The number of sunspots is controlled by the amount of distortion of the Sun's magnetic field. The magnetic field becomes distorted because the Sun's equator and core rotate more quickly than its other parts. As a result, sunspot activity varies over an average 11-year cycle. Over approximately 11 years, the Sun goes from a solar minimum (fewer spots) to a solar maximum (more spots) and back to a minimum again.
Photo: Sunspots as seen by the SOHO probe (NASA/Goddard Space Flight Center Scientific Visualization Studio)
Sunspots can be as big as the Earth.
Images of the Sun are created with a huge solar telescope.
In the late 1960s, images of the Sun are created at Kitt Peak National Observatory with the McMath-Pierce Solar Telescope.
George Hale discovers sunspots' magnetic origins.
Through careful spectrographic study, early 20th century American astronomer George Hale discovered that sunspots are caused by distortions in the Sun's powerful magnetic field.
The 17th century scientist sees blemishes on the Sun.
The Italian physicist and mathematician Galileo Galilei was the first person to observe the Sun through a telescope. He saw sunspots moving across the Sun's surface and realised that it was rotating.
Nineteenth century astronomers split the Sun's light to see more of its structures.
By splitting the light of the Sun into its component colours with a technique called spectroscopy, early astronomers saw more of the Sun's features. They also worked out what chemical elements form the Sun.
The solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change in the Sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number of sunspots, flares, and other manifestations).
They have been observed (by changes in the sun's appearance and by changes seen on Earth, such as auroras) for centuries.
The changes on the sun cause effects in space, in the atmosphere, and on Earth's surface. While it is the dominant variable in solar activity, aperiodic fluctuations also occur.