Wolf number calculator

About this tool

The Wolf number (also called the relative sunspot number) was introduced in 1849 by Rudolf Wolf at the Zurich observatory. It combines two counts made on the solar disc: W = k × (10 × G + S), where G is the number of sunspot groups, S the total number of individual spots, and k a personal coefficient. Giving each group a weight of ten reflects that the appearance of a new group says more about activity than one extra spot in an existing one.

The coefficient k corrects for the instrument, the observer’s eye and the sky: a small refractor shows fewer tiny spots than a large telescope, so its k is above 1. Leave it at 1 if you do not know yours; a personal value is usually established by comparing many observations with a reference series. Explorer shows the raw daily mean of the community, without any smoothing.

The calculator gives the number and its position on the colour scale used throughout the site; nothing is published from here. To contribute an observation to the community series, log it on Wolf.Solar — it will appear in the activity heatmap once synchronised.