Are you flipping out yet? The transit of Venus is coming!

A transit of Venus is when Venus crosses the disk of the sun from earth's perspective. (Source: Jan Herold)
A transit of Venus is when Venus crosses the disk of the sun from earth's perspective. (Source: Jan Herold)

TUCSON, AZ (Tucson News Now) -

The upcoming transit of Venus will be the last one any of earth's human inhabitants will see in their lifetimes.

Transits of Venus are rare.

What is a transit of Venus? A transit of Venus is when Venus crosses the disk of the sun from earth's perspective. (See photo at bottom of story.)

When is the 2012 Transit of Venus? First contact (when it all begins) is at3:07PM on June 5, 2012. The transit will be in progress at sunset (7:28PM).

How often do they happen? The answer to the frequency question is quite complicated. On average, transits of Venus happen every 80 years or so. However, this average figure is messy. You see, Venus transits occur in a 'pair of pairs' pattern that repeats every 243 years. First, two transits take place in December (around Dec 8th), 8 years apart. There follows a wait of 121.5 years, after which two June transits occur (around June 7th), again 8 years apart. After 105.5 years, the pattern repeats.

Why do they happen so infrequently? Venus, Earth and the Sun actually line up about every 584 days, but the alignment is not perfect. Usually, Venus is above or below the sun from Earth's vantage point. Think about it in terms of things closer to home: the Moon doesn't block out the sun (eclipse) every month because, even when they are roughly aligned with Earth, the Moon is above or below the Sun from the Earth's perspective. The perfect alignment of the Earth, Venus and the Sun (transit of Venus) has only occurred seven times since the telescope was invented in 1610!

When was the last transit of Venus? The most recent transit of Venus was on June 8, 2004. Prior to that, no living human had seen a transit of Venus. The one prior to the most recent was on December 6, 1882!

Why is there so much excitement about the transit of Venus? In the modern era, the transit of Venus is exciting mostly because of its rarity. However, it has a historical link that can teach us about discovery. Humans have used the transit of Venus to understand the solar system and even the universe. Although it's just a dot crossing the disk of the sun from earth's view, Edmund Halley (of the famed comet) determined that the distance from Earth to the Sun could be determined from a Venus transit.

How did Halley's theories lead to the discovery of the distance from Earth to the Sun? Halley (pronounced like "valley", picture at bottom) knew that the position of the dot (Venus) on the Sun's disk would be different from different places on Earth. He sent expeditions around the world to observe the transits of 1761 and 1769. By measuring the change in location of the dot (Venus) on the Sun from location to location on Earth, Halley's theory used simple geometry to calculate the distance from the Sun to Earth and from Venus to Earth. Unfortunately, the timing of the measurements taken at different places on Earth was not precise enough to allow Halley to make the calculations. His theory, and his experimental measurements in the 1700s, allowed the next generation of observers (transits of Venus in 1874 and 1882) to accurately measure the distance from Earth to the Sun (called the "astronomical Unit" or "AU") to within 0.20%!

How can we safely observe the 2012 transit of Venus? It's never safe to look at the sun. You must use a device or method to observe the transit indirectly. Venus is large enough for its "dot" to be visible to someone with normal eyesight, without the help of binoculars or a telescope. Its diameter will appear about 1/32 of the diameter of the Sun. For safe viewing of the transit, the same rules apply as those for observing an eclipse of the Sun. Eclipse viewing glasses can be used, as long as they are undamaged and observing is limited to a few minutes at a time. Note that they must NOT be used with binoculars or a telescope. For an enlarged view, an image of the Sun can be projected onto a screen by a small telescope. Pinhole projection, which worked well for the recent solar eclipse, will not produce a sharp enough image to show Venus clearly. KOLD and KMSB will broadcast the transit live on Tucson News Now's Transit of Venus page.

Where in Tucson are there viewing opportunities with experts and all the equipment needed? The following is a list of viewing opportunities.

Mt. Lemmon SkyCenter (fee involved)

Flandrau Science Center (viewing glasses for sale)

Starizona (viewing glasses for sale)

Kitt Peak National Observatory

and on Tucson News Now's Transit of Venus page! (Check back here for a link to the page.)

Enjoy! And, if you have any questions, feel free to Facebook them to me!

Edmund Halley
Edmund Halley
Transit of Venus, June of 2004
Transit of Venus, June of 2004

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