FLORIDA, USA (REUTERS) - Mercury's journey, which astronomers refer to as a "transit", began with what looked to be a small, black dot on the edge of the sun at 7.12am EDT (1112 GMT), images relayed live on Nasa TV showed.

Over the next 7½ hours, Mercury, which travels at a speed of 48km a second, crossed the face of the sun, a spectacle last seen in 2006, occurring every 10 minutes or so, as Earth and its smaller neighbouring planet come into perfect alignment.

But Mercury is too small to see without high-powered binoculars or a telescope, and looking directly at the sun, even with sunglasses, could cause permanent eye damage.

Fortunately Nasa and astronomy organisations were providing virtual ringside seats for the show by live-streaming images of the transit in its entirety and providing expert commentary.

The tiny planet, slightly larger than Earth's moon, started off as a small black dot on the edge of the sun at 7.12am Eastern (1112 GMT). Travelling 48km a second, Mercury was expected to take 7½ hours to cross the face of the sun, which is about 1.39 million km in diameter, or about 109 times larger than Earth.

Lika Guhathakurta, a heliophysics scientist, described Mercury's traversing of the sun.

"What happens here is two celestial bodies approach each other from our vantage point and so during a transit you see, a smaller celestial body gets in front of a larger body, cutting across the disk and so the size of the two are distinctly different, Mercury is a dot and the sun is huge."