The celestial show will see, or would have already seen, the lunar disc almost completely cast in shadow as it moves behind the Earth, reddening 99% of its face.

The spectacle was visible for all of North America and parts of South America from 6.02am GMT (2pm in Malaysia) today, and might thereafter be seen in Polynesia, Australia and northeast Asia.

By 7.50am GMT, sky-watchers with a cloud-free view in those regions saw the moon half-covered by the Earth’s penumbra, the outer shadow.

Space scientists yesterday said that by 8.45am GMT the moon would appear red, with the most vivid colouring visible at peak eclipse 18 minutes later.

The dramatic shade is caused by a phenomenon known as “Rayleigh scattering”, where shorter blue lightwaves from the sun are dispersed by particles in the Earth’s atmosphere.

Red lightwaves, which are longer, pass easily through these particles.

“The more dust or clouds in Earth’s atmosphere during the eclipse, the redder the moon will appear,” Nasa explained. “It’s as if all the world’s sunrises and sunsets are projected onto the moon.”

The moment the eclipse begins – when the moon enters the Earth’s shadow – to when it ends will take more than three hours and 28 minutes.

That is the longest partial eclipse since 1440 – around the time the printing press was invented – and won’t be beaten until 2669.

But moonwatchers won’t have to wait that long for another show: there will be a longer total lunar eclipse on Nov 8 next year, Nasa said.

Even better news for anyone wanting to watch is that no special equipment is necessary, unlike for solar eclipses. Binoculars, telescopes or the naked eye will give a decent view of the spectacle, as long as there is good weather here on Earth.

After it passes into the umbra – the full shadow – the whole process will go into reverse as the moon slithers out of the dark and carries on its endless journey around the planet.