The transit time means watchers on the East Coast will be able to see the whole thing, but viewers nearly anywhere in North America won't miss out, since Mercury will still be making its journey when the sun is up on the West Coast.
The transit itself doesn't look like much due to the size difference between Mercury and the sun, Paulson said - you're seeing an itty-bitty black dot. Earthlings get treated to just 13 or 14 Mercury transits a century.
The transit begins at 1235 GMT and ends at 1804 GMT, when Mercury appears to enter on one edge of the Sun and exit on the other. The event will begin at 4:35 a.m. PT, but if you don't fancy getting up that early then don't worry - the transit lasts for almost six hours so you have plenty of time to catch it later in the morning. However, because the Earth and Mercury take different amounts of time to complete a full circle around the sun, we don't see transits every year.
Avid amateur astronomer Blake Nancarrow says a solar transit will be visible Monday. It was by studying the transits of Mercury and Venus in the early 1600s that Johannes Kepler was able to see that the orbits of the planets are elliptical, not circular as previously believed.
Mercury is 4,800 kilometres in diameter, compared with the sun's 1.4 million kilometres.
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Mercury is the smallest planet in the solar system and is a bit smaller than Earth's moon, Florian said.
A transit is when a planet or a satellite passes between the Sun and the Earth.
I'll be using my Baader Planetarium Mylar visual-grade solar filter on the front of my telescope. During a transit, the interceding body is not sufficiently large to cover whatever it's moving in front of.
Now remember, you should never look directly at the Sun without proper protection, as it can permanently damage your eyes. Venus transits are even rarer and won't happen until 2117. NASA and the European Space Agency also will provide updates from the Solar and Heliospheric Observatory (SOHO). There's no harm in pulling out the eclipse glasses from the total solar eclipse across the USA two years ago, but it would take "exceptional vision" to spot minuscule Mercury, said NASA solar astrophysicist Alex Young.
"Fortunately, transits of Mercury are considerably more common than transits of Venus".