Why don’t we see galaxies rotating?

Christopher S. Baird

Humans do not notice the rotational motion of galaxies because galaxies are so unimaginably large. Galaxies are so large it takes them a long time to complete one rotation. For instance, our galaxy takes about 240 million years to complete one full rotation at the location of our Sun. The rotation rate of a galaxy is extremely slow. Even though the stars in our galaxy are travelling at about 500,000 miles [804,672 kilometres] per hour relative to the centre of our galaxy, the galaxy is so large that it still takes hundreds of millions of years to complete a lap. Humans have eyes and brains that are built to notice motion involving changes in position and orientation that happen on the scales of milliseconds to hours, not years, and definitely…

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All About Space (Digital) - 1 Issue, No. 161

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