The sun, like Earth and other celestial bodies, spins on its axis. But unlike our planet’s roughly 24-hour day, the sun’s rotation is far from uniform. The length of a full spin depends on where on the sun you measure from, and even how you measure it. Early observations by Galileo Galilei in the 17th century first revealed that the sun rotates, but centuries of further study have shown the reality is much more intricate than a simple number.
The Early Discoveries: Sunspots and Rotation
Galileo’s observations of sunspots—dark areas on the sun’s surface—moving across the solar disk led him to conclude the sun rotates roughly once every 28 days. Later, in the mid-1800s, astronomer Richard Carrington refined this estimate to approximately 27.3 days by studying sunspot movement around 30 degrees latitude.
However, these early measurements were complicated by Earth’s own movement. Because we observe the sun from a moving platform, the rotation rate appears slightly slower than it actually is. This effect, known as synodic rotation, adds nearly two days to the observed period. The true rotation rate—relative to distant stars—is faster.
Differential Rotation: The Sun Doesn’t Spin as a Solid Body
The key is that the sun isn’t solid. It’s a massive ball of gas, and different parts rotate at different speeds. This phenomenon, called differential rotation, is why there’s no single answer to how long the sun takes to spin. The sun rotates fastest at its equator, completing a rotation in just 24.5 days. At the poles, it takes over 34 days.
This variation isn’t just surface-level. The sun’s rotation rate changes with depth, too. The outer layers, including the convection zone, rotate at varying speeds depending on latitude. But deeper inside, in the radiative zone, the sun rotates more uniformly, taking about 26.6 days regardless of latitude.
Modern Methods: Beyond Visible Light
Today, scientists use advanced methods beyond observing sunspots. Helioseismology studies sound waves traveling inside the sun to map its internal rotation, while Doppler shifts measure changes in sunlight frequency caused by the sun’s spin. These techniques confirm the differential rotation and provide a more complete picture than was possible centuries ago.
The sun’s core rotation remains a mystery. Current methods can’t provide precise measurements, leaving it as an area for future research.
Ultimately, the sun’s rotation is a complex phenomenon. The standard “Carrington rotation” of 27.3 days is still widely used for tracking solar activity, but scientists now recognize that the true rotation period varies significantly across the sun’s surface and even within its layers.
