The most precise clocks on Earth tick to the rhythm of atoms trapped in carefully controlled laboratories. A team in Colorado has now shown that the same recipe can work with a far messier ingredient: ordinary sunlight piped in from the sky.
Scientists from Boulder’s university campus and the federal measurement lab NIST, working under Scott Diddams, gather sunlight into a single thin optical fibre. Within its spectrum they target one dark line, the fingerprint left when iron atoms in the solar atmosphere absorb a very specific shade of infrared light. A laser is held firmly on that line and a frequency comb, a kind of optical ruler, counts its oscillations. Its builders have named the device a solar optical clock.
It will not replace the world’s best timekeepers. Its value lies elsewhere. Any shift in the iron line reveals how fast the glowing gas is moving towards or away from us, and after averaging for a few thousand seconds the instrument pinned that speed down to roughly 30 centimetres per second. The data even picked up the Sun’s natural surface oscillations, which repeat about every five and a half minutes.
An elegant test arrived with the eclipse of April 2024, when the Moon covered much of the Sun as seen from Colorado. Our star rotates, so one rim is always approaching and the other retreating. As the Moon slid across, it masked those rims in turn, and the iron signal lurched in opposite directions by as much as about 500 metres per second.
The longer-term prize is planet hunting. A world like ours makes its star wobble by only a few centimetres per second. In the team’s estimate, the next generation of giant observatories, with mirrors between 25 and 40 metres across, might bring the method to nearby stars similar to the Sun in a few hours of observing. Faint starlight and the restless, boiling surfaces of stars remain serious hurdles, so for now it is a promising tool rather than a ready-made planet finder.
