Round and round goes the Sun

13 March 2026

Gira, gira il Sole

When we think of the rotation of celestial bodies, we immediately picture the Earth spinning once every 24 hours, or the Moon rotating so slowly that it always shows us the same face. The Sun rotates too, but not in the simple way a solid planet does. Its rotation is a fascinating phenomenon that reflects the complex nature of this enormous star at the centre of our Solar System.

The first evidence that the Sun rotates came from Galileo Galilei in 1612. Observing the solar disc through a telescope, he noticed that darker spots, called sunspots, moved slowly from one side of the surface to the other. By tracking their movement, Galileo realised that the Sun must complete a full rotation in about 28 days. Later, in the nineteenth century, the English astronomer Richard Carrington obtained a more precise estimate: sunspots in a temperate region of the solar surface take about 27.3 days to return to the same position. However, this number does not tell the whole story, because it depends on how we measure it. When we observe the Sun’s rotation from Earth, we must take into account the fact that the Earth itself moves around the Sun. This means that the apparent length of the rotation changes depending on our point of view. Astronomers distinguish between:

  • Synodic period: the time observed from Earth for the Sun to return to the same position in our sky (about 27.3 days).
  • Sidereal period: the time measured relative to the distant stars, which is unaffected by the Earth’s motion. This is the more “scientific” value for solar rotation: about 25.4 days at a mid-latitude on the solar surface.

Why is there no single rotation period?

Unlike the Earth, which as a solid body rotates as one, the Sun is made mainly of hot gas and plasma. For this reason, it does not rotate uniformly: some parts spin faster than others, a phenomenon known as differential rotation.

  • At the equator, rotation is faster: it takes about 24.5 days to complete one full turn.
  • Towards the poles, rotation slows considerably: it can take more than 34 days.

This difference occurs because the Sun is a fluid rather than a rigid body: different regions can move at different speeds. This behaviour is unique and is not seen in rocky planets. The Sun’s rotation also varies with depth. In the convective zone, the outer layer where heat moves towards the surface, the speed changes greatly with latitude. Deeper down, in the radiative zone, the behaviour is more uniform, and the rotation period approaches about 26.6 days. It remains a mystery, however, how the core — the inner heart of the star — rotates: scientists still do not have precise measurements for this central region. Understanding how the Sun rotates matters. Differential rotation influences the solar magnetic field, which in turn drives phenomena such as sunspots, solar storms and flares. These events can affect the Earth, for example by interfering with satellite communications or producing spectacular polar auroras.