# Naked Eye Visibility of Mars.

by Solon
Tags: mars, naked, visibility
 P: 25 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?
Mentor
P: 21,674
Welcome to PF!
 Quote by Solon 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.
"Resolution" is being able to tell two objects apart or being able to see the size of one object. It isn't required for viewing a point object, such as a planet or start. Only brightness matters in visibility.
 but why is it so apparently large?
Point objects that are bright can appear large due to blooming or saturation. Basically, you're seeing extra glare and atmospheric refraction that isn't exactly along the line of sight of the planet, making for a larger apparent diameter.
P: 25
 Only brightness matters in visibility.
Well that's where I can't understand this model, as the tiny looking Sun seen from the surface of Mars surely is not lighting Mars to such a brightness that it can be seen so easily from Earth? I'd really like to see some figures, my results fall far short, in fact Mars would only be visible by eye out to about 300,000 miles. Or less.

 Point objects that are bright can appear large due to blooming or saturation.
Fair enough, but Mars is by no means bright by my understanding, that's why I am here, cap in hand, wondering if anyone can provide an end-to-end explanation of the mechanisms, formulas and calculations required to quell the doubts I'm having. I've asked around, no satisfactory responses so far, maybe all the really smart folk hang out on PF?

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## Naked Eye Visibility of Mars.

 Quote by Solon Well that's where I can't understand this model, as the tiny looking Sun seen from the surface of Mars surely is not lighting Mars to such a brightness that it can be seen so easily from Earth? I'd really like to see some figures, my results fall far short, in fact Mars would only be visible by eye out to about 300,000 miles. Or less.
It would help if we knew how you were calculating this figure so we can correct where you've gone wrong.
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P: 1,958
 Quote by Solon Well that's where I can't understand this model, as the tiny looking Sun seen from the surface of Mars surely is not lighting Mars to such a brightness that it can be seen so easily from Earth? I'd really like to see some figures, my results fall far short, in fact Mars would only be visible by eye out to about 300,000 miles. Or less.

how on earth did you come up with that figure ??

do you realise how big Mars is, say compared to the Moon ?
Do you realise the Moon is ~ 240,000 miles distant from the earth ?
then consider how huge and bright Mars would be in our sky if it was in the same position as the moon

Dave
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 Quote by drakkith it would help if we knew how you were calculating this figure so we can correct where you've gone wrong.
snap :)

d
 Mentor P: 21,674 To do the calc, I'd start with the moon's magnitude, then ratio the square of the earth-sun to mars-sun distances and earth-moon and earth-mars distances. Edit: using that method with the numbers pulled out of the air, I get a mag of -1.5, which sounds pretty reasonable. So let's see how your calc compares.
P: 25