Tune the solar clock

1 · Lock the laser onto the line

Light from the whole solar disk arrives through the fibre. A laser is waiting for it on the iron line at 1,565.28 nm.

Beginner

Once the laser is locked onto the centre of the iron line, what does the instrument count to track the Sun's velocity?

The laser's frequency, against the frequency comb The position of the line on the detector, in pixels The number of photons the fibre receives each second

2 · Let the Moon pass

8 April 2024, Boulder. The Moon is about to cover 80% of the disk.

Beginner

During the eclipse, the iron line shifted by up to 500 m/s, one way and then the other. Where does this shift come from?

From the Sun's rotation: the Moon hides the approaching edge, then the receding one From the Moon's gravity, which pulls the Sun slightly toward it as it passes From air cooled by the Moon's shadow, which bends the sunlight on its way into the tracker

3 · Average down to an Earth

Three megahertz in one second. Now time is on your side.

Intermediate

If only photons mattered, how much averaging time would it take to go from 3 MHz to 200 kHz, the precision the article reports?

About 15 seconds About 4 minutes About 1 hour

4 · And on another star?

A 30-metre telescope could bring the precision shown here to a nearby Sun-like star in a few hours.

Expert

On that star, would averaging even longer be enough to reach the 8.9 cm/s of an Earth?

Yes: the photon limit keeps falling as the square root of time, you just have to wait long enough No: the star's granulation imposes a floor of several tens of cm/s, even over an hour No: an Earth's signal takes a year to build up, and no telescope follows a star for that long

Clock tuned

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