How fast is the Milky Way rotating?
Our galaxy is rotating its own axis at a speed of 864,000 km/h, corresponding to the Solar System taking about 220 million years to complete one rotation, also known as a galactic year. In its 4.6 billion years of existence, the Solar System has only rotated the centre of the Milky Way 21 times. So, Earth is 21 galactic years old.
Astronomers primarily use radio telescopes to determine the Milky Way’s rotation. Heavenly bodies emit radio signals, and by studying the signals of specific heavenly bodies over a number of years, scientists can see how different parts of the galaxy move as compared to us. Subsequently, astronomers can calculate how fast the galaxy itself is rotating.