Admit it: At some point you've probably thrust the tip of your index finger (2° across, more or less) skyward to cover up the Sun's disk (½°, more or less). Isn't it amazing how a simple body part can momentarily obliterate a star nearly a million miles across?
Using gadgetry far more sophisticated than a fingertip, astronomers have been measuring the Sun's diameter ever since the 17th century — and they've gotten very good at it.
So has our star changed size in all that time? Hard to tell. You'll get a different result depending on the technique used, the wavelength of light used, and what exactly you define as the Sun's "edge."
A new result, announced this week by a team of solar physicists, tries to sidestep all of these vagaries by using only measurements taken by a single instrument over 12 years simply to see if the Sun's diameter has changed — and the answer, apparently, is "no."
Jeff Kuhn (University of Hawaii) and three colleagues find that the Sun's mean diameter hasn't changed more than one part in a million since 1998. And what makes them so sure? They've sifted through some 500,000 frames taken by the Michelson Doppler Imager aboard the Solar and Heliospheric Observatory. Launched in December 1995, SOHO has followed the Sun through a full 11-year solar cycle and then some. Moreover, it doesn't have to contend with turbulence in Earth's atmosphere.
"Getting above the atmosphere is an enormous advantage for astrometry, more than an order of magnitude improvement," Kuhn explains, "and we just don't see any solar-cycle variability." His team's analysis of SOHO observations will appear soon in Astrophysical Journal.
The same can't be said about measurements taken closer to home. Measurements using astrolabes, Mercury transits, and eclipse timings, among others, reveal variations 10 to 100 times larger. Yet they all show bigger swings in the Sun's size, which suggests that the ground-based measurements are indeed correct but systematically influenced by some kind of subtle atmospheric effect. (That said, measurements taken from high-altitude balloons in 1992-96 inexplicably show the same trend.)
Kuhn has a hunch that this apparent discrepancy between ground and space is "telling us something interesting about how the solar cycle affects the stratosphere."
"All mechanisms are speculative," he notes, but pointedly adds, "That the solar diameter is rock-solid is not speculation."
Fortunately, even better measurements will be in hand soon. The researchers plan to repeat their work using images from NASA's just-launched Solar Dynamics Observatory. By 2017, they'll have access to the exquisitely detailed views from Advanced Technology Solar Telescope being built at the summit of Haleakala in Hawaii.