Overall, the
Earth isn’t even spiraling in toward the Sun; it’s spiraling outward, away from it. So are all the planets of the Solar System. With every year that goes by, we find ourselves just slightly — 1.5 centimeters, or 0.00000000001% the Earth-Sun distance — farther away from the Sun than the year before.
If the pull had remained unchanged, there would be a very, very slow inward spiral due to the effects of friction, collisions, and gravitational radiation. But with the changes we actually experience, the Earth, like all planets, is compelled to slowly drift away and spiral outwards from the Sun. Although the effect is small, this change of 1.5 centimeters per year is easily calculable and is unambiguous.
What we haven’t been able to do is measure that change in distance directly, however. We know it must occur; we know what its magnitude must be; we know that we’re spiraling away from the Sun; we know that this is happening to all the planets.
But what we’d love to do is measure it, directly, as yet another test of the laws of physics as we know them.
