What is the Sunspot Number?
It is
not the number of sunspots.
"What?? How do we interpret this ubiquitous number then? How in the blazes is Sunspot Number not the same as Number of Sunspots?"
"Pull up chair.. I'll tell you.."
The Sunspot Number is actually derived from a formula.
The formula yields a number.
That number -- the result of the calculation is what we call the Sunspot Number.
The formula is as follows
R = k (10g + s)
- R is the sunspot number.
- g is the number of sunspot groups on the solar disk.
- s is the total number of individual spots in all the groups.
- and k is a variable scaling factor (usually <1) that accounts for observing conditions and the type of telescope (binoculars, space telescopes, etc.).
To do this yourself, find s, and g. Assume k = 1 for sake of argument.
Enter your calculator these steps:
- 10 multiplied by g
- add that product to s
- multiple that sum by k
That's the Sunspot Number.
Scientists combine data from lots of observatories -- each with its own k factor -- to arrive at a daily value.
So, observatories count the number of groups (g), they count the number of spots in each group and add those up (s), and based on the conditions and type of telescope used they assign a scale factor (k).
R = k (10g + s)
When folks talk about the Sunspot Number, what they are referring to is usually R. Unless they are explaining the value s (total number of spots), the R value is what we usually are talking about when we say Sunspot Number.
According to historical data and approximations, if the question still is "Well, how many sunspots are actually there?" The rule of thumb is to take R, (the Sunspot Number) and divide by 15. That's a close approximation of the number of actual sunspots based on historical records.
