Tests of General Relativity

Albert Einstein's general theory of relativity explained a lot when it was introduced in 1915, but there was little empirical evidence to suggest its validity at the time. In the 98 years since then extensive experimental testing has removed virtually all doubt about the practical implications of relativity. You can read more about that testing here:
Tests of general relativity

General relativity makes some trippy claims about the universe that, to say the least, are not intuitively obvious. The idea that gravity can bend and distort the color of light, slow the passage of time, and disturb the orbits of planets all by itself seems out of whack with our casual observations, but like any good scientific theory, relativity made specific quantified predictions about these odd phenomena. That left the door open to experimental verification.

In a sense, the first experimental test of general relativity was the motion of Mercury. As astrometry became more and more accurate during the 1800s astronomers noticed something odd about Mercury's orbit. Every century the location of Mercury's closest approach to the Sun (or perihelion) backtracks by about 43 arc-seconds more than it should according to the laws of Newton. The reason for this, Einstein explained, is that as a planet approaches its parent star and speeds up, its mass increases and time slows down just a little bit relative to the rest of the cosmos. This action combines to alter the orbit a hair each revolution, and since Merucry's orbit is closer to the Sun and more elliptical than the other planets, the effect is most noticeable there. Right out the gate, relativity looked good, but the error bars were large in those days before radar, and physicists wanted more evidence of the machinery of relativity in action.

Since the Sun contains almost 99% of the solar system's mass in a circle of the sky less than half a degree wide, it's the most obvious choice for looking for gravity's bending effect on light. Typically looking at stars within a few degrees of the Sun is problematic due to the intense glow of sunshine, but as luck would have it a new Moon is almost exactly the same apparent size as the Sun when viewed from Earth's surface, and makes a natural sunshade for a few minutes a time or two a year if you know where to look for it. In 1919 simultaneous observations of a solar eclipse from Brazil and Sao Tome and Principe showed that gravity seemed to do just what Einstein thought it did. The results were sensational, but also ambiguous due to the possibility of systematic error, and it wasn't until the 1950s that eclipse observations were widely accepted as a verification of relativity.

Over the years the tests have become more specific to narrow down the range of uncertainty on the empirical factors that remain in our picture of physics. Astronauts have placed lasers on the Moon to more closely track its position, extraordinarily sensitive space telescopes have observed the gravitational twinkling of stars, and ever-more-accurate clocks have shown the quirky relationship between speed, gravity, and time. It's an odd universe we live in, but one remarkably amenable to understanding.