By Andy Veh, for the Redoubt Reporter
Tuesday, June 5, will herald an unusual event, rare enough to only happen every 120 years — Venus, from our vantage point, will pass in front of the sun.
What is a Venus transit?
Venus moves in front of the sun’s disk. Only the inner planets, Mercury and Venus, both orbiting between Earth and sun, are able to transit.
Why is this Venus transit special?
A Venus transit is extremely rare, as they happen in pairs only every 120 years. The phrase “once in a blue moon” should be replaced by “once in Venus transit.”
Johannes Kepler discovered the laws of planetary motion in the 1610s, allowing astronomers to predict the positions of planets, transits and eclipses very accurately. The invention of the telescope around the same time helped the cause, too. Hence, prior to Kepler, no transits had ever been observed. In contrast, eclipses were somewhat easier to predict, even with the knowledge of accurate planetary motions.
Mercury, on average, transits every seven years. Thirty-six such transits have been observed since 1631 (Kepler’s first prediction). Personally, I’ve observed the 1999, 2003 and 2006 Mercury transits.
There are only six Venus transits that have been observed historically, in 1639, 1761, 1769, 1874, 1882 and 2004.
This list excludes 1631, predicted by Kepler but not visible from Europe and no astronomer made the effort to book a flight from Paris to Chicago. Personally, I’ve seen this one from Barrow.
The next Venus transits after 2012 will occur in 2117 and 2125.
Why is a Venus transit so rare?
All planets orbit the sun in almost the same plane, i.e. almost edge-on. But that’s only “almost.” Each orbit is slightly tilted with respect to the Earth’s orbit (Venus’ orbit by 3.4 degrees), with each two orbits’ line of sight crossing each other twice. These are called nodes, and they are always in the same place in Earth’s and Venus’ orbit around the same time in either June or December.
As Venus needs about 7.5 months to orbit the sun, it and Earth meet up every 584 days. Usually that conjunction means that when all three are lined up, Venus appears higher or lower than the sun’s disk (due to the orbit tilt) and therefore doesn’t pass in front of the sun. But twice every 120 years, the sun, Venus and Earth line up when Venus crosses one of its nodes and a transit occurs.
What is the scientific value of a Venus transit?
An eastern location on Earth sees Venus come into contact with the sun’s edge several minutes ahead of a more western location. In Anchorage it starts at 14:06:30 June 5, but in Seoul at 7:10:31 June 6. Adjusting for the 17-hour time difference, the transit in Anchorage starts four minutes, one second earlier. By knowing the distance between the two locales, one can triangulate the distances from Earth to Venus and Earth to sun.
Hence, the 1760s transits produced the first accurate measurements of the scale of the solar system. At 153 million kilometers, 95 million miles, it was only a little larger than the actual 149.6 million kilometers, 93 million miles, measured by radar and telemetry.
During the 2004 Venus transit, 1,510 registered observers from around the world collected data that yielded a distance of 92.983 million miles, compared to the actual 92.976 million miles, a measurement error of 0.007 percent.
How can I observe Venus transit safely?
Basically, in the same way as observing a solar eclipse — be very careful. Solar eclipse glasses and welder’s filters of shades 14 and higher may be used. Other filters may not be safe (regular sunglasses are out of the question) and may lead to temporary or permanent eye damage. Under no circumstances should you use binoculars or a telescope to view our sun, not even with solar filters.
The larger lens or mirror will intensify the 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 the sun for too long, even with safe solar filters.
What should I expect to see?
Provided that the weather will cooperate, you will see a black circle (which is Venus’ night side) transit across the sun’s disk, plus a few sunspots.
What should I bring to view Venus transit?
You could bring a photo or video camera to take pictures of the sun’s projection, or even try to shoot through a telescope equipped with a solar filter.
Should I be punctual?
Unlike a total solar eclipse, which is over within a couple of minutes, a Venus transit lasts for hours. Projected times for the Kenai Peninsula are from 2:06 to 8:48 p.m. June 5.
Andy Veh is a professor of astronomy and physics at Kenai Peninsula College’s Kenai River Campus.
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