Image file size of the Earth

TL;DR The file size of a 72ppi PNG image of Earth's surface is 3.37 zettabytes, or 3,705,354,190,000 gigabytes.

	Assuming image resolution:
	72ppi = 5184 pixels per square inch
	Surface area of the Earth:
	510,072,000 km2 = (7.90613181 × 10 to the 17th) square inches
	Assuming (for a lossless PNG) an average storage size of:
	~7.76 bits per pixel
	7.90613181 × 10 to the 17th square inches
	* 5184 pixels per square inch
	_______________________________
	4.09853873 × 10 to the 21st pixels
	4.09853873 × 10 to the 21st pixels
	* ~7.76 bits
	_______________________________
	3.36744942 zettabytes
	((7.90613181 × 10 to the 17th) * 5184) * 7.76 bits
	3.36744942 zettabytes
	or
	3.70254979 × 10 to the 12th gigabytes
	or
	3,702,549,790,000 gigabytes
	Note:
	I realize that ppi is somewhat contentious. I only chose
	the 72ppi figure because it was convenient-ish. According
	to Wikipedia, photographs should be printed at 300 ppi to
	look decent.
	http://en.wikipedia.org/wiki/Pixel_density
	In that case, a 300ppi image will weigh in at:
	58.5 zettabytes

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