Calculating the reflected power from solar radiation on the moon

[mell0r]

Question: The Moon has an albedo of 0.05. What is the total power of the solar radiation reflected from the Moon in units of 10^15 W?

My Answer: - To get my answer i multiplied the flux density of the sun measured on the moon, then multiplied it by half of the moon's total surface area, followed by the multiplication of the albedo value.

However, the mark scheme reads: 'Flux density of the Sun as seen on Moon times the cross-sectional area of the moon gives the total power of the solar radiation incident on the Moon. Multiplied by the albedo gives you the total power reflected.'

Could anybody explain why i should multiply by the cross-sectional area rather than half of the moon's surface area?

Cheers in advance!

 

[tiny-tim]

hi mell0r!

Could anybody explain why i should multiply by the cross-sectional area rather than half of the moon's surface area?

because the total sunlight falling on the moon is the same as that falling perpendicularly onto a flat disc of the same diameter as the moon

 

[mell0r]

because the total sunlight falling on the moon is the same as that falling perpendicularly onto a flat disc of the same diameter as the moon

Thanks for such a sudden reply! It seems so obvious now, but my mind would just not accept it at the time haha... Thanks to the help of a handy ping pong ball lying around the house it's overcome it though now