Today’s Transit Of Venus: The Sight Of A Lifetime

The full moon rises over a hotel on Duluth's Park Point last night at 9:45 p.m. Photo: Bob King

I’ve been preoccupied with the Venus transit and sometimes forget there are other things happening in the sky. Last night the rising moon brought this to my attention. Like an unexpected gift, it stood beautifully orange and oval when I left work for home. Of course I had to change course to stop and enjoy the view.

Edmond Halley

In yesterday’s blog we talked about things to watch for during today’s passage of Venus across the sun. You might wonder if a transit is still of any scientific interest. It was Edmund Halley of Halley’s Comet fame who first suggested that Venus transits could help us determine Earth’s distance from the sun.

Back in the early 18th century, astronomers using Kepler’s laws of planetary motion could only determine the relative distances of the planets from the sun. Earth was understood to be one astronomical unit distant (1.0 A.U.), Mercury 0.4 A.U.,  Jupiter 5.2 A.U.s and so forth. No one knew the real or absolute distances in miles.

In 1677 when Halley was watching a transit of Mercury, it occurred to him that widely spaced observers across the globe could use a transit of Venus to determine its distance from the sun. Armed with that number, simple multiplication would then give the distances to the other planets. Halley suggested that observers record small variations in the position and timing of Venus’ entry and exit from the sun’s disk from various locations around the Earth during the next transit on June 6, 1761.

My daughter Katherine demonstrates how to see parallax using her finger and eyes. Photo: Bob King

The apparent shift of Venus against the more distant sun as seen from widely spaced locations is called parallax. You can see parallax at close range by holding up your index finger at arm’s length from your face. Close one eye and then the other and watch your finger shift against the distant backdrop. If you measure the amount of shift and know the distance between your eyes, you can determine the distance to your index finger with simple trigonometry.

18th century French cartoon depicting the transit of Venus. The first known observation of a Venus transit was on December 4, 1639 by Englishman Jeremiah Horrocks. He used his telescope to project an image of the sun on white paper.

Halley figured the same idea would work on Venus provided observers were spaced far apart (in effect, making for a wide set of multiple “eyes”) , knew their locations exactly and could time Venus entry and exit from the sun precisely. Although Halley didn’t live to see the 1761 transit, his idea was embraced by the scientific community. Astronomers were dispatched around the world in the first international scientific enterprise of its kind.

The black drop effect frustrated the efforts of astronomers to derive accurate distances to the planets. Credit: NASA/TRACE/LMSAL

Despite careful timings, the infamous black drop effect made them anything but precise. If Venus had cleanly separated from the sun when it entered the disk, accuracy would have been excellent. Instead, the planet appeared to “stick” to the inner limb (an atmospheric affect) anywhere from a few seconds up to a full minute for some observers, confounding the timings. When the results were analyzed, astronomers could do no better than place Earth’s distance from the sun at between 77.8 and 96.2 million miles.

Observers met with similar frustrations during the 1769, 1874 and 1882 transits. An accurate Earth-sun distance was finally determined by Scottish astronomer David Gill who photographically measured Mars’ parallax against the distant background stars in 1877. For an excellent, in-depth explanation of how to measure parallax using Venus, click HERE.

The Venusian "ring of fire" is visible just beyond the sun's edge in this satellite photo taken during the 2004 transit. Credit: NASA

During the current transit, another worldwide effort is underway to study Venus miniature “ring of fire”. A narrow ring of light caused by sunlight scattered by the planet’s thick carbon dioxide atmosphere briefly makes an appearance when the planet enters and the exits the sun’s disk. Astronomers will use nine different coronagraphs spaced around the world to probe the temperature and density structure of Venus’s middle atmosphere, or “mesosphere,” where the sunlight is refracted. A coronagraph is a special telescope designed to block the sun and its glare, so astronomers can study the inner corona or in this case, Venus and its atmosphere.

By analyzing how the ring brightens and fades with time, researchers can figure out the temperature and density of this critical layer from pole to pole. To read more about the effort, please check out the full NASA press release.

Extrasolar planets are discovered by measuring the change in brightness of the host star when a planet orbits in front of it.

To date 775 extrasolar planets beyond the solar system have been discovered. Scientists find many of them when they transit their host stars, robbing them of a tiny fraction of their light in a repeated and predictable way.

Observing extrasolar transits, astronomers can determine sizes, masses and orbits of planets we can’t see directly because they’re so far away. Starlight filtering through an exoplanet’s atmosphere can even leave its imprint on the star’s light, revealing the composition of the air.

In another transit study, astronomers will point the Hubble Space Telescope  at the moon to observe the transit of Venus. What? The sun’s too bright and would damage Hubble’s sensitive instruments. Instead, the scope will measure the tiny drop in the amount of sunlight bouncing off the moon as Venus passes in front of the sun.

A spectrum of the sun shows dark lines where elements like calcium and hydrogen reveal themselves by absorbing a bit of sunlight. Scientists will look for similar absorptions in light reflected off the moon to study the composition of Venus' atmosphere.

During the transit, Venus’ atmosphere will absorb specific wavelengths of sunlight that will go missing from the light reflected off the moon. A spectrograph, an instrument which spreads light into its component rainbow colors, will spot these missing colors and determine what chemical components in Venus’ atmosphere cause them. The Hubble effort will help inform efforts to study the atmospheres of exoplanets – an idea nothing short of brilliant.

Venus almost ready to go on stage. Sunlight scattered from the planet's atmosphere reveals the full outline. This picture was taken yesterday June 4 at Observatory Rimavska Sobota in Slovakia. Thanks and credit to: Pavol Rapavy

I hope you have clear skies today. If you’re threatened with clouds and mobile, check out the Clear Sky Chart to find a better location. If you’re socked in, you can always watch the transit via live webcast at Planethunters or NASA EDGE. Remember, the transit starts just a few minutes past 6 p.m. Eastern time, 5 p.m. Central, 4 p.m. Mountain and 3 p.m. Pacific. You can watch it at the Alworth Planetarium on the UMD campus here in Duluth, Minn. from 5 p.m. till sunset. If you take a photo, please share it with us by e-mailing me at:

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