I have been trying to figure out why I can see Mars, by eye, even when it is very close to a bright, almost full Moon. Firstly, Mars at its closest to Earth only has an angular diameter of less than 1/2 an arc minute, while the accepted limit of resolution of the eye is 1 minute. That equates, by my recconing, to seeing a basketball at about 2,000 meters. With a self luminous body such as a star, I am told it is due to luminosity, but Mars is only reflecting the Suns light. The apparent diameter of Mars, using Celestia, is over 4 minutes, which explains why it is so easily visible, but why is it so apparently large? Mars has an albedo not much greater than the Moon, averaging about .15, and I have used a Lambertian model of reflection. Total solar irradiance at Mars averages less than half of Earth (and the Moon) value. Where I have run into difficulties, and can find nobody so far to calculate, is how many photons per second would reach an observer on Earth, and how many per second are required for them to consciously register. I have read that the eye may have single photon sensitivity, but between 150 and 500,000 photon/sec are needed for us to become aware of. Just wondering if I am missing something obvious here, or if we really don't understand light, vision and perception well enough to be able to fully explain the process?