The 2012 Transit of Venus

Transit of the Century? Venus, June 5, 2012

On Tuesday, June 5, 2012, viewers across the world can watch as our neighbor planet Venus passes between the Earth and the Sun. The event is called the "transit of Venus"—the term "transit" referring to the appearance of the planet crawling across the surface of the sun.

For viewers in the Delaware Valley, the event begins at 6:03PM EDT, with Venus encroaching onto the upper edge of the sun. While the entire event takes nearly 7 hours to complete, the sun sets for us (8:24PM) as the transit is in progress. We'll see about a third of the total event. Viewers farther west will see more; viewers in the mid-Pacific Ocean will see the entire event.

How can I see the transit?
Come to The Franklin Institute where two planetary geologists, Dr. Vicki Hanson (Univ. of North Dakota) and Dr. Tracy Gregg (Univ. of Buffalo), specialists on the volcanology of Venus, will give a 45-minute presentation in the Fels Planetarium beginning at 4:30PM. At 5:30PM, we'll all go up to the Institute's spectacular rooftop deck where a flock of astronomical telescopes will be set up to provide various kinds of views of the transit. We'll watch from the beginning at 6:03PM until sunset at 8:24PM. NASA Solar System Ambassador and Franklin Institute Chief Astronomer Derrick Pitts will lead the program, including the observing session on the roof. The program ends at 9:00PM.

What is a "Transit of Venus" anyway?
The effect of a solar eclipse is caused by a match between the apparent size of the sun as seen in the sky and the apparent size of the moon as seen in the sky. We know the sun is much bigger than the moon but it just so happens that they can appear to be the same size because of the ratios of their size and distance when compared to Earth. Because of these factors, the moon will occasionally just match the sun's apparent size. If this occurs at just the right time, the moon will completely block the sun's light and create a shadow that can race across our planet—a total solar eclipse!

On June 5, the Earth will be "eclipsed" when Venus passes between the Earth and the Sun. The difference here is that Venus is so much farther from us, that its physical size won't block very much light from the sun at all.

What will we see?
Covering just 1/30 of the Sun's apparent diameter, Venus will appear as a dark spot against the Sun. Properly protected naked-eye observers will just barely see the spot of Venus and will not see any subtle effects of the event. Viewed under magnification, as through a properly filtered telescope, Venus will appear as a larger dark dot and subtle effects like the "Black Drop" at the beginning and ending of the event will be discernable.

How rare is this event?
Both rare and not so rare, because of the unique cycle of the intersections between the orbits of Venus and the Earth that make this possible, if you catch a transit alignment of Sun, Venus, and Earth at just the right time and see a transit, the next one is just 8 years later, but if you miss that "just right time," you're in deep trouble. The cycles of triple alignment at 8-year spacing coincide every 105 years and 121 years! So at those long intervals you'll see two transits 8 years apart. This year's transit is the second of a pair; the first of the next pair of transits comes along 105 years from now—in 2117!

When the transit was visible here in Philadelphia in 1769, American mathematician and astronomer David Rittenhouse organized observing teams for the American Philosophical Society and observed from his estate in Norriton. The APS president at the time, Benjamin Franklin, was in England attending to affairs of state.

So whether you see it online or "live" through a telescope, it's worth a squint just to say you didn't come to this planet without having seen one of the rarest astro events possible!

For more information, contact Derrick Pitts, Chief Astronomer,, 215.448.1234.

The simulator below projects the transit's trajectory as visible from Philadelphia.

CLICK HERE if the simulator is not working in your browser.

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