Archimedes attempts to calculate how many grains of sand it would take to fill the Known Universe, which, while much smaller than the Universe of modern physics is much larger than most people would suppose. Ptolemy, whose geocentric system is much derided, was aware that the Earth is a sphere and that the distance to the "fixed" stars was so great that the Earth could be considered a mathematical point to all intents and purposes.
Archimedes is, in The Sand Reckoner, the inventor of a number system which can deal with what we now call astronomical numbers.
External link to full text: http://web.fccj.org/~ethall/archmede/sandreck.htm