Astronomers should beware of the danger to their eyes if they try to catch a glimpse of a comet which won't be seen on earth again until the year 112,000.
The C/2011 L4, also known as Comet Pan-Starrs, is making a rare visit to the inner solar system in the middle of this week, and will fly past the sun.
But experts have warned that amateur stargazers not to try and see it too soon.
Rev Kate Kay, chief astronomer at the Norman Lockyer Observatory in Sidmouth, Devon, said: "It's going around the sun, and there is always a danger that if you try to look at it when it is near the sun, you are going to do some damage to your eyes.
"We're hoping to get our first glimpses and photographs of it on Tuesday and Wednesday when it's away from the sun, and near the new moon. If people look then for the moon, just to the right of it, they may see a little white smudge, hopefully with a nice tail.
"With any luck this bad weather may have improved by then, allowing young people with stronger eyes to see it, or older people with binoculars."
The comet takes its name from the Hawaiian telescope which astronomers used to spot it in June 2011.
Members of the Flamsteed Astronomy Society, part of the Royal Observatory, planned to look for the comet from Blackheath in south east London on Friday, or Saturday if the weather is bad on Friday.
Joint vice chairman Grey Lipley said: "If you are going to try to find the comet while it is near the sun, you should wait until the sun is fully set. But it is going to be difficult to see it given the poor weather at the moment."