Seleucus even proved the heliocentric system, but it is not known what arguments he used. Mathematics Babylonian mathematical texts are plentiful and well edited. In respect of time they fall in two distinct groups: one from the First Babylonian dynasty period (1830–1531 BC), the other mainly Seleucid from the last three or four centuries BC. In respect of content there is scarcely any difference between the two groups of texts. Thus Babylonian mathematics remained stale in character and content, with very little progress or innovation, for nearly two millennia. The Babylonian system of mathematics was sexagesimal, or a base 60 numeral system. From this we derive the modern-day usage of 60 seconds in a minute, 60 minutes in an hour, and 360 (60 × 6) degrees in a circle. The Babylonians were able to make great advances in mathematics for two reasons. First, the number 60 has many divisors (2, 3, 4, 5, 6, 10, 12, 15, 20, and 30), making calculations easier. Additionally, unlike the Egyptians and Romans, the Babylonians had a true place-value system, where digits written in the left column represented larger values (much as in our base-ten system: 734 = 7×100 + 3×10 + 4×1). Among the Babylonians' mathematical accomplishments were the determination of the square root of two correctly to seven places (YBC 7289). They also demonstrated knowledge of the Pythagorean theorem well before Pythagoras, as evidenced by this tablet translated by Dennis Ramsey and dating to c. 1900 BC: 4 is the length and 5 is the diagonal. What is the breadth? Its size is not known. 4 times 4 is 16. And 5 times 5 is 25. You take 16 from 25 and there remains 9. What times what shall I take in order to get 9? 3 times 3 is 9. 3 is the breadth. The ner of 600 and the sar of 3600 were formed from the unit of 60, corresponding with a degree of the equator. Tablets of squares and cubes, calculated from 1 to 60, have been found at Senkera, and a people acquainted with the sun-dial, the clepsydra, the lever and the pulley, must have had no mean knowledge of mechanics. A crystal lens, turned on the lathe, was discovered by Austen Henry Layard at Nimrud along with glass vases bearing the name of Sargon; this could explain the excessive minuteness of some of the writing on the