Mercury appears as a small black dot passing across the face of the Sun.

Credit: NASA

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SYDNEY: Just after dawn tomorrow, Australians will have the chance to catch a glimpse of Mercury moving across the Sun, the only time they’ll be able to see it until 2032.

This phenomenon, called a transit, occurs when a planet moves in front of a larger body. The only two planets that ever pass between the Earth and the Sun are Mercury and Venus, according to astronomer Nick Lomb from Sydney Observatory.

“You’ll see the planet Mercury move across the face of the Sun,” said Lomb. “It will be seen in silhouette and what you’ll see is a very small dot moving … from right to left across the top part of the Sun.”

People from Australia’s east coast and Tasmania will be able to witness the entire transit, which will begin at 6.12am and conclude at 11.10am. People in New Zealand, the Pacific Ocean and the west coast of the U.S. will also be able to watch the transit from start to finish.

Residents farther west in Australia will also be able to view the transit, but for a shorter time, explained Lomb. They will already see Mercury on the Sun when it rises.

While astronomers and avid stargazers gear up to see the transit, Lomb warns that proper measures should be taken when observing the event. “It needs to be watched with extra care because just looking directly at the Sun is very dangerous," he said.

Lomb suggests that amateur astronomers project the image from their telescope onto a white screen for viewing. The planet will only appear as a black dot, just 1/194th the width of the Sun, but it will be visible with amateur telescopes, he said.

So where's the best location for viewing the transit? “Basically, anywhere as long we get a break from clouds,” said Lomb. Clouds have been forecast for tomorrow, but Lomb remains optimistic.

The observatory is hosting a transit-viewing breakfast from 6am to 7.30am tomorrow. While the breakfast is fully booked, people will be able to use the observatory’s telescopes to view the transit later on in the morning.

Lomb said the breakfast session will commemorate the transit by serving guests “a yellow custard tart with a blueberry, representing Mercury moving across the Sun”.

“Comparing what happens during a transit of Mercury or a transit of Venus helps us understand what we’re looking at when we are trying to discover planets around other distant stars,” said Lomb.