By Dean Regas
Enquirer contributor

The world was a very different place the last time a transit of Venus occurred on Dec. 6, 1882. This astronomical event is so rare, that no one alive has seen it.

A transit of Venus occurs when the planet comes directly between the Earth and the sun. Venus is then visible as a small beauty mark on the face of the sun.

IF YOU GO

Venus Friday Includes hourly classes about our closest planetary neighbor and tours of the Observatory. Then, see the clouded, shrouded goddess of beauty, plus Jupiter and Saturn, through our historic telescopes (weather permitting). When: 8-11 p.m. Friday Where: Cincinnati Observatory Center, Mount Lookout Cost per program: $5, $3 for children Reservations: Required, 321-5186. Transit of Venus Class Delve into Venus' mystery and explore the history of viewing transits. Includes viewing of the moon, Saturn, Jupiter, and Venus (weather permitting). When: 8-10 p.m. May 3 Where: Cincinnati Observatory Cost: $10 per person. Transit glasses are included. Reservations: Required: 321-5186.

The 1882 transit was visible throughout most of the Western Hemisphere. Viewing this transit took on extra urgency for astronomers and the general public because the next transit would not occur until this year.

So a long wait is over. On June 8, Venus will once again transit the sun and the eastern half of the United States can witness it.

Transits of Venus crop up in pairs eight years apart, but more than a century separates any two pairs. The transit this June will be followed by another in 2012 (which will be visible in the western half of the United States). But don't wait for 2012 - it could be cloudy on one or both of those days. And I'm sorry to say that you won't live to see the next pair in 2117 and 2125.

1631 and 1639

Johannes Kepler predicted the first transit of Venus in 1631. No one saw it. The Thirty Year's War, cloudy weather, and the short duration of the transit made observing impossible.

The next transit in 1639 was only viewed by two astronomers in England. Jeremiah Horrocks calculated the transit and employed his friend William Crabtree to observe from a neighboring city. The day of the transit started off cloudy. But late in the afternoon the clouds parted and Horrocks "beheld a most agreeable spectacle, the object of my sanguine wishes, a spot of unusual magnitude and of perfectly circular shape."

1761 and 1769

Not everyone was as lucky as Horrocks. European countries mounted great expeditions to the far corners of the globe to see the transits of the 18th century. If accurate measurements of the transit could be made at opposite ends of the Earth, astronomers could calculate the distance from the Earth to the Sun (and also the distance to each planet).

Astronomers traveled to Siberia, India, Madagascar, South Africa and the South Pacific. Some were welcomed with clear skies. Others missed the entire thing.

One of the greatest adventures to see the transit of 1769 was led by Captain James Cook, who took his crew to the island of Tahiti. From Point Venus, Cook and company had perfect weather and a nice stay in a tropical paradise.

1874 and 1882

After 105 years, astronomers eagerly awaited the transit of 1874. Russia, Britain, France, Germany, Italy, Holland, and America funded 62 expeditions to see it.

Astronomers used a new invention: the camera. The American teams alone captured 350 pictures of the 1874 transit and 1,700 images of the 1882 transit.

The transit of 1882 was especially viewable from the United States. And not just astronomers were geared up for the event. This was one of the most talked about cultural events of the year. This transit was continually reported in newspapers and sales for smoked glass (the "safer" method of observing the sun in 1882) skyrocketed. The New York Times reported: "This is the first time within the memory of man that the unlearned common people have been permitted to observe a transit."

Safety

Looking at the sun will damage your eyes, so the easiest and cheapest way to observe it safely is by looking through eclipse shades. For about $3, these glasses will allow you to observe the sun, transits, partial eclipses, and maybe even a large sunspot. Eclipse shades are available at the Cincinnati Observatory or from Rainbow Symphony at http://www.rainbowsymphony.com/eclipse/soleclipse.html

Don't wait for June 8 to see Venus. You can watch it blazing in the western sky after sunset, brighter than any other star-like object.

Dean Regas is the Outreach Astronomer at the Cincinnati Observatory Center. E-mail: deanobservatory@zoomtown.com.