Astronomy
The Use of the Tully-Fisher Relation to Measure Galaxy Distances
Quick fact
Brighter spiral galaxies rotate faster, and this simple relationship lets astronomers measure distances to galaxies billions of light-years away with an accuracy of about 10%.
Why this is interesting
You know how you can tell how far away a car is by how bright its headlights appear? Astronomers use a similar trick with entire galaxies—but first they have to figure out the bulb's true wattage.