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Why is the transit of Venus so rare?

Why do transits of Venus happen twice in eight years, then not again for over 100 years?

If you miss the transit of Venus in early June you'll have to wait another 105 years until December 2117 for another chance to witness one of the rarest and most famous events in astronomy.

"You can blame that long wait on the geometry and interplay of the orbits of Earth and Venus," says planetary scientist Dr Craig O'Neill from Sydney's Macquarie University.

"It's purely just a consequence of how long it takes the two planets to go around the Sun and how long it takes them to be in alignment again."

A transit occurs when Venus crosses the disc of the Sun as seen from Earth.

But instead of covering the whole Sun as the Moon does during a total eclipse, Venus being so far away and looking so much smaller from our perspective, will appear as a tiny dark dot passing across the golden face of the Sun.

The transit of Venus doesn't happen every year because the orbits of Venus and Earth are slightly out of alignment with each other, says O'Neill.

"Compared to Earth's orbital plane, Venus is about three degrees out."

"When we see Venus, which usually happens in the early morning or late evening, it generally seems to pass a little above or below the Sun because its orbital plane is a little bit out from our perspective."

"So Venus and Earth only line up in the exact same orbital plane in relation to the Sun, a couple of times every few hundred years," he says.

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Orbital dynamics

The alignment of the Earth and Venus depends on the amount of time it takes both planets to go around the Sun.

Earth takes around 365.25 Earth days to complete one orbit around the Sun.

"Venus goes around the Sun every 224.7 Earth days, so it's buzzing around far quicker than Earth is," says O'Neill.

"By the time it takes the Earth to complete one orbit of the Sun, Venus has gone around almost one-and-a-half times. That's why they don't meet up on exactly the same alignment very often."

"And the fact that Venus is on a slightly elliptical orbit makes matters worse," says O'Neil

"Venus will arrive at the point where our two ecliptic planes line up a bit earlier this time than eight years ago," says O'Neill.

"In another eight years time, Venus will arrive at that point before Earth gets there, and we won't see a transit. Another eight years on, it will be a little more ahead again, and that will continue for another 105-and-a-half years — until it eventually catches Earth again, at the other side of the Sun."

The next transit after the 6 June 2012 will be in 2117.

"Then there will be another cycle of eight years, so there will be another transit in 2125. After that, Venus will inch ahead again, and won't catch Earth again for another 121-and-a-half years."

"So the entire cycle goes on a 243-year basis. By the time it takes Earth to go around the Sun 243 times, Venus has gone around 395 times and they almost exactly line up again give or take about nine hours or so."

Dr Craig O'Neill is an astronomer and planetary scientist at Macquarie University in Sydney. He was interviewed by Stuart Gary