What will you be doing in a year, April 8, 2024, at 3:27 p.m. ? Admittedly, this is difficult to determine, so far in advance. But at this precise moment, a total solar eclipse will occur in Montreal and its South Shore – a rare phenomenon.
Indeed, if a partial solar eclipse can be relatively frequent, the total disappearance of the Sun, hidden by the Moon, only occurs when the latter is closer to our planet or the Sun is farthest from it. It’s a matter of the size of the Moon compared to the size of the Sun. When the two stars are perfectly aligned, this creates a shadow cone. People who live on Earth, inside this narrow band, can then admire the unique spectacle of the total eclipse.
On average, such alignment does not occur that every 375 yearsbut this may vary. Thus, the last total eclipse to affect Montreal occurred on August 31, 1932. But other regions in Canada are not as blessed! In St. John’s (Newfoundland), the last total eclipse dates from February 3, 1440 and the next will not take place until July 17, 2205, a wait of 765 years! The record belongs to the city of Regina, Saskatchewan, which was exposed to a total eclipse in 54 BC. AD, and will not be again until October 17, 2153, a period of 2207 years!
So don’t miss the total eclipse of Montreal in 2024. Otherwise, you’ll have to go in Calgary for the next one, in 20 years.
However, this phenomenon is accompanied by significant risks in terms of eye health. As an optometrist, I am very concerned about issues related to eye health. I certainly wouldn’t want anyone to go blind after watching a solar eclipse without proper protection!
Observe, but protect yourself
Observing a total solar eclipse is always fascinating. During the phase when the Moon completely obstructs the Sun, daylight gives way to a deep twilight sky. The outer atmosphere of the Sun (called the solar corona) appears gradually, shining like a halo around the Moon facing it. Bright stars and planets become more visible in the sky.
In daylight, the Sun usually emits intense visible light, so intense that we cannot stare at it directly for very long. If ever our eye looks directly at the Sun, we immediately have the reflex to turn away average after 0.25 sec). This reflex constitutes a natural protection for ocular health against the harmful rays of the solar star, some of which are not visible. We are talking here about ultraviolet and infrared.
Ultraviolet (UV) radiation
UVs count for 7% of solar radiation. They are partly absorbed by the cornea (clear part, in front of the eye) and the crystalline lens (natural lens inside the eye), without causing any damage, unless the exposure is too important.
In such cases, depending on the amount of UV absorbed, the cornea may develop inflammation, called keratitis. For its part, the lens will lose its transparency – this is a cataract. Other impacts are also to be expected, such as the development of small cysts (pinguecula) on the conjunctiva (white of the eye) or a membrane invading the cornea (pterygium).
The eyelids can also develop skin cancers. The upper eyelid, whose exterior is usually little exposed when our eyes are open, is particularly at risk when we lie on the beach, eyes closed, without protection. Finally, UV rays predispose to macular degeneration, an attack on our best retinal cells, which can result in a more or less significant loss of vision.
These pathologies develop by direct radiation, but also when the sun’s rays are strongly reflected by surfaces such as snow (snow blindness), sand, or water. This is why it is recommended to wear protective glasses that cut off all UV rays (UV400 protection), when exposure to the Sun for more than a few minutes is envisaged. As much for children as for adults, the frame must envelop the eyes well, so as not to let rays pass from the side or from above.
Infrared radiation (IR)
IRs make up the majority of the rays emitted by the Sun, i.e. 54%. We feel its effects because it is thermal radiation, which is accompanied by heat.
If the cornea (burn) and the lens (cataract) can also be affected by IRs, it is more the retina that can suffer from inappropriate exposure to them. Again, it’s about intensity and duration. As with UVs, the more intense the radiation, the more exposure to the rays will cause permanent damage in a short period of time.
Damage to the retina by IRs destroys the cells that allow us to see and ultimately creates a scotoma, a permanent black spot in our field of vision. It is a cause of blindness.
Eclipse and radiation
When the Sun is only partially hidden (partial eclipse), UV and IR radiation are as important as in full Sun. However, the brightness being reduced, our natural reflex is no longer exercised. It may then seem more comfortable to observe the Sun for several seconds or even minutes. Without protection, this type of exposure can lead to the pathologies described above, and contribute to blindness, if the central retina is affected. That’s what the President Trump was recalled in 2017as he watched a partial eclipse unprotected, putting his vision at risk.
During a total eclipse, on the other hand, it is possible, during the short duration of the total obstruction of the Sun (1 min 37 sec), to look at the solar corona without protection. It is then necessary to be very vigilant and to think of putting the protection back in place as soon as the Moon begins to move and that the radiation is again present, although the ambient luminosity is still reduced.
The same precautions must be exercised if one looks directly at the eclipse through binoculars, a telescope, a camera or another optical means. For example, don’t look at your phone screen with the naked eye while trying to take pictures of the eclipse! The rays are not blocked by these instruments and can cause significant eye damage.
A matter of protection
What protection are we talking about? These are solar filters that can be mounted in glasses or even in temporary glasses, made of cardboard, but perfectly covering the entire surface of the eye. Again, care should be taken to avoid leaving a space between the eye and the protective screen through which harmful radiation could enter. Authorized filters must meet the ISO-12312-2 standard.
Before wearing such filters, be sure to follow the instructions provided with this material. It is very important for parents to ensure that children wear the filters properly and do not play with them. When the observation is finished, they should not be removed while continuing to look at the Sun: look away, face the Sun and remove the filters. Then, don’t look up to the sky!
Corneal and retinal damage may appear hours after exposure; they are not always immediate. If you have ever been exposed, inadvertently or carelessly, watch your vision for hours after the eclipse. Any blurring or change in your vision should prompt you to see an optometrist or ophthalmologist as soon as possible.
Several activities will surround the arrival of the total eclipse. To make the most of this unique event, watch for announcements from organizations such as Space for Life, institutions such as the University of Montreal, or your local astronomy clubs. These organizations will distribute information, can provide protective glasses/filters and, above all, will allow you to better understand the phenomenon.
It’s a date in a year! But in the meantime, young and old, from now on, let’s protect our eyes properly.