An annular eclipse, whilst lacking the magnificence of totality, is still an impressive sight with its 'ring of fire'. Image: Francisco Diego.

There is a rare treat for US observers on 20 May 2012 when an annular eclipse is visible from parts of seven western US states, with a partial eclipse visible across much of North America, except for areas near and along the Atlantic coast. Although lacking the sheer magnificence of a total solar eclipse, without the Sun's corona, chromosphere and prominences being visible, the 'ring of fire' is still an amazing sight and well worth making a special effort to see.

Because the Moon's orbit around the Earth is not a perfect circle but an oval or elliptical in shape, the Earth-Moon distance varies by 13 percent. When the Moon is on the far side of its orbit, it appears slightly smaller than the Sun and therefore doesn't completely cover the solar disc, leaving a ring of bright sunlight around the Moon when at its 'new' phase and we are denied the 'awesome totality'.

This annular eclipse has a 240-300 kilometre wide track and begins in China at 22:06 UT on 20 May. The shadow sweeps eastwards across Japan with Tokyo being the first major city lying under the track and close to the centre line at that. The point of greatest annular eclipse occurs over the Bering Sea, where the duration is five minutes and forty-six seconds and the annular track finally hits North America on the southern Oregon and northern Californian coast at 1:23am UT, the late afternoon on 20 May, local time. Unfortunately the Sun has already sunk to 20 degrees above the horizon at this time. In Redding, California, 30 kilometres south of the centre line the annular phase starts on 20 May at 6:26pm PDT (1:26am UT on 20 May) and lasts for four and a half minutes. The track of annularity then crosses central Nevada (Reno, 6:31pm PDT, 17 degrees altitude), southern Utah, northern Arizona and central New Mexico (Albuquerque, 7:36pm MDT, altitude five degrees). The annular track finally ends in Texas.

If you are going to view this eclipse then special eye protection will be needed at all times. Unlike total eclipses, at no time is it safe to view this eclipse with unprotected eyes. The safest method is always to project the solar image onto a piece of white card or a wall through a small telescope or binoculars with one lens covered. Do not be tempted to ignore these essential safety guidelines otherwise you could risk blindness, even though the Sun will be low to the horizon near the end of the track.