What Is Solar Diameter?
The astronomical measure of the angular size of the sun as seen from the Earth. It refers to the angle subtended by the sun's disk at the observer's location.
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Angular Size
The average angular diameter of the sun is approximately 32 arcminutes (or about half a degree). This is roughly the same angular size as the Moon.
Visual Effect
The fact that the sun and moon have nearly the same angular diameter is what allows for total solar eclipses, where the moon perfectly obscures the sun's disk.
Relevance to Retinopathy
When a person looks at the sun, the entire solar disk (32 arcminutes) is focused onto the macula, covering an area large enough to cause severe, focused thermal damage.
Does it change?
Yes. Since the Earth's orbit is elliptical, the angular diameter of the sun appears slightly larger in January (perihelion) and slightly smaller in July (aphelion).
How does the eye focus this size?
The cornea and lens of the eye act as a magnifying system, concentrating the sun's image onto the retina.
Can I cover the sun with my thumb?
Yes. Held at arm's length, your thumb will subtend approximately 1.5 to 2 degrees of visual angle, which is large enough to completely cover the sun (0.5 degrees).