From early evening, the Moon will begin to move across the Sun. Across the UK roughly 90 to 96 per cent of the solar disc will eventually be obscured, making this the most significant eclipse visible from Britain and Ireland since 1999. In London, the eclipse begins at about 6.17pm, reaches its maximum at around 7.12pm and ends shortly after 8pm. Cornwall will experience closer to 95 per cent coverage.

It should be spectacular.

It could also generate a rather less celestial phenomenon: a queue of people wondering whether they have damaged their eyes.

That is why today’s eclipse matters to the NHS as well as to astronomers. The overwhelming majority of people will watch it safely and remember it for years. But history shows that a relatively small number will stare directly at the Sun, use inadequate protection, develop visual symptoms and turn to eye casualty, urgent care or emergency services.

Britain has been here before.

After the total solar eclipse of August 1999, around 70 people were reported to have developed visual problems associated with viewing the event. About 40 per cent had looked at the eclipse for less than a minute. More than half of those affected had used no eye protection, while others had relied on filters that were not adequate for direct solar viewing.

The impact was particularly visible at specialist eye services. Contemporary reporting recorded 220 people attending Moorfields’ accident and emergency department after the eclipse, while Birmingham Eye Hospital saw 64 people through special eclipse clinics.

A prospective Leicester study provides an even clearer picture of what happens when public anxiety meets a rare astronomical event. Forty-five people attended its eye casualty service after the 1999 eclipse. Twenty reported visual symptoms and five had visible retinal changes. Four were still experiencing symptoms seven months later.

That does not mean Britain should expect emergency departments to be overwhelmed tonight. Quite the opposite may be true.

A large US study examining emergency department attendances surrounding the 2024 total eclipse found 853 eye-injury visits in the week before the eclipse and 921 in the eclipse and post-eclipse period. The difference was not statistically significant. The researchers concluded that there was no measurable surge in emergency presentations for ocular pain or photokeratitis after the event, although they cautioned that their method might not capture every case of solar retinal injury.

That is encouraging because it suggests public information works.

It is also worth being precise about NHS 111. There is no comparable national published dataset I can find showing that previous British eclipses caused a specific spike in NHS 111 calls. The most important British comparison is the 1999 eclipse, which predates NHS 111 altogether. The service was not available nationally until years later. Nor is there good published evidence demonstrating that the 2015 partial eclipse produced a material national increase in either 111 demand or A&E attendance.

What we do have is much stronger evidence of increased presentations to specialist eye services after unsafe viewing.

That distinction matters.

Today’s NHS challenge is probably less about an avalanche of emergency demand and more about preventing unnecessary harm, giving people clear advice and making sure those with genuine symptoms know where to go.

The underlying injury is solar retinopathy.

Because retinal damage is not always painful, it is particularly dangerous.

During an eclipse the Sun looks less bright, making it psychologically easier to stare at it for longer. But enough intense visible and ultraviolet radiation can still reach the retina to injure its light-sensitive cells. The damage may occur without pain and symptoms can take hours to become apparent.

The symptoms are not especially subtle once they appear. Moorfields warns about blurred or distorted vision, difficulty reading, changes in colour perception, sudden changes to sight and new dark spots in the centre of the visual field. In some cases vision improves with time. In others, retinal damage can be permanent.

There is no clever trick that makes staring at a partial eclipse safe.

Ordinary sunglasses do not do it. Darker sunglasses do not do it. Cloud does not do it. And an unfiltered camera, telescope or binoculars can make things considerably more dangerous by concentrating sunlight.

The UK Health Security Agency recommends specially designed solar viewers meeting the ISO 12312-2 safety standard, or indirect viewing methods such as pinhole projectors, projecting the Sun through a colander or simply watching the projected crescents created through gaps between leaves.

The Royal Observatory Greenwich makes the same point. Regular sunglasses are inadequate, damaged eclipse glasses should not be used and ordinary telescopes must never be pointed at the Sun without the correct solar filter.

For parents, schools and families, that message deserves particular emphasis. Children are naturally curious and today’s eclipse will occur at exactly the sort of hour when families are together, outside and carrying phones. The temptation to take a quick look because the Sun appears almost covered will be enormous.

Almost covered is not safe.

Unlike people inside the path of totality in Iceland, Greenland and northern Spain, observers in Britain will see a partial eclipse throughout. There is therefore no period during today’s UK event when people should remove proper eye protection and stare directly at the Sun.

That is perhaps the most useful public-health message of the day.

Look up, certainly.

Just do it properly.

For the NHS, today’s event is also a small case study in prevention. A few weeks of simple communication, distributed through schools, hospitals, social media and the press, can prevent injuries that may otherwise result in urgent consultations and, occasionally, lasting visual impairment.

The American experience in 2024 suggests that good preparation can make a difference. Tens of millions of people watched one of the most widely anticipated eclipses in modern history without producing a statistically significant increase in the eye-related emergency attendances examined by researchers.

Britain should aim for the same result tonight.

The spectacle itself will last only a couple of hours. The memories should last considerably longer.

The retinal damage does not need to.