A total solar eclipse is a celestial event where the Moon passes directly between the Sun and Earth, completely obscuring the Sun’s bright face. This rare alignment casts a shadow on our planet, transforming a daytime sky into a fleeting twilight. The visual splendor of this event offers a unique opportunity to view the Sun’s outer atmosphere, the corona. Viewing an eclipse presents a significant safety concern because of the intense solar radiation.
Viewing Safely During Totality
Directly looking at the Sun is only safe during the brief period of totality, and only if you are within the narrow path of the Moon’s darkest shadow. Totality begins when the Moon fully covers the Sun’s bright surface (photosphere), requiring the removal of certified eye protection. During this short window, which may last from seconds to a few minutes, the Sun’s faint corona becomes visible.
The unaided eye can safely observe the corona, which is millions of times dimmer than the Sun’s disk. Viewers must know the exact predicted start and end times of totality for their specific location. The critical moment to stop direct viewing is marked by the reappearance of the Sun’s intense light, known as the “diamond ring” effect. This sudden flash of sunlight means totality has ended, and protective filters must be immediately placed back over the eyes to prevent injury.
Understanding Retinal Damage During Partial Phases
The primary danger during the partial phases of an eclipse, before and after totality, is solar retinopathy, a condition resulting from photochemical injury to the eye’s retina. This damage occurs when intense light focuses on the fovea, the center of the macula responsible for sharp central vision. The retina lacks pain receptors, meaning a person will not feel the damage occurring in real-time, leading to a false sense of security.
Even when the Sun is 99% obscured, the remaining crescent emits enough focused light to cause injury. The paradox of an eclipse is that as the sky darkens during the partial phases, the eye’s pupil dilates to let in more light. This wider opening allows a greater amount of the Sun’s intense, unfiltered light to reach and damage the retinal tissue. Damage can range from temporary vision loss to permanent central blind spots or distorted vision. Standard sunglasses, regardless of darkness, are not adequate protection, nor are homemade filters like smoked glass or photographic film.
Certified Eye Protection Requirements
Safe viewing of the partial phases requires specialized solar filters that meet an international standard. All certified eclipse glasses or handheld viewers must comply with the ISO 12312-2 specification for filters intended for direct solar observation. This standard ensures the filter reduces the Sun’s visible, ultraviolet (UV), and infrared (IR) radiation to safe transmission levels.
Before using any filter, inspect it thoroughly for scratches, tears, or damage, as even a small defect can compromise safety. The market may contain counterfeit products, so viewers should only obtain filters from reputable manufacturers and vendors. For those without certified viewers, an alternative is using indirect methods, such as a pinhole projector, which safely projects an image of the Sun onto a surface without requiring direct eye contact.