Dark spot on the Sun

By Jonathan Nally May 5, 2016
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Amateur and professional astronomers in the Northern Hemisphere are preparing for a rare ‘transit of Mercury’ on 9 May.

A TRANSIT OF Mercury will be seen across the northern hemisphere on Monday, 9 May, with people in the eastern half of the United States and Western Europe getting the best views.

For the rest of us, there will be live streams of the rare event online.

A transit occurs when one of the two inner planets, Mercury or Venus, moves in front of the Sun as seen from Earth, looking like a black dot slowly crawling across the solar disc.

Transits of Mercury occur 13 or 14 times per century, while the rarer transits of Venus are far less frequent — they come in pairs eight years apart, but with the pairs separated by more than a century.

Before the advent of modern methods, astronomers used observations of transits to try to estimate the distance from the Earth to the Sun. It turned out that Mercury transits weren’t great for this, but Venus transits were.

In fact, it was to witness a transit of Venus that Captain James Cook was sent to the South Pacific in the 1769. He successfully observed it from Tahiti, and on his way back to England just happened to bump into some landmasses that would later be known as New Zealand and Australia.

When to see it

As mentioned, those in the eastern half of the USA and Western Europe, including the UK, will be able to witness the entire event, weather permitting. Unfortunately, Australasian observers will miss out this time. The next one for us will be in 2032!

The transit will start at 7:12am US Eastern Daylight time and end at 2:42pm.

Planet Mercury

The planet Mercury, as seen by NASA’s MESSENGER spacecraft. (Image: NASA)

This means that for people in the western half of the US, the transit will already by underway by the time the Sun rises.

For those observing from Eastern Europe, Russia, most of Asia and most of Africa, the Sun will have set before the transit has finished.

Solar safety

It must be stressed that if you want to try and witness the transit, you must take the same precautions as you would with a solar eclipse. That means no looking directly at the Sun, with or without optical aid.

The best thing to do is construct a simple pinhole camera using some cardboard.

If you are in the northern hemisphere but miss this transit due to inclement weather or other unforeseen circumstances, don’t worry — there’ll be another one along on 11 November 2019!

Mercury facts:

  • Mercury is the closest planet to the Sun;
  • At just 2440km across, it is the smallest of the major planets. By comparison, Australia is about 4000km wide;
  • Mercury’s year (the time taken to go around the Sun) is 88 Earth days;
  • It rotates very slowly, and so spins on its own axis only three times every two Mercurian years;
  • It is named after a mythological Roman god.

Jonathan Nally is editor of Australian Sky & Telescope magazine and SpaceInfo.com.au, and has been writing about astronomy for 30 years. He can also be seen regularly on Channel 9’s Today Show.