A PARTIAL solar eclipse will be visible across the UK just before sunset today as the moon appears to take a 'bite' out of the sun.

The phenomenon will last roughly 40 minutes with the mid-point occurring at different times around the country.

For observers in Edinburgh, the peak of the eclipse will be at 7.58pm and 8.04pm in London today (August 21). Overcast skies will be a problem for many although the Met Office says there may be a break in the clouds in the south west.

The movement of the moon between the Earth and sun will produce a much more dramatic event in the US, where a total eclipse will turn day to night for two minutes.

Up to five solar eclipses occur each year, but each one is visible only within a limited band across the Earth's surface where the moon's shadow happens to fall.

In the US this year, 14 states will experience a total eclipse along a path stretching from the east to the west coast.

The Royal Astronomical Society warned anyone hoping to catch the phenomenon not to look directly at the sun.

A spokesman said: "Extreme care must be taken when observing the eclipse, because of the blinding brilliance of the sun.

"Never, ever look directly at the sun through binoculars or a telescope, for you will risk permanent eye damage.

"Even staring at the sun is dangerous, and sunglasses are no protection. Hospitals regularly see patients who have damaged their eyes while watching eclipses. Don't be among them."

Met Office forecaster Martin Bowles said: "It doesn't look very promising. It is only going to be about 4% of the sun which will be blotted out, so even if it is perfect weather conditions you won't see a lot.

"From a meteorological point of view it is not looking very good because of the cloud - most people won't be able to see a thing.

"There will be some breaks in the cloud in the south-west of the country - South Wales and south west England - there will be enough breaks that people who are looking specifically might be able to see a little chip out of the corner of the sun.

"Anywhere in the east, including London, won't see anything because it will just be clouded over; also Scotland and Northern Ireland."

Due to the partial eclipse occurring near sunset, there is unlikely to be an observable reduction in light, he added.

Millions of Americans are gathering along a stretch from Oregon to South Carolina to watch the spectacle - the first total solar eclipse to sweep coast to coast across the US in 99 years.

Southern-most Illinois will have the longest period of darkness at two minutes and 44 seconds. It is expected to be the most observed and most photographed eclipse in history.

How to view the partial eclipse safely

A partial solar eclipse can pose more risk to eyesight than a full one, specialists have warned.

British stargazers may think "they don't need the same protection as they would do during a total eclipse" and could suffer long-term retinal damage, the College of Optometrists said.

The leading optometry body has offered guidance to Britons wanting to watch the lunar event safely.

:: Do not look directly at the sun even if wearing sunglasses, which do not protect eyes enough;

:: Watching through telescopes, binoculars and cameras is risky and should be avoided;

:: TV or webcam broadcasts are a "reliable and safe alternative";

:: You can watch directly with specially designed solar filter glasses (with an appropriate CE mark);

:: Use the "pinhole projection method". This involves putting a hole in a piece of cardboard, and holding it up - with your back to the sun - so the sun's image is projected on to another piece of paper or card.

Daniel Hardiman-McCartney, of the College of Optometrists, said: "While there will be a full solar eclipse in the US, we will only have a partial eclipse in the UK, and in some ways this can be more dangerous as many people may feel that they don't need the same protection as they would do during a total eclipse.

"You should never look directly at the sun during a total or partial eclipse. This is because the radiation emitted by the sun is so powerful it may cause long-term harm to the retina."

The guidance was reiterated by Nasa, which warned: "It is never safe to look directly at the sun's rays - even if the sun is partly obscured."

The space agency is providing several live-streams of the lunar event through Facebook, Twitter and YouTube.