Skywatchers have been trying to gauge the Sun-Earth distance for thousands of years. In the 3rd century B.C., Aristarchus of Samos, notable as the first to argue for a heliocentric solar system, estimated the Sun to be 20 times farther away than the Moon. It wasn't his best work, as the real factor is more like 400.
By the late 20th century, astronomers had a much better grip on this fundamental cosmic metric — what came to be called the astronomical unit. In fact, thanks to radar beams pinging off various solar-system bodies and to tracking of interplanetary spacecraft, the Sun-Earth distance has been pegged with remarkable accuracy. The current value stands at 149,597,870.696 km — with an uncertainty of just 0.1 meter (4 inches).
Having such a precise yardstick allowed Russian dynamicists Gregoriy A. Krasinsky and Victor A. Brumberg to calculate, in 2004, that the Sun and Earth are gradually moving apart. It's not much — just 15 cm (6 inches) per year — but since that's 100 times greater than the measurement error, something must really be pushing Earth outward. But what?
One idea is that the Sun is losing enough mass, via fusion and the solar wind, to gradually be losing its gravitational grip. Other possible explanations include a change in the gravitational constant G, the effects of cosmic expansion, and even the influence of dark matter. None have proved satisfactory.
But Takaho Miura and three colleagues think they have the answer. In an article submitted to the European journal Astronomy & Astrophysics, they argue that the Sun and Earth are literally pushing each other away due to their tidal interaction.
It's the same process that's gradually driving the Moon's orbit outward: Tides raised by the Moon in our oceans are gradually transferring Earth's rotational energy to lunar motion. As a consequence, each year the Moon's orbit expands by about 4 cm and Earth's rotation slows by 0.000017 second.
Likewise, Miura's team assumes that our planet's mass is raising a tiny but sustained tidal bulge in the Sun. They calculate that, thanks to Earth, the Sun's rotation rate is slowing by 3 milliseconds per century (0.00003 second per year). In other words, as an answer to the question "Why is the a.u. increasing?", the four researchers conclude it's "because the Sun is losing its angular momentum."