The orbital plane of Venus lies at an angle of just under 3 1/2 degrees to the Earth’s orbital plane, known as the ecliptic. Transits, and eclipses, can only happen when the third body lies on, or very close to, that plane. This condition is only realised at two points on Venus’s orbit where it crosses the ecliptic going from below it to above it, called the ascending node, or from above to below it, called the descending node. Transits of Venus happen in pairs eight years apart. The interval between the second of one pair and the first of the next alternates between 122 years and 105 years.

Total transits of Venus, when its crossing can be seen from beginning to end, are extremely rare. The last complete transit of Venus occurred on 23rd May 1283 and lasted from about 2pm until 7.25pm when the Sun was still 16˚ above the horizon. The next Venus Transit entirely visible from the UK will not take place until 11th June 2247 when mid transit will be at noon GMT. The last incomplete transit of Venus, when part of its path across the Sun could be observed, occurred in 1882.

Transits, which should never be observed directly without proper filtration, have only occurred six times since telescopes were first used in the early 17th century. This time, schools and amateur astronomy clubs across the British Isles and Europe will take part in a mass observation that repeats an experiment performed by a little-known English astronomer called Jeremiah Horrocks, who in 1639 was the first person to predict and observe a transit of Venus.

In the 17th century the true distances between the planets and the Sun were not known. They could only be known if the true distance of one planet from the Sun, in kilometres, was known. This could be found by timing the transit of Venus from several places on the Earth and using the parallax effect with trigonometry to find the distance to the Sun. This distance is known as the Astronomical Unit or AU and we will be joining with others across Europe on 8th June to precisely time the ingress and egress of Venus on the disk of the Sun. The astronomical unit is the baseline for stellar parallax determinations, and is thus the first step of the cosmic distance scale. Below are the approximate times of contact for Belfast.

1st Contact 0519hrs GMT
2nd Contact 0539hrs GMT
3rd Contact 1103hrs GMT
4th Contact 1123hrs GMT

This time, the transit begins at about 6.20am BST on 8th June, shortly after sunrise, when the black disc of Venus appears to kiss the outer edge of the Sun. The entire transit takes about six hours, with mid-transit due at 9.22am and Venus leaving the Sun at about 12.04pm.