In 630 AD, Brahmagupta, an Indian mathematician was 32 years old.  Brahmagupta was head of  the astronomical observatory in Ujjain.  He just finished completed "Brahma- sphuta- siddhanta" ("The Opening of the Universe") two years earlier.  This book states the belief in a static Earth and approximates the length of a year as being 365 days 6 hours 5 minutes 19 seconds.  Later he wrote "Khandakhadyaka" which also deals with astronomy and mathematics.  In astronomy he worked on solar and lunar eclipses, planetary conjunctions and positions of the planets (as well as the length of a year).  He also made advancements in algebraic notation and in areas of trigonometric shape.  He came up with an almost modern version of the quadratic equation, which is able to take negative
values.  He used symbols for identifying an unknown variable (usually an initial letter of a color).
Sometimes he even used more than one unknown in a single problem.  In areas of trigonometry, he
determined formulas for a cyclic quadrilateral and the lengths of diagonals in terms of the sides.  He
also studied theorems on volumes and surfaces.

Author: Charles DeBoer

References:
http://www-groups.dcs.st-and.ac.uk/~history/Mathematical_MacTutor.html

http://aleph0.clarku.edu/~djoyce/mathhist/chronology.html#toc