Why is the Sun so round?

Chris Lintott

Nasa’s Parker Solar Probe is now the fastest object ever built, hitting 101 miles per second as it passed within six million miles of the Sun last month, setting a record for the closest approach to our star on its tenth planned orbit. The views during its slingshots are spectacular, with Parker’s camera catching Mercury, Venus, Jupiter, Saturn, the Milky Way and the Earth sliding past the window on a recent passage.

These familiar objects are seen in an unfamiliar setting. The flickering light in the background is from the Sun’s outer atmosphere, the corona, through which Parker is flying. It is the first spacecraft to enter this region of tenuous hot gas, normally only visible from Earth during the special conditions of a total solar eclipse.

That such opportunities exist at all is the result of what seems to be a cosmic coincidence, that the Moon and the Sun are approximately the same size in the sky. If the Moon were any larger, or any closer to Earth, during an eclipse it would block the corona as well as the main disc of the Sun. If it were any smaller, or more distant, we would enjoy only annular eclipses. Instead, a celestial configuration that may be extremely rare has made the corona an object of fascination at least modern eclipse science began in 1715.

The eclipse in April of that year had a track which crossed Southern England, and it was much anticipated. Rival astronomers vied to make predictions of its circumstances. The Painted Hall of the Royal Naval College in Greenwich has a contemporary portrait of the astronomer royal John Flamsteed hiding in the ceiling’s baroque splendour, waving a chart with his forecast.

Repeated observations of eclipses throughout the eighteenth and nineteenth centuries made it clear that the Sun was surprisingly dynamic, with the shape of the corona changing each time. The Antarctic eclipse enjoyed by a few hardy tourists and several million penguins earlier this month displayed the symmetrical form typical of periods where the Sun is less active. Eclipses near solar maxima reveal sculpted, extended shapes. The Parker images reveal this dynamic region in detail, showing myriad small streamers threading the corona.

Observing such fine structures may, it’s hoped, solve a longstanding problem. The corona is several million degrees hotter than the solar atmosphere beneath it, and the source of this energy is unclear. Future voyages close to the Sun’s surface may help, but it’s still the case that despite its familiarity, much about the Sun is confusing. It is, for example, too round; a rapidly rotating gaseous body should be flattened and yet our star is the most perfect sphere found in nature.

Such mysteries have inspired a generation of new instruments, including a giant telescope in Arizona dedicated to solar science and the European Space Agency’s Solar Orbiter. These projects are technological marvels, and of great scientific importance. The fact that studying the Sun produces images like those from Parker, full of wonder and strange beauty, is a welcome side-effect.


  • 24 December 2021 at 10:31pm
    Laurie Strachan says:
    Miles? What they?

    • 25 December 2021 at 6:21am
      Joe Morison says: @ Laurie Strachan
      Just be grateful Jacob Rees-Mogg hasn’t yet succeeded in making leagues, furlongs, and chains the official units of distance.

    • 25 December 2021 at 12:00pm
      Chris Lintott says: @ Joe Morison
      The speed of light is a foot per nanosecond, which is useful to know.

    • 26 December 2021 at 1:18pm
      Jake Bharier says: @ Chris Lintott
      Aeronautical practice follows nautical, so nautical miles, and a speed of 363 000 knots?

  • 26 December 2021 at 1:11pm
    MattG says:
    The only proper scientific measure for speed - in my opinion is - "Furlong per fortnight".

    The author claims that the corona is "several million degrees hotter". But everybody know a circle has only 360 of them.

  • 27 December 2021 at 2:19am
    nlowhim says:
    Beautiful video. Wonder if there's a conspiracy theory as to what it's actually showing. That being said, every now and then I take a look at the pics over at NASA and am absolutely amazed. I should keep doing that. This internet can be some addictive scrolling of comments and thoughts that have no right to one's attention (but it works). Sometimes it's good to go to places that deserve it and let the mind breathe.

  • 29 December 2021 at 9:53pm
    Jim Payne says:
    The speed of light
    three hundred thousand kilometers per second.
    Isn't it???

    • 30 December 2021 at 7:57am
      Jake Bharier says: @ Jim Payne
      Sorry to have confused you: I was referring back to the speed of the craft in the original post. Anyway, if you try to measure the speed of light in knots, doesn't something odd appear to happen to the rope?

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