1. A solar eclipse occurs when the moon passes between the sun and the earth. During a total solar eclipse, the moon completely obscures the sun. At this time, both the sun and the moon - which is 400 times smaller than the sun, but 400 times closer to Earth - appear the same size in the sky while the moon casts a small, circular shadow on Earth.
2. As the moon orbits Earth, the shadow moves in a narrow ribbon across it - known as the path of totality. To view a total solar eclipse, you must be within this path. The shadow of tomorrow's eclipse will be about 140 kilometres wide and begin in the Northern Territory at sunrise, cross the top of far north Queensland and exit the east coast between Innisfail and Port Douglas. It will then brush the top of New Zealand, traverse the south Pacific Ocean and end at sunset off South America.
3. In Cairns, where about 60,000 people have travelled to watch the eclipse, the first phase will begin at 5:45am on Wednesday and reach totality at 6:38am for two minutes.
4. Wednesday's solar spectacle is the first total eclipse visible from Australia since 2002, when one passed over Ceduna in South Australia. The last solar eclipse over the Great Barrier Reef was in 710AD and the next one will occur in 2237. The exact date, time and place of an eclipse can be determined one to two hundred years into the future.
5. The rest of Australia, including people in Sydney and Melbourne, will be able to witness a partial eclipse, where part of the sun is blocked by the moon. The effect is not as grand, and in many regions will be hard to notice.
6. A total solar eclipse is the only time the sun's outer layers of gas - the corona - can be viewed from Earth. Usually the corona is too faint to see against the blue sky. An American astronomer, Jay Pasachoff, who travels the globe to study the corona during eclipses, will witness his 56th solar eclipse tomorrow.
7. Scientists also use eclipses to measure the diameter of the sun and assess changes in its size over time.
8. People who travel the world to watch eclipses are known as eclipse hunters, eclipse chasers or umbraphiles (umbra meaning "shadow", phile meaning "lover of"). Terry Cuttle, an amateur astronomer with the Astromical Association of Queensland, has been to French Polynesia, France, Africa, Antarctica and Egypt to witness 11 solar eclipses. "They're addictive," he says.
9. Do not look at the sun or any type of eclipse without proper eclipse eye protection; otherwise, serious eye damage may occur. You can view an eclipse directly with welding glasses with a lens category higher than 14 or approved solar eclipse glasses. Make sure the lens is not cracked or damaged. Do not look through binoculars, telescopes or camera viewfinders without appropriate filters. More information here.
You can also observe the sun indirectly by projecting the eclipse onto a piece of card or concrete. See here for details.
10. One of the earliest known records of an eclipse comes from ancient China, when the moon is thought to have blocked the sun on October 22, 2134BC.
11. In places where a total eclipse can be viewed, the sky will darken enough that other planets will also be visible. During totality, Venus and Saturn will glow higher than the sun in the sky, while Mercury will sit between the sun and the horizon.
12. Tomorrow's eclipse will occur just after sunrise and be visible low in the eastern sky, about 14 degrees above the horizon. As the eclipse will occur during the beginning of north Queensland's wet season, there is a chance clouds will obscure people's view. As a precaution, Professor Pasachoff and his students have set up three viewing sites, and hired a helicopter to ensure they don't miss the big event.
13. A total solar eclipse occurs somewhere on Earth about once every 18 months.
Sources: Astronomical Association of Queensland, CSIRO and NASA