Facts of the Matter

Richard Brill

Sunday, June 6, 2004

Isle history recalled
as Venus eclipses the sun

On Tuesday, a small black dot will drift for six hours across the face of the sun for the first time since 1882. It is not a sunspot, but a rare transit of Venus as Earth's inferior sister eclipses the sun.

Transits of Venus recur at intervals of 8, 121.5, 8 and 105.5 years in a repeating cycle (skipping the eighth year every fourth cycle). This pattern emerges because the orbits of Earth and Venus are in resonance, but not on the same plane. Therefore, Venus only comes between Earth and the sun when both planets are at the node where the two orbital planes intersect.

The transit itself is not particularly exciting for the casual viewer, who will not see it anyway since it requires special viewing precautions to avoid damage to the eye. Sky-geeks of all types are interested in the event for the sake of the event, and there is much for astronomers to learn from it as well.

But the real significance is historical. The story begins in 1627, when Johannes Kepler published the most accurate data on the motions of the planets, the "Rudolfine Tables," culminating 2,500 years of record-keeping, dating back to the ancient Babylonians.

Using data from 20 years of precise nightly observations by Tycho Brahe, a Danish astronomer, Kepler painstakingly discovered the first mathematical relationships between the planets and was the first to correctly describe their orbits. From these relationships, known today as Kepler's Laws, he calculated that both Mercury and Venus would appear as black dots traversing the bright face of the sun in 1631.

Kepler died in 1630, so he missed the transit. On the other hand, it couldn't be seen from Europe anyway because it occurred at night.

Due to a small error in his calculations, Kepler never realized that another transit of Venus would occur in 1639. The omission was discovered by a young amateur English astronomer, Jeremiah Horrocks only a few weeks before the Dec. 4 transit. Horrocks nearly missed it, but observed at least some of it and later wrote up his observations, thereby becoming the first recorded witness of the phenomenon. Later Horrocks calculated the cycle of transits and predicted the next occurrences in 1761 and 1769.

Newton's "Principia," published In 1687, contained the theory of gravitation and methods that made such calculations easier and gave results that had much greater precision.

In 1716 Edmund Halley, as in Halley's comet -- a friend of Newton and also one of Horrocks' star students -- suggested that one could use trigonometry to find the size of the solar system if the transit could be observed from different points on the globe simultaneously, the more widely separated the better.

Halley would not live to see the transit, just as he would not live to see the return of the comet whose return he predicted, using his friend Newton's calculus. The idea was intriguing nonetheless, even though no scientist alive in his time would be there to see it.

The distance between the planets was the Holy Grail of scientists in the time of Newton and Halley and the best estimates of the distance between the earth and the sun ranged from 5 million to over 150 million miles.

This distance, known as the "astronomical unit" or AU, is the standard unit for all solar system measurements.

It was known from Kepler's laws that Venus was 0.7 AU from the sun, but without a precise value for the AU the size and extent of the solar system could never be known.

Kepler determined that there was a rather unusual relationship between the period of revolution (the planet's "year") and its average distance from the sun (the third power of the planet's distance from the sun divided by the second power of its period of revolution equals one AU per year).

It would be a simple matter to calculate the distance of any planet from the sun merely by knowing the length of its year, if only the size of the AU were known.

When Venus appeared as a little black spot on the sun in 1761 as predicted, plans were made to put Halley's suggestion into action for the 1769 event. Captain James Cook was dispatched to obtain data on the transit.

Technical problems abounded, as great as those confronting space missions today. Sea travel was hazardous, with fatalities often exceeding 50 percent on long voyages. The timing of the transit and the location of the observation needed to be known because the precision of both impacted the accuracy of the calculations of the AU by Halley's trigonometry.

In the mid-18th century, finding location at sea was a shaky proposition at best. Latitude could be determined to a reasonable precision by observing the altitude of the sun, but longitude was a different matter.

Longitude is intimately connected to time, so the solution to the longitude problem was to carry an accurate clock showing the reference time, which could be compared to local noon by sun time.

There were no clocks then that could keep accurate time for months under the harsh conditions at sea. Christian Huygens had invented the pendulum clock in 1665, hoping it would solve the longitude problem, but sea trials in 1662 and again in 1686 proved ineffective for keeping shipboard time due to the pitching, yawing and rolling of the ship on the waves.

Spring driven clocks were primitive and less accurate to begin with, but changing temperature or humidity would cause the metals in the spring to expand and contract, adding to the inaccuracies.

The longitude problem was so pressing that in 1714 the British government offered a 20,000-pound reward (roughly equivalent to half a million dollars today) for a practical solution to it. The prize was eventually awarded, after much conflict and controversy, to John Harrison, a poor, uneducated watchmaker who took on the might of the Oxford and Cambridge professors who sat on the Board of Longitude.

When Cook set sail on the Endeavour he carried three of Harrison's very expensive clocks (at $75,000 in today's dollars), three being deemed the minimum necessary to assure accuracy. He also carried a 140 power reflector telescope with a micrometer for accurate measurement.

Accompanying him were two astronomers: Lt. James Cook (no relation) and Charles Green. The younger Cook was a career seaman, an expert in nautical astronomy and coastal mapping. Green was a former assistant at the Greenwich Observatory who had observed the 1761 transit of Venus, and in 1763 had tested Harrison's clock on a voyage of evaluation.

Having arrived in Tahiti after a voyage of eight months, they set up an observatory in a tent at what is now called Venus Point.

The data was collected, but observation problems made them inaccurate.

One was the atmosphere of Venus, the other was what Capt. Cook dubbed the "black drop." He stated it this way:

"Very difficult to judge precisely of the times that the internal contacts of the body of Venus happened, by reason of the darkness of the penumbra (actually the atmosphere of Venus) at the sun's limb, it being there nearly, if not quite, as dark as the planet. At this time a faint light, much weaker than the rest appeared to converge towards the point of contact, but did not quite reach."

(A similar "black drop" can be seen just before the thumb and index finger touch when brought together slowly at arm's length.)

The difficulty was to decide precisely when Venus "broke free" from the sun's limb because even the smallest inaccuracy in the timing of the transit would be magnified in the calculation of the AU.

One might say that the operation was a success but the patient died.

Because of the small inaccuracies in observation and in the clock, the voyage contributed little to calculating the AU. It did, however, amount to a major event in history, and added much to scientific knowledge otherwise.

Cook had been give secret, sealed orders to find and chart all unknown lands in the South Seas and annex them for Britain. He "discovered" New Zealand and Australia on this voyage. He also insisted on strict health and dietary regimens, especially the eating of Vitamin C-rich foods, which prevented scurvy, a terrifying problem on long voyages.

After circumnavigating the planet, he returned to England a hero, was given two more ships and dispatched on subsequent voyages.

On the third voyage he sailed the west coast of North America in search of the legendary northwest passage after spending a year in the South Pacific. It was on his way north that he stumbled upon Hawaii, and on his return south that he met his demise in Kealakekua Bay.

Unfortunately this upcoming transit of Venus will not be visible from the Pacific Rim (including Hawaii), the western United States and southern South America. Ironically the black drop begins just after the green flash in Honolulu Monday evening. But don't sweat the irony. Next time, June 6, 2012 the transit will begin around noon and end right at sunset. If you miss that one you'll have to wait until Dec. 11, 2117!

Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at


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