Sky Almanac


Venus, our sister planet, more beautiful in the past. Now fried by run-away global warming.
photo courtesy NASA

Pairs of planets grace our evening and morning skies. At dusk Mars and Saturn lie near the star Spica. Before dawn Venus and Jupiter are close to Aldebaran and the Pleiades. Throughout the month there will be a dramatic rush of Mars toward Saturn and Spica.

July 3: The full moon of July is known as the Buck Moon because this is around the time when the new antlers of buck deer push out of their foreheads.

July 4: Although this day is in the midst of our northern summer, the earth reaches its greatest distance from the sun.

July 11: Last quarter moon

July 19: Dark moon; Look for meteors, coming up out of the southeastern horizon from Aquarius. They peak some 10 days later, but during these warm evening nights the sky will be darkest for the next week and meteor hunting should be good.

July 26: First quarter moon.

The transits of Venus

We were treated to a transit of Venus on June 5, 243 years, almost to the day, of the most famous transit, which occurred in 1769. For that watershed event, some 400 observers scattered across the globe to use the transit to determine the size of the solar system. The most famous was Captain Cook.

On August 12, 1768, the Endeavour sailed from England with Lt. James Cook in command, bound for Tahiti. The island had been discovered by Europeans just a year earlier. Reaching this little speck of land 20 miles across was as challenging at that time as a space craft reaching Mars. They needed to reach Tahiti before June 1769, establish themselves among the (hopefully) friendly islanders, and construct an astronomical observatory.

Transit of Venus

Transit of Venus on June 5 photographed by the Japanese spacecraft, Hinode. Note the bright rim produced by the atmosphere of Venus.

The size of the solar system was one of the chief puzzles of 18th century science, like dark matter and dark energy is today. Astronomers knew that six planets orbited the sun (Uranus and Neptune hadn’t been discovered yet; Pluto hadn’t been found yet, but it doesn’t count). Astronomers knew the relative spacing of those planets, and all one needed was just one good measurement of our distance to another planet to determine the earth’s distance to the sun.

Edmund Halley set this project in motion in1716. He showed how by measuring the two times Venus touched the edge of the sun from difference places on the earth, the distance to Venus could be measured. Transits of Venus are rare. They come in pairs, 8 years apart, separated by approximately 120 years. King George III (who is associated with loss of the Colonies and insanity) even got interested, donated 4000£ to the voyage of the Endeavor, and built an observatory in the Kew Gardens to observe the transit of Venus in 1769.

The crew of the Endeavor were also to be guinea pigs in the Navy’s fight against scurvy, which produced lassitude, rotted gums, hemorrhaging. Cook carried a variety of experimental foods onboard, such as sauerkraut. English sailors were not particularly enthusiastic about the German dish, and anyone who refused to chow down on sour, fermented cabbage was whipped. Cook flogged some 15 members of his crew.

By the time Cook reached Tahiti in 1769, he’d been sailing west for 8 months—about as long as modern astronauts might spend en route to Mars. Five crewmen were lost when the ship rounded stormy Cape Horn, and another despairing marine threw himself overboard—one wonders if it was because of sauerkraut—during the 10-week voyage across the Pacific. Cook navigated using hourglasses and knotted ropes to measure ship’s speed, a sextant, and the moons of Jupiter as a celestial clock. Scurvy turned out not to be a problem,  not because of sauerkraut, which doesn’t have much vitamin C, but because of lemons they also carried. They arrived  at Tahiti on April 13, 1769, almost two months before the transit. Telescopes were set up at a location still known as “Point Venus”.

The sky was clear at the time of the transit and their observations came off without a hitch except for an optical effect known as the black drop effect, which made it difficult to determine precisely when Venus touched the edge of the sun. Cook’s observations disagreed with those of ship’s astronomer Charles Green by as much as 42 seconds. Cook was alarmed and terribly disappointed by that discrepancy, but the combined results still were still valuable. Half a world away at the King’s observatory, it was a different story. The times for the transit of Venus from the Kew are remarkable for their agreement, with only one second difference between all of the observers. It must have been difficult to disagree with King George III.

For much of the next year Endeavor and her crew scoured the South Pacific, searching for a continent that some 18th century scientists claimed was necessary to balance the great land masses of the northern hemisphere. Later, during a 10-week stopover in Jakarta for repairs, seven seamen died of malaria. Cook left the city as quickly as possible, but not soon enough: 38 of the Endeavour’s original company perished, including astronomer Charles Green. On July 11, 1771, Cook returned to England, a journey one month short of three years.

The analysis of the transit data proved to be worth the time away from home and those endless meals of sauerkraut. The combined transit data permitted measurement of the distance from the earth to the sun to within 1% of the modern value. In 1771, the analysis of data gave a distance of 153 ±1 million kilometers. A true watershed in astronomy had been crossed. Never before had the solar system been measured so precisely. During the late 20th century, direct radar measurements of the distances of Venus accumulated over 40 years helped to refine the estimate of the Astronomical Unit to its modern value of 149,597,870.691±0.030 kilometers. Pretty impressive! That’s an uncertainty of only100 feet.

Captain James Cook

Captain James courtesy National Maritime Museum, Greenwich, England