Next live webcast: Total Lunar Eclipse of February 20-21, 2008

VENUS TRANSIT
June 8, 2004

I shall try to show some of the results that are sent to Astronet soon. For the time being you might have a look already at the resulats that are shown at Venusvoordezon.nl or the Venus Transit Photo Gallery of SpaceWeather.com.

Archive images by Paul Dolk at Space Expo, Noordwijk, The Netherlands

Soon: archive images by Mira Public Obervatory, Belgium

Between June 3 and June 13 Venus is visible in SOHO's coronagraph LASCO.

Venus takes more than six hours to track across the Sun. Times shown are Universal Time. Greatest transit is the instant Venus passes closest to the Sun's center. Astronomy magazine illustration by Roen Kelly.

The principal events occurring during a transit are characterized by contacts. The event begins with contact I which is the instant when the planet's disk is externally tangent with the Sun. The entire disk of the Venus is first seen at contact II when the planet is internally tangent with the Sun. During the next several hours, Venus gradually traverses the solar disk at a relative angular rate of approximately 4 arc-min/hr. At contact III, the planet reaches the opposite limb and is once again internally tangent with the Sun. The transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress. Greatest transit is the instant of minimum angular separation between Venus and the Sun as seen from Earth's geocenter.

Courtesy: Fred Espenak

Courtesy: ESO

This is one frame of an animation that shows both the track of Venus across the Sun and the regions on Earth where the transit will be visible. Contact times and the exact path Venus will take across the Sun differ slightly for observers in different locations. The path and times shown here are correct for an observer located at Earth's center, but contact times will not differ from these by more than 7 minutes anywhere in which the contacts are visible. The Earth image is centered on the geographical location where Venus lies directly overhead.
Click for the Astronomy magazine animation (AVI, 1,3 Mb) by Francis Reddy.

SAFETY

NEVER LOOK DIRECTLY AT THE SUN WITH UNPROTECTED EYES - THIS MAY CAUSE TOTAL BLINDNESS WITHIN SECONDS! ALWAYS BE SURE TO USE PROPER OPTICAL FILTERS TO PROTECT YOUR EYES.

The use of #14 shade welding glass or eclipse shades will permit a large number of people who do not have specialized equipment to observe this event. However, as the planet approaches the limb of the sun, subtleties like the black drop effect may not be discernible.Pinhole projectors are a safe, indirect viewing technique for observing an image of the sun. While popular for viewing solar eclipses, pinhole projectors suffer from the same shortcomings as unmagnified views when Venus approaches the edges of the sun. Small features like the black drop effect and the halo around Venus while it straddles the solar edge may not be discernible.

Courtesy: European Southern Observatory

You may project a magnified view of the sun through a telescope onto a surface, but the technique often has its own limitations. For example, large reflector telescopes can generate too much heat by concentrating a lot of the sun's energy on the secondary mirror and eyepiece. Likewise, Schmidt-Cassegrain telescopes can experience too much heat build-up as the light bounces internally. Also, magnified projections usually have an exposed focal point beyond the eyepiece where bystanders could inadvertently burn themselves. Constant attention is required.

The transit of Venus is perhaps best viewed directly when magnified, which demands an appropriate solar filter over the large end of the telescope. Do not use small filters that fit over the eyepiece, for the concentrated sunlight can shatter them. The sun's energy must be attenuated before it enters the telescope. A filtered, magnified view will show the planet Venus, the "black drop" effect, and sunspots.

No matter what technique you use for viewing the sun, do not stare continuously at the sun! Always give your eyes a break.

MEASURING THE SOLAR SYSTEM

The first astronomer to recognize the importance of observing transits of Mercury and Venus was Edmund Halley (1656-1742). It appeared to the observer, using a telescope as the lens of a camera obscura (never look directly at the Sun through a telescope!) as a black dot crossing the surface of the Sun. The first astronomers to use telescopes to observe the transit of Venus were Jeremiah Horrocks (1618-41) and William Crabtree (1610-44) in 1639.

Astronomers travelled to remote parts of the world to observe the transits of Venus in 1761 and 1769. To observe the transit of 1769, Captain Cook sailed from England to Tahiti. He discovered Hawaii and a few other places as bonuses along the way; it is not often that the side benefits of astronomical research are so apparent.

How accurate were the measurements of Cook? The "dusky shade round the body of the Planet" as he descrtibed the appearance of Venus was a problem. Intense sunlight filtering through Venus' atmosphere fuzzed the edge of the disk and decreased the precision with which Cook could time the transit. For this reason, his measurements disagreed with those of ship's astronomer Charles Green, who observed the transit beside Cook, by as much as 42 seconds.

Cook's observations

Black drop effect

In recent years, radio signals emitted by spacecraft as they pass behind Venus have enabled us to obtain very accurate planetary positions and masses, as well as the distance between the Earth and the Sun. Because of these results from space exploration, observing this and future planetary transits will be of less scientific importance, but they will of course continue to be great public and educational interest.

What else is there to be seen? Look before and during ingress and during and after egress for a spectacular 'light ring' around the black silhoutte of Venus. This is caused bu sunlight that is refracted by Venus' thick atmosphere. It was after observing this light ring in 1761 that Russian astronomer Mikhail Lomonósov realized that Venus is surrounded by an atmosphere.

The light ring in 1874

General information:

The Black Drop Effect

Weather:

Mercury transit, May 7, 2003

---