Nicole Bajic, an ophthalmologist at the Cleveland Clinic Cole Eye Institute in Ohio, who is among the eye doctors, specialists and eclipse experts advising enthusiastic viewers across the country on how they can make April’s total solar eclipse as safe – and as fun – as possible.

During a total solar eclipse, the moon appears to completely cover the sun, casting its shadow along its trajectory above the Earth. This year’s eclipse will be the second of its kind in the 21st century to touch the contiguous U.S. There won’t be another one until 2044.

Experts say anyone in northern Mexico, parts of Canada, or one of 15 U.S. states from which the eclipse can be seen on April 8 should also prepare.

Ordinary sunglasses won’t do that day. Anyone looking up to the sky should wear a pair of safe solar viewing glasses to protect from the sun’s ultraviolet radiation and infrared light, which can cause serious injury to vision and possibly blindness. Even a glance at the sun can be risky.

Even though most people in the U.S. will be able to see at least a partial solar eclipse that day, only those in the 115-mile-wide (185.07-kilometer-wide) path of totality – right under the moon’s shadow – will be able to take their glasses off for approximately four minutes briefly.

For specialists, safety is at the top of their minds. But understanding how the eyes absorb light and perceive color can help viewers enjoy a short-lived, possibly once-in-a-lifetime, stargazing event.

The eye will notice light dimming five to 10 minutes before the eclipse’s totality. Two minutes before the main event, red colors will fade and contrast with greens and blues in a biological phenomenon called the Purkinje effect.

Normally, as day transitions into night, the eyes adjust to the dark as rod cells become more active than cone cells. This period, called dark adaptation, can take anywhere between 30 minutes and two hours.

So the retina takes a while to adjust to the quick change. This part of the eye has two types of photosensitive cells that convert light into signals sent to the brain that allow humans to see the world. Cones are the receptors responsible for colorful vision in the daytime. Rods are the receptors that work in low-light conditions and allow for nighttime vision.

This effect, named after the Czech physiologist who discovered it, occurs as cones struggle to absorb light under the moon’s shadow and rods activate nighttime vision. This desaturates the colors the human eye perceives, decreasing the reds, slightly increasing the greens and blues and creating a gray hue.

Those who want to look at the sun as the moon partially or completely obscures it should secure a pair of safe solar glasses in advance. But eclipse viewers should also keep other things in mind, especially if they have eye conditions.

Often associated with eclipse viewing is a serious injury to the eye’s retina that can cause temporary or permanent blurry vision from just a few seconds of looking at the sun. Those at higher risk for this retinal burn, called solar retinopathy, include people taking medication like tetracycline for acne or rosacea.

On the flip side, Bajic said those with a history of cataracts might be less sensitive to the sun because their cloudy vision would technically act as a filter. But that’s not to say they shouldn’t take the additional and necessary step of protecting their eyesight with viewing glasses.

Those who prefer not to venture a peek can get creative with the easy pinhole projection method, which works best during a partial eclipse or when a total solar eclipse has yet to reach totality.

When sunlight passes through small holes –punched in an index card or cardboard box, on a pasta colander or straw hat, or through outstretched fingers on crossed hands – and lands on a flat surface, it projects the sun’s crescent shape in a grid pattern. Spaces between tree leaves can also create pinhole projections.

Importantly, this method of indirect viewing does not entail spectators looking directly at the sun through these holes but rather at its projections as they face away from the star. An Adler Planetarium blog post at adlerplanetarium.org/blog offers instructions on making a projector with a cardboard box.