The 2004 Venus transit as seen by the Solar and Helospheric Observatory (SOHO).SOHO/EIT/ESA/NASA
— The transit will occur on the evening of June 5 for the Americas; the morning of June 6 for Europe, Africa and western Australia.
— Barring any huge advances in medical science, this will be the last Venus transit anyone alive today will witness.
It last happened in our skies in 2004 and happens again this year on June 6. If you miss it, you'll have to wait until 2117 for the next chance to see the elusive 'transit of Venus.'
This astronomical event was first recorded in December 1639 by English astronomer Jeremiah Horrocks and subsequently by other astronomers who used it to change our view of the Universe.
The transits occur when the sun, Earth and Venus all line up and, from our vantage point, we see Venus as a tiny black spot slowly moving across the fiery bright solar disk.
Venus races across the sun in a repeating pattern with pairs of transits 8 years apart with the following pair appearing 121.5 years later and the subsequent ones after a further 105.5 years. Needless to say, barring any huge advances in medical science, it's highly unlikely that many people alive today will get a chance to see the next transit.
Following the first observed transit in 1639, the pair of transits that occurred in 1761 and 1769 received much more attention and even encouraged Captain James Cook to plan expeditions across the world to study them.
These observations, and those of subsequent transits, allowed calculations to be made of the distance between the Earth and sun that were impressively close to today's precise figure. The Venus transits set the foundations for calculating the vast distances between other celestial bodies.
The transit is visible on the evening of June 5 from the Americas and in the morning of June 6 for parts of Europe, Africa and western Australia. A map of its visibility can be found on the NASA Venus transit website.
Of course, the usual words of caution need to be said when it comes to observing the sun: don't look directly at it. It's possible to safely view the transit by projecting an image of it through binoculars or a telescope onto white card, or use special eclipse glasses. For more on observing the sun safely, follow my solar observing guide: "Observing Our Nearest Star, Safely."