How Big is the Sun, Really?

With all the attention that astronomers have lavished on old Sol over the centuries, you'd think that by now they'd know its diameter to, oh, 10 or 12 significant digits.

Nope. While the Sun's girth has indeed been measured dozens of times over the past 40 years, the results haven't converged on a single value and scatter by as much as ± 0.1%. One big reason is that, though some measurement techniques are extremely precise, their accuracy suffers because of the turbulence induced by Earth's atmosphere. Most often astronomers use a compromise value of 865,000 miles (1,392,000 km).

So Marcelo Emilio (State university of Ponta Grossa, Brazil) has teamed with observers at the University of Hawaii and Stanford to approach this measurement with, literally, space-age techniques. They used images taken by the Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory (SOHO), homing in on transits of the planet Mercury across the solar disk in 2003 and 2006.

This makes perfect sense. The spacecraft sits at the L₁ Lagrange point, a million miles from Earth, and Mercury has nothing but the barest wisps of atmosphere — a made-to-order combination for crisp images. (The SOHO team has posted a movie of 2006 transit).

However, as the team describes in a forthcoming issue of Astrophysical Journal, the technique wasn't without its complications.

For example, the MDI takes images that are 1,024 pixels on a side — state of the art when the spacecraft was built during the 1990s, but still yielding a modest resolution of 4 arcseconds in its full-Sun imaging mode. That's a bit less than half of Mercury's diameter during each transit. Also picking the instrument's focus point also affects the crispness of the planet's silhouette against the solar disk. And then there's the matter of where, exactly, to place the "edge" of the of the Sun's photosphere.

Taking all these factors into consideration, Emilio and his collaborators peg the diameter of the Sun at 865,374 miles (1,392,684 km), plus or minus about 0.01%. And they got the same solar size during both transits, confirming (as this same group reported two years ago) that the Sun's diameter is "rock steady."

It should be possible for researchers to improve on this new value when Venus transits the Sun in just a couple of months (76 days to be exact). This time astronomers can use the Solar Dynamics Observatory, which can take images with four times better resolution than SOHO. "SDO will be able to see the transit of Venus in June and we have several activities planned," according to project scientist Dean Pesnell. In fact, the SDO team has launched its own transit website, though it's still under development.