The smallest of the solar system’s major planets is hiding in the Sun’s glare right now. That’s because Mercury is at superior conjunction — it lines up behind the Sun as seen from Earth. As it moves away from the Sun, it’ll climb into decent view in the early evening sky in a couple of weeks.

Mercury’s motion around the Sun played a big role in the development of Albert Einstein’s theory of gravity, known as General Relativity.

Mercury’s path around the Sun is lopsided, so the little planet’s distance from the Sun varies by quite a bit. Astronomers plotted that path, and used the laws of gravity devised by Isaac Newton to predict where Mercury should appear in the future. But they discovered that each time Mercury comes closest to the Sun, it’s a little farther along in its orbit than predicted.

Einstein saw gravity not as a force — something that pulls two objects together — but as a warp in space caused a massive object — in this case, the Sun. The closer to the Sun you get, the more pronounced that warp becomes. So each time Mercury comes closest to the Sun, it gets pulled forward a bit by the warp in space.

Mercury’s orbit was the first bit of tangible evidence in support of General Relativity. In fact, Einstein discussed it in his early papers on the subject. Since then, much more evidence has confirmed Einstein’s ideas — showing us that, for now, General Relativity is the best explanation of gravity.

Script by Damond Benningfield