4 Astronomical tests
The inverse-square law can be tested by carefully measuring the orbit of the Moon around the Earth. It takes 2.6 s for a laser beam (green dashed line) to travel from the Earth (blue) to a mirror on the Moon (grey) and back. Precision measurements of this time over a 30-year period have characterized the shape of the Moon's orbit with an accuracy of about 4 mm. These measurements show that major axis of the orbit precesses by 19 milliarcseconds (φ) per year, which is in agreement with the predictions of general relativity (within errors). A violation of the inverse-square law at the 108 m length scale would appear as a deviation from the prediction. The precession of the axis is exaggerated in this figure.