June offers glimpse of rare event

Posted: Sunday, June 06, 2004

Dark nights are virtually nonexistent, though some bright stars and planets can be seen, especially around local midnight which is at 2 a.m. when our sun is lowest beneath the horizon and the sky takes on its darkest blue hue.

The brightest of those planets should be Jupiter in the west. Among the stars, Vega and Capella should be visible in the east and north. And as Venus is in inferior conjunction between us and our sun it should not be visible. But this is the month for the famed Venus Transit.

The magic date is Tuesday. However, since Alaska is situated in a western time zone, for us it's Monday evening leading into Tuesday. On Monday starting at 9:15 p.m., Venus will glide across the sun's face for the ensuing six hours. This is called a Venus Transit. The last such event occurred in 1882. It is a rare but predictable event, and in past centuries a Venus Transit had astronomical significance since the distance from the sun to Earth could be determined.

While a Mercury Transit happens every seven years on the average, though right around now they occur rather often in 1999, 2003 and 2006, Venus Transits are indeed very rare as they come in pairs eight years apart and then again either 105 or 121 years later. Venus orbits our sun every 224 days and passes Earth every 584 days. But since Venus' orbit is tilted by 3.4 degrees with respect to the Earth's orbit, the Earth, sun and Venus rarely are lying exactly on a straight line. That's only the case when Venus is exactly at its nodes, where its orbit crosses the plane of the Earth's orbit, and we're viewing Venus and sun together.

The Venus Transit can be seen from Europe, Asia and most of Africa, as well as parts of eastern North America because it happens during their daytime. Of course, in Alaska daytime is extended into the night and the further north you travel, the more you can see it as our sun skims along the horizon.

You may view the Venus Transit safely with special solar eclipse glasses or, according to the August 1999 issue of Sky & Telescope, welder's filters of shades 14 and higher may be used. Other so-called filters may not be safe (of course, regular sunglasses are out of the question) and may lead to temporary or permanent eye damage.

Under no circumstances, use binoculars or a telescope to view our sun, not even with the safe solar filters, since the lenses or mirrors will intensify our sun's light and burn a hole right through the solar filter and then through your retina! You have to make do with the small image that the eclipse glasses or the welder's filter will give you. Just to be on the safe side, use caution and do not look at our sun for too long with the safe solar filters.

In 1627, German astron-omer Johannes Kepler became the first person to predict a Venus Transit, having achieved the mathematical feat with the help of his newly discovered Laws of Planetary Motion. He predicted the upcoming transits for 1631 and 1639, though he died a year prior to seeing one himself and the 1631 transit wasn't visible from Europe anyway. On Dec. 4, 1639, the English Jeremiah Horrocks and William Crabtree were the first to view a Venus Transit, using the projection method. Fortunately, they had recalculated Kepler's computations and found that the transit would occur a few hours before Kepler's predictions, otherwise they would have missed it. Still, it's quite an accomplishment for Kepler to have computed a transit 12 years prior with such a high accuracy. And this was before hand-held calculators were available.

A hundred years later, Edmund Halley proposed that a Venus Transit could be used to determine the scale of the solar system since observing a transit from different vantage points on Earth would yield slightly different observing times and using parallax and simple trigonometry, distances in the solar system could be measured.

Several expeditions were sent out for the June 1761 and 1769 transits. Some met with misfortune, such as Guillaume LeGentil's mission which in 1761 had to observe the transit from a ship during heavy seas on the Indian Ocean.

The French frigate could not land in Pondicherry, India, because the English had just occupied the region. In 1769, after having built an observatory in India, he encountered cloudy skies during the day of transit. Others were more successful, such as the Americans John Winthrop in Newfoundland and David Rittenhouse in Pennsylvania. James Cook himself on the island of Tahiti.

One of the scientific accomplishments was the discovery by the Russian Mikhail Lomonosov in St. Petersburg in 1761 that Venus has an atmosphere as sunlight was slightly smeared around the edges of Venus. That proved a drawback for timing the transit exactly because a smeared image doesn't make for accurate measurements.

Compiling all measurements, though, led to a sun-Earth distance of 95.4 million miles, which was the standard used until the December 1874 and 1882 transits when photography aided astronomers and a corrected figure of 92.8 million miles was accepted. Newer measurements using radar give an average distance of 93 million miles, 91.4 in January and 94.5 in July.

In my quest to get as far north as I can and to have an unobstructed northern horizon, I will pack a small telescope with the proper filter, binoculars with proper filters, two projection screen telescopes and some eclipse glasses and head out on the Dalton Highway toward the Tors near Finger Mountain, just 18 miles short of the Arctic Circle.

I also have a plane ticket to Barrow, but the weather prospects aren't good. Actually, they aren't good near Fairbanks either. From the Kenai Peninsula, the first two hours of the transit can be observed, but then our sun sets around 11 p.m. on Monday.

Andy Veh is the physics and astronomy instructor at Kenai Peninsula College. This column appears on the first Sunday of each month. He can be reached at aveh@uaa.alaska.edu.

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