When the sun is very low (nearly touching or even partly below the horizon), it’s typically shining through such a large amount of atmosphere that the sunlight is significantly weakened by the time it reaches your eyes. This isn’t always true though, for ex if the air is unusually dry, clear, or thin (such as near the poles). Good rule of thumb is that if it looks red rather than yellow or white, it’s likely safe to look at for at least a few seconds.
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