead by his family who were in the process of dividing up his property.

Similar but generally less exciting expeditions were mounted to observe the 19th century transits.

After all this, however, it turns out that the results of the observations, even when clouds did not intervene, were never accurate enough for modern calculations. Today radar and laser imaging provide us with the kind of extremely accurate values necessary for applications to such activities as international space programs.

Observing a Transit of Venus no longer carries the important weight it did in earlier times. Still it is exciting to observe an event that will occur only twice in our lifetime.



The paths of Venus across the sun during transits. All paths go from left to right.


-- Gerry Rising


Happily, the last half of the June 8, 2004 Transit of Venus was easily observed in western New York. With Jeremiah Horrocks (1618-1641), the first observer of a Venus Transit, we too can claim, "I then beheld a most agreeable spectacle, the object of my sanguine wishes, a spot of unusual magnitude and of perfectly circular shape". And we can appreciate the small poem he wrote after witnessing this marvelous event:


Oh! then farewell, thou beauteous queen!
Thy sway may soften natures yet untamed,
Whose breasts, bereft of the native fury,
Then shall learn the milder virtues.
We, with anxious mind, follow thy latest footsteps here,
And far as thought can carry us;
My labours now bedeck the monument for future times
Which thou at parting left us. Thy return
Posterity shall witness; years must roll away,
But then at length the splendid sight
Again shall greet our distant children's eyes.