Observing the Moon dancing with Venus

Did you notice, in the days between Christmas and New Year, the pleasing juxtaposition of the planet Venus and the crescent Moon…

Did you notice, in the days between Christmas and New Year, the pleasing juxtaposition of the planet Venus and the crescent Moon?

In the early evenings they were the dominant features in the southern sky, and there they performed a slow-motion pas de deux, about half a dozen moon diameters apart. It was a fitting display to mark the approaching anniversary of the death of Jeremiah Horrocks.

Horrocks, son of an English farmer, was born in 1619, and from an early age developed an interest in astronomy and mathematics.

By 1639 he was tutor to the children of the local gentry in the village of Much Hoole in Lancashire, a position which allowed him to indulge his hobby. His great achievement was to predict, and then observe, a "transit" of Venus on November 24th that year.

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Now, the planet Venus is nearer to the Sun than we are. Every now and then it passes between Earth and Sun, and obscures for some little time the solar disc.

Such an event is much less spectacular than a solar eclipse by the Moon; instead of blocking out the sun completely, Venus is seen - if you happen to be looking - as a tiny dot that moves across the sun. This phenomenon is called a "transit", and is very rare.

In the last 400 years only five such alignments have occurred - in 1631, 1639, 1761, 1769, 1874 and 1882. There were no transits of Venus during the 1900s, and our next opportunities to view such a spectacle will be on June 8th, 2004, and on June 6th, 2012.

Johannes Kepler correctly predicted a transit of Venus for 1631, but it took place when the sun was below the horizon for a European observer.

Horrocks, however, calculated that there ought to be another transit eight years later - and so there was. He observed it by projecting the solar image onto a screen with a small refracting telescope.

It was cloudy for most of that astronomically eventful Sunday, and not until 3.15 p.m. did the sky clear - but when it did the round black dot of the planet Venus could be clearly seen silhouetted against the bright disc of the sun.

Horrocks's description of the event, Venus in Sub Sole Visa, became one of the classics of astronomical literature.

But for his early death, he might well have made many other substantial contributions to astronomy. However, it was not to be: at the age of only 22 Jeremiah Horrocks died unexpectedly, 360 years ago today.