On Wednesday, the planet Venus will appear to cross in front of the sun, appearing as a small black dot. This historic skywatching event, called a transit of Venus by astronomers, is among the rarest of predictable phenomena and one that has amazed scientists and observers throughout history.

Transits come in pairs – the last was June 8, 2004 – but the pairs are separated by over 100 years. The last pair was between 1874 and 1882.

Astronomer Edmund Halley proposed in 1716 that the transits, viewed from two widely-separated positions on Earth, could be used to measure the distance from the Earth to Venus and to the sun by mathematical triangulation. He never lived to see a transit, but in 1769, two teams were sent to measure that year’s transit from Tahiti and northern Norway.

The planet Venus will be visible as a tiny black dot crossing the face of the sun, a process which allows the experts to fix the place of the planet Earth amongst the galaxies and within our solar system. If Venus were large enough, or close enough to the Earth to completely cover the face of the sun we would have a solar eclipse instead.

However, Venus is about 40 million kilometres away from the Earth and so it only appears as a bright ‘star’ in the sky.

This month’s image by Winston Grech shows the planet Venus as it appeared last week – a crescent very low in the west after sunset.

The Astronomical Society will be organising two observing sessions to view the Venus transit.

One group will meet in the car park at Żonqor Point in Marsascala, while another will meet along the St Julian’s seafront near the Balluta waterpolo pitch. Observations will start at sunrise (5.46 a.m.) and will last until an hour later when Venus moves out of the sun’s disc at 6.56 a.m.

Do not look directly at the sun with the naked eye at anytime while attempting to viewthe transit.

For up-to-date information join our Facebook group at www.facebook.com/groups/maltastro.

Mr Pace is president of the Astronomical Society of Malta.

Astronomical events this month