Sunspots: what they are and how to observe them safely

What a sunspot is

The Sun’s visible surface, the **photosphere**, has a characteristic temperature around 5,800 kelvin. Convection transports energy upwards beneath it. Where concentrated magnetic fields emerge, they inhibit this transport, allowing a region to become cooler than the surrounding surface.

A developed spot has a darker central umbra and a lighter, filamentary penumbra around it. Those names describe the structure of the spot here, rather than eclipse shadows. A spot is dark by comparison, not intrinsically unlit; isolated from its bright surroundings, it would still be intensely luminous.

Spots often form groups with opposite magnetic polarities. Small spots can last hours or days, while large groups can survive several solar rotations. Some are larger than Earth. Near the solar limb, bright faculae can stand out as the disk itself darkens towards its edge. With suitable equipment and steady air, the surface’s fine granulation reveals convection cells roughly a thousand kilometres across, continually changing on timescales of minutes.

Near the limb, a spot’s appearance also reflects foreshortening and the depressed structure of the cooler region. These are features to interpret through a properly filtered observation, not reasons to look at the Sun without protection.

The solar cycle

Sunspot counts rise and fall on an average rhythm of about eleven years, with individual cycles commonly ranging roughly from nine to fourteen years. Near minimum the Sun can be spotless for days; near maximum many active regions may appear together. The global magnetic pattern reverses each cycle, giving a full magnetic cycle of about twenty-two years.

Early in a cycle, spots tend to emerge at moderate heliographic latitudes, around thirty to thirty-five degrees, then appear progressively closer to the equator. Plotting their latitudes through time produces the butterfly diagram.

The numbered cycle record begins in the eighteenth century; Solar Cycle 25 began in late 2019. A cycle’s broad phase does not predict the exact groups visible on a chosen day, and major activity can occur outside a nominal maximum. The Maunder Minimum, approximately 1645–1715, is a historical example of an extended period with very few recorded sunspots.

The **sunspot number**, associated with Rudolf Wolf, combines group counts and individual spot counts with observer and instrument corrections. Groups receive more weight than an additional spot within an existing group. This standardisation allows long historical records to be compared, while the index itself remains different from a prediction of a particular flare.

How active regions are classified

The Mount Wilson classification describes magnetic structure. Alpha identifies a unipolar group; beta indicates separated positive and negative polarities; gamma marks more complex distributions. Delta identifies opposite-polarity umbrae sharing a penumbra. Complex beta-gamma-delta regions can be associated with strong activity, without guaranteeing a specific event.

The McIntosh classification describes visible structure with three characters: the group’s basic form, the penumbral form of its largest spot and the distribution of spots inside the group. It is complementary to a magnetic classification, not another name for the same quantity.

To orient an observation, three solar angles are useful: P, the position angle of the rotation axis; B0, the heliographic latitude of the disk centre; and L0, its heliographic longitude. The Sun turns in roughly twenty-seven days as seen from Earth, so a persistent region moves across the visible disk over about two weeks. The apparent orientation on your image also depends on the instrument and its optical path.

Observing the Sun safely

Never look directly at the Sun through unfiltered binoculars, a telescope or a camera viewfinder. Appropriate protection must match the way you observe.

• Unaided-eye solar viewers: Intact eclipse glasses or handheld viewers meeting ISO 12312-2 can reveal exceptionally large spot groups. Ordinary sunglasses are not substitutes.
• Optical instruments: Use a purpose-built solar filter securely fitted over the front of the optics, following the instrument and filter manufacturers’ instructions. Eclipse glasses worn at your eyes do not protect you when looking through unfiltered magnifying optics. Eyepiece filters exposed to concentrated solar energy are unsafe substitutes.
• Projection: A purpose-designed solar projection setup can place an image on a screen. Suitable equipment, supervision and covered or removed optical finders are essential; do not assume any ordinary telescope or binocular is safe for solar projection.
• Dedicated solar equipment: Specialist solar telescopes and compatible, correctly configured systems such as a Herschel wedge provide other observing methods. They require the complete specified filtration system and competent setup, rather than improvisation.

Steadier morning air can help, but local conditions vary. A log of date, time, group count and spot appearance lets you follow rotation and evolution across several sessions. Safety precautions are needed every time, regardless of the Sun’s height or apparent brightness.