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Transits

A transit is what happens when a celestial body appears to move across the face of another. Examples are a planet moving across the face of a star or a moon moving across the face of the planet it is orbiting.

If a celestial body completely or mostly obscures the other object it is moving in front of it is called an occultation. Solar and lunar eclipses are types of occultation.

From Earth the only planets that can be seen transitting the Sun are Mercury and Venus. Transits of Venus are particularly notable because they happen very infrequently and are quite impressive – Venus appears as an easy to see disc. When Mercury transits it's visible as just a very small dot on the face of the Sun.

Transits of Venus occur in pairs eight years apart, with two pairs every 243 years, spaced at 121.5 years and 105.5 years. The latest pair was in 2004 (not visible from New Zealand) and 2012 (visible in New Zealand – see below). The previous pair were in 1874 and 1882. The next pair of transits will be in 2117 and 2125.

Transits of Mercury are much more common than transits of Venus, and for the last several hundred years have always occurred in May and November. The next Transit of Mercury will occur in 2016 – a partial transit viewed from New Zealand.

History

After observing the transit of Venus from Tahiti in June 1769, Captain James Cook proceeded to New Zealand (which had not been visited by Europeans since Abel Tasman in 1642), which he reached in October.

In November 1769 Captain Cook observed a transit of Mercury from Mercury Bay on the eastern coast of the Coromandel Peninsula. It was because of this transit that people realised that Mercury has no atmosphere.

Safety while viewing transits of the Sun

Do not look at the Sun directly – it can cause permanent eye damage.

Do not look at the Sun through exposed photographic black and white film – it doesn't block all the UV light.

Do not look at the Sun through a telescope or binoculars without a proper solar filter(s) attached to the front end – it can cause instant blindness.

Viewing is possible using a pair of binoculars or pinhole to project an image of the Sun onto a screen (eg, a piece of paper, a white wall, etc). Alternatively, use a "reflected pinhole", where a 5mm "pinhole" on a mirror is reflected into a dark room.

Solar viewing glasses can also be bought from observatories but these should not be used for extended viewing and of course do not provide any magnification.

6 June 2012 Transit of Venus

On Wednesday, 6 June 2012, a transit of Venus was visible from New Zealand, although the weather wasn't very good in most parts of the country.

Venus appeared as a small black disc moving in front of the much larger disc of the Sun. In Auckland it started at 10:15am and was finished by 4:44pm.

When Venus transits the Sun it moves across the face of the Sun in a straight line, but because of Earth's rotation it appeared to move in a curve as this diagram from the Royal Astronomical Society of New Zealand shows.

See www.rasnz.org.nz/2012Transit/Venus2012.html for more information.

I took some photographs near the start of the transit when I could in clear breaks and before the clouds closed in completely, although I was also able to catch a brief glimpse a bit over an hour before the end of the transit. The sun was quite active at the time, so sunspots were also visible.

To take the photographs I mounted a pair of binoculars on a tripod with a bungie cord and projected an image of the Sun onto a sheet of white plastic a couple of metres away. The projected image was upside down and the plastic sheet wasn't as clean or scratch free as it could have been. A large plain white piece of paper would be better. Drawing the curtains mostly closed provided a reasonably dark viewing environment.

10:25 am – Venus showing as a bite out of the edge of the Sun.

10:34am – Venus fully in front of the Sun, still visible through the clouds. The image was coming and going a bit because of the clouds. I may have been getting some black drop effect here – also observed by Captain James Cook in 1769.

10:35pm – Venus and the Sun clear of clouds again. Tripod visible at bottom left, aimed through a crack in the curtains.

11:07am – Venus making its way across the face of the Sun.

In my last sighting at around 3:35pm Venus was more to the left and a little further from the edge than the last photo above shows. It was on it's way out again, and exactly where predicted.