My Country has Oldest Astronomical Book in the World.
Soorya Siddhant. (At least 2500 years old)
This book contains knowledge which was given to Asur Maya by Lord Sooryadev in Tretayug. The present Soorya Siddhanta may nevertheless be considered a direct descendant of the text available to Varahamihira. (who lived between 505–587 CE)
Now if we calculate the size of a year by Soorya Siddhant, it is 365.2421756 days, which is only 1.4 seconds shorter than the modern value of 365.2421904 days !
Similarly if we calculate solar/lunar eclipse as per Soorya Siddhant, it's at par with the current time by 99.8%.
Soorya Siddhanta also estimates the diameters of the planets. The estimate for the diameter of Mercury is 3,008 miles, an error of less than 1% from the currently accepted diameter of 3,032 miles.
It also estimates the diameter of Saturn as 73,882 miles, which again has an error of less than 1% from the currently accepted diameter of 74,580.
Surya Siddhanta also contains the roots of Trigonometry.
It uses sine (jya), cosine (kojya or “perpendicular sine”) and inverse sine (otkram jya) for the first time, and also contains the earliest use of the tangent and secant when discussing the shadow cast by a gnomon in verses 21–22 of Chapter 3:
Of [the sun’s meridian zenith distance] find the jya(“base sine”) and kojya (cosine or “perpendicular sine”). If then the jya and radius be multiplied respectively by the measure of the gnomon in digits, and divided by the kojya, the results are the shadow and hypotenuse at mid-day.
In modern notation, this gives the shadow of the gnomon at midday as :
Even today most of the authentic astrologers in India use Surya Siddhanta as base to compute their Panchangs (Almanacs) in many languages.
Such is the knowledge in the ancient Bharat!
India is known as Vishwa Guru for some reason I believe!