As we have seen, testability is crucial for any scientific theory. However, as crucial as it might be, it is not enough to test a theory against the observations that led to its creation. A scientific theory must also be able to predict unknown facts that can only be confirmed by fresh observations. If any of the predictions are proven wrong, the theory must be deemed incorrect. This potential refutability is a powerful criterion that can help distinguish scientific from most non-scientific theories, as can be demonstrated by comparing astronomy with astrology.

For thousands of years astrologers had been using the apparent movement of the sun, moon, and the five known extraterrestrial planets (Mercury, Venus, Mars, Jupiter and Saturn) to foretell earthly events. Once the sixth planet, Uranus, was discovered in 1781, astrologers adjusted their charts to include the new planet and continued with their forecasting as before.

Astronomers, on the other hand, were fervidly striving to calculate the orbit of the new planet. Their calculations, however, did not match the actual path of Uranus. The discrepancy could be explained by assuming that either Newton’s theory of gravity was wrong, or that the gravity of an unknown heavenly object, farther from Uranus, was responsible for the deviation. Using the law of gravitation, astronomers calculated the assumed celestial position of the undiscovered planet. They confirmed their calculations in 1846 when, by means of a telescope, they managed to view Neptune at the predicted location.

Just as discrepancies in the orbit of Uranus had led to the search for Neptune, irregularities in the orbit of Neptune, and to a lesser extent in the orbits of Uranus and Saturn, had led scientists to suspect the existence of a ninth planet. Again, it was theoretical calculations that led to the discovery of Pluto in 1930. The theory of Gravity is therefore a scientific theory.