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June 2012 – What have Captain Cook, Charles Mason of Mason-Dixon Line fame, and Ireland Got in Common?

On June 6th 2012, weather permitting, Ireland will witness a transit of Venus when the planet will cross in front of the Sun. Transits of Venus are very rare and if you don’t see this one, you will have to wait until 2117 for the next and even then you will have to travel to the other side of the World to view it.

Back in 1769, it was realised that by recording a transit from several well-chosen locations on the Earth it was possible to get the distance from the Earth to Venus and then, using Newton’s Theory of Gravity, our distance to the Sun. Even in the 18th century astronomers knew that if we could determine how far away is Sun is, we could use it, as we do today, to measure the scale of the Universe.

Such was the importance of this task that Captain Cook was dispatched to the Pacific island of Tahiti to record the transit, but observations were also needed closer to home. This is where Charles Mason and Ireland comes in. Mason had recently returned to England after completing his survey in the American Colonies with Jeremiah Dixon. The resultant Mason-Dixon line subsequently became famous during the American Civil War as the dividing line between the northern and southern (Dixie) states. Nevil Maskelyne, then Astronomer Royal in Britain, asked Charles Mason to go to Ireland to observe the transit. Mason agreed and, travelling on the appropriately named ship Venus, he landed near Balbriggan with an array of scientific instruments. Mason decided to set up a temporary observatory near Lifford in Donegal so as to view the transit for the maximum period.  On the day, 3rd June 1769, weather conditions were favourable and Mason managed to make some excellent observations. No only did he observe the transit but he also determined his position with respect to Greenwich with remarkable accuracy for the time to within 50 metres. Not bad even in comparison to modern GPS!

The transit of Venus can be seen on Wednesday morning 6th June immediately after sunrise, which occurs around 5 am. It will last for approximately an hour. Incidentally under no circumstances look directly at the sun with the naked eye, through binoculars or a telescope. The safest way to observe the transit is by using a telescope or a pair of binoculars to project an image of the Sun onto a white card.

While transits of Venus are now largely of historical interests, as we have much better ways of measuring distance in the Solar System, planetary transits are still important.  They are our main method of detecting planets around other stars and also of observing what the planet’s atmosphere is like.

Finally one of the best places to observe the transit in the Republic of Ireland is Skerries in North Dublin as it is sufficiently north and east to give maximum time of visibility. The Dublin Institute for Advanced Studies, the Dublin Institute of Technology and Astronomy Ireland will set up telescopes in Red Island Car Park, Skerries from 5 am onwards to observe it.  Ironically this is within sight of where Charles Mason landed his instruments, 243 years ago almost to the day.


Tom Ray, Astronomy & Astrophysics Section, School of Cosmic Physics, DIAS.