# How do you calculate the Milky Way’s galactic year? [closed]

The Solar system moves at a speed of 220 km / s around the galaxy. It’s about 27,000 light years from the Galactic Centre. How long does it take for the solar system to orbit around the Milky Way?

I first calculated the circumference = $\mathrm{2 \times 3.14 \times 27,000 \times 9.4605284 \times 10^{15}\ m}$

Then, I divided the circumference by 220 km/s. However, I get $7.2 \times 10^8$

Can someone give me idea what I did wrong?

## closed as too localized by akhmeteli, Brandon Enright, Michael Brown, Emilio Pisanty, twistor59Jun 7 '13 at 16:12

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• Speed is 230 Km/s, not 220. – Schrödinger's Cat May 27 '13 at 8:34
• In circumference calculation, its 10 raised to the power 15, not 5. – Schrödinger's Cat May 27 '13 at 8:37

You've got the right idea, but, as pointed out in the comments, you've punched in the number for light years incorrectly. One light year is $9.461\times10^{15}$ meters. Also, you might not be converting km/s into m/s. Remember, all the units for a given dimension must be the same.
• Units, units, units. $1.604\times10^{21}$ what's? And the answer you get? What unit is that in? You should find that the only missing ingredient is a conversion from one unit of time to another unit of time. – Warrick May 27 '13 at 13:11