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.

Full Earth from Apollo 17

The crew of Apollo 17 captured this iconic view of the full Earth as they coasted toward the Moon in December 1972.


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."


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June 1, 2009 at 11:34 am

I wonder what these 'solar tides' turn into? If it is building up, it must go somewhere, right?

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June 1, 2009 at 2:40 pm

not to mention that every time we use the planet to sling shot a space craft we also transfer momentum that we don't get back.

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Dennis Moore

June 1, 2009 at 4:35 pm

Is it possible that the ditance between Earth and Sun is increasing because as the matter in which the Sun is made burns, the Sun loses mass, therefore creating more distance between the outer crust of the Earth and the outer limits of the Sun.

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Peter Wilson

June 1, 2009 at 9:00 pm

...and the expansion is accelerating. See for complete explanation.

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June 3, 2009 at 8:45 am

It's a bit early to link an increase in Sun-Earth distance to dark energy, especially at such a small scale in such a tightly gravitationally bound system.

More likely it's some combination of decreasing Solar mass (mass loss due to fusion and solar wind, less infalling comets and other space junk) and tidal effects (similar to Earth-Moon). What would be interesting would be to see (if and) how much the Earth's orbital velocity is speeding up to put us in a more distant orbit. If we're not speeding up at all, it would have to be due to decreased solar mass. Although it would be difficult to directly measure Earth's velocity, we /can/ accurately measure the length of the year and fairly accurately measure the A.U.

It would be good to have the A.U. increasing, so that we don't heat up as much as the Sun's luminosity increases over time. Even so, it probably won't be enough to save us from serious overheating in the next billion years.

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Gerald Nordley

June 4, 2009 at 9:55 pm

A constantly expanding orbit is (on average) a spiral with a flight path angle and a radial velocity component, which can be constant, increasing or decreasing. Any outward force would be the dynamic equivalent of reduced gravity. A very slight azimuthal force (magnetic drag, tides, or relativistic frame drag, for instance) could also have constant velocity solutions as azimuthal acceleration is balanced by radial deceleration, though in that case the flight path angel would increase with time. So I’m afraid precise solar-frame total velocity measurements won’t necessarily solve the problem. Anyway, it might be interesting to send a few Lageos type retro-reflector balls into a deep space orbit for lidar measurements of Earth’s orbital velocity. I understand laser velocity measurements can get down to hundredth of a millimeter accuracy, and this is what would be necessary the velocity changes associated with a semimajor axis increase of a few centimeters per year.

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Thomas Hanson

June 7, 2009 at 6:38 am

I don't think the distance is related to the loss of mass of the sun.
If you were to look at it in reverse, then the addition of six inches a year over the lifespan of the sun so far would add up to about 400 million miles!
That would be a very large sun, and no earth.

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June 7, 2009 at 9:42 am

Wouldn't these tidal motions be slowing the orbit of the Earth around the Sun as well? If so there would be less inertial force keeping the two apart.

If you ask me it just makes sense that matter would spread further apart and increase more in speed the further it gets from it's source of origin.

Imagine a big tap in space whence all matter in the universe spews forth.

Say all matter emanates outward from this 4th dimensional tap evenly in all directions. That would mean that the concentration of matter would diminish in a exponential pattern as it travels further from it's source. As there would always be more pressure being exerted on the source side of the matter particles moving away from it, and less on the other, those particles would also be increasing their speed exponentially the further they get from the source.

There may be constants like for example, a gravitational constant in the omniverse, but I'd have to say they'd be on a sliding logarithmic scale of sorts.

I think this is sort of what you are trying to say Gerald, just that you seem to have a much better grasp of the scientific lingo.

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