What is a transit of Venus? A transit of Venus is the passage of the planet directly across the face of the Sun as viewed from Earth. Although Venus passes between the Earth and the Sun every 1.6 years (inferior conjunction), the chances that the alignment is precise enough to produce a transit is very small because of the tilt of the orbit of Venus and the small size of the planet compared to its orbit. When the calculations are done it turns out that transits tend to occur in pairs separated by eight years and spaced by over a hundred years. The pair of transits visible to our generation occur in 2004 and 2012. The next pair will not occur until 2117 and 2125.
How can the June 5, 2012 transit be observed in Grand Rapids? The circumstances for viewing the Venus transit from any location on Earth can be found from the US Naval Observatory web site. From Grand Rapids, we will see the beginning of transit at 6:04 pm EDT while the Sun is still fairly high in the sky (33 degrees altitude, 272 degrees azimuth or almost due west). Venus will entirely be in front of the Sun (interior contact) by 6:22 pm. Although the transit will last six and a half hours, our viewing will be cut short by sunset at 9:18 pm (at which point the sun will be a 303 degrees azimuth or nearly northwest). For maximum viewing, seek a location with an unobstructed northwestern horizon. As always, care must be taken when viewing the Sun to protect your eyes. Either use solar eclipse glasses (which block the bulk of the light), a telescope with a solar filter, or view a projection of the event onto a screen.
The Calvin College observatory will host a special open house from 6 to 9 pm that evening. The general public is invited, no admission fee. To find us see the directions. Viewing will be possible with eclipse glasses (which we will provide) or with several telescopes (viewing through the eyepiece or projected on a screen). We will also post images on our website at intervals through the event.
Viewing depends, of course, on weather. The forecast as of Friday June 1 is for partly cloudy skies. We will hope for the parts that are not cloudy!
What did we see during the June 8, 2004 transit? The June 2004 transit was already in progress when the Sun rose in Grand Rapids at 6:04 am EDT. The end of complete blockage (interior contact) was only an hour later at 7:05 am, with the end of all contact at 7:25 am.
Sunrise came with nominally clear skies but a thick haze on the horizon, as often happens in summer in Michigan. The obscuration was enough that one could at first view the Sun directly without protective filters. The back side of Venus is truly black making the transit much more dramatic than sunspots, which are not nearly as dark and generally smaller. This was the first time in history a Venus transit was seen with the unaided eye.
Images of those first minutes did not show Venus well as the Sun itself was so distorted: much dimmer and redder at the bottom than the top. The four images below were taken by L. Molnar during the final minutes of the transit. The spacing between them was about three minutes. They were taken with a standard home camera at the eyepiece of the 16 inch telescope (viewing with a solar filter). North is up and east to the left. Note how the color of the Sun continued to change as it rose higher with each successive image.
What is the history of Venus transits? The possibility of a Venus transit was first realized by Johannes Kepler (discoverer of the elliptical nature of planetary orbits) in 1627. The first successful observation was made by Jeremiah Horrocks in 1638. Edmund Halley suggested in 1716 that careful observations of a transit made from various points on Earth could be used to relate the distance to the Sun to the distances between the observers, in short to determine the size scale of the solar system. This led to coordinated observations of transits in the 1760s. In many ways this was the beginning of "big science", coordinated projects involving many people.
Other, more precise means are now used to determine the scale of the solar system, so a transit of Venus is no longer a special event scientifically. However, it remains a rare and beautiful event. It has been an inspiration to the imagination from Thomas Pynchon's Mason and Dixon to PBS's Inspector Lewis.
posted 6/1/2012 by L. A. Molnar