KEY POINTS:
For five hours next Thursday, the Earth will be bathed in the shadow of the planet Mercury.
Looking from Earth, the planet will be seen as a small black disc silhouetted against the dazzling face of the Sun.
Astronomers call this rare event a transit of Mercury. The next will not be seen from New Zealand until 2052.
The transit will start at 8.12am and end at 1.10pm and be visible from all parts of the country. Most public observatories will be open, offering the opportunity to safely view this special event - weather permitting.
Mercury is very small compared with the Sun, so the planet will appear like a small black dot about 1/200th the size of the solar disc.
It is possible that a few sunspots may also be visible on the Sun's surface, but they will be more irregular in shape.
Why transits happen
The only planets that can be seen in transit from Earth are Venus and Mercury because only they have orbits closer to the Sun. Mercury completes an orbit of the Sun every 88 days but allowing for the Earth's own motion, the Sun, Earth and Mercury align every 116 days.
However, most of these alignments don't result in a transit because Mercury's orbit is inclined by 7 degrees relative to Earth's. This means that for most alignments, Mercury appears to pass above or below the Sun in the sky - and not across the solar disc.
Transits are only seen if the three bodies are in alignment at the same time that Mercury is crossing the Earth's orbital plane. There are two points - called nodes - where this happens, one falling in June and the other in November. Mercury passes through each node point on every orbit, but unless the Earth also happens to be in alignment with the Sun and Mercury at that moment, no transit will be seen.
On average there are about 13 transits of Mercury every century, but some occur when New Zealand is on the side of Earth facing away from the Sun (during our night time). This is what happened when the last transit occurred in 2003.
The next planetary transit visible from New Zealand will be when Venus passes in front of the Sun in 2012.
Size of the solar system
Transits of the planets Venus and Mercury have been very significant events because they offered early astronomers a way of measuring the true size of the Solar System.
This measurement was of great scientific importance and involved numerous heroic expeditions, yet it was not satisfactorily determined until nearly 1900.
In 1619, Johannes Kepler published his three laws of planetary motion. In particular, his third law defined the relationship between the orbital period of any planet and the size of its orbit - but only as a proportion. The orbital period is easy to measure but the actual dimension of any one planet's orbit was still needed to set the true scale of the Solar System.
A direct approach was to measure the size of the Earth's orbit, which had been roughly estimated by various methods dating back to antiquity.
The astronomer Edmond Halley proposed in 1716 that transits of Venus or Mercury could be used to determine the Earth-Sun distance. Venus, being larger and closer to the Earth, was the more favoured of the two, but using Mercury was at least marginally possible.
The essence of Halley's idea was simple. Two transit observers, widely separated in latitude, would see the planet take a slightly different track across the Sun's face. The precise position of the track could be established by accurately recording the time taken for the transit - to within a few seconds.
Armed with that information and knowing the size of the Earth, astronomers would then be able to calculate the Earth-Sun distance.
James Cook was dispatched to the South Pacific to observe the transit of Venus from Tahiti on June 3, 1769, before sailing to New Zealand to observe the transit of Mercury on November 9.
Cook and his astronomer, Charles Green, made their observations from a site at Cook's Beach in Mercury Bay. They also used these observations to accurately establish their longitude, thereby fixing the position of New Zealand on the map of the globe.
Other astronomers also observed the transits from other places on the globe, in the expectation that by combining all their observations, an accurate value would be obtained for the true size of the Solar System.
In practice, they found that it was very difficult using the simple telescopes of the time to judge by eye the exact moments when the transit started and ended.
The observations of the 1769 transits failed to deliver the accurate Earth-Sun distance, so astronomers tried again during the 1874 and 1882 transits of Venus. New Zealand was used as an observation site on both occasions.
The modern era
By the dawn of the 20th century, astronomers had developed other, more accurate, ways to measure the Earth-Sun distance and today it is known to within about 30 metres using radar and satellite telemetry. It is about 149.598 million kilometres (or 499 light-seconds). The only scientific value of a transit of Mercury today is that it provides an opportunity to study the extremely thin atmosphere of the planet. This requires sophisticated instruments on specialised telescopes.
HOW TO SEE IT
* Stardome Observatory in Auckland will be open to the public and provide safe viewing of the transit of Mercury (weather permitting) on Thursday, November 9 from 8am. Entry fee is $5.
* Safety warning: Looking at the Sun with any optical instrument can be very dangerous and cause serious damage to the eyes. A good telescope equipped with special solar filters is required.
