# Measuring How Many Days Are in a Year - Comments

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• Janus
In summary, this post discusses various ways to measure a year, including human observation of the moon, Earth's rotation rate, atomic clocks, and ways to measure a year using celestial bodies. Various issues with these methods are also discussed, such as the rotation rate of Earth and the need for fundamental constants with dimensions.
Think I’ve figured out how Wikipedia got 365.256363004 SI days for the Sidereal year?
Earths semi-major axis a = 1.4959974465490E+11 Metres.
Gravitational constant G = 6.67408E-11 Newton Metres squared / kilogram square
Suns mass M1 = 1.98855E+30kg
Earths mass M2 = 5.97237E+24kg
T = 2*PI()*SQRT(a^3/G*(M1+M2))
Orbital period T = 31558149.76354540 SI seconds

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