The Sunspot Number
track solar cycles by counting sunspots -- cool planet-sized areas
on the Sun where intense magnetic loops poke through the star's
sunspots is not as straightforward as it sounds. Suppose you looked
at the Sun through a pair of (properly filtered) low power binoculars
-- you might be able to see two or three large spots. An observer
peering through a high-powered telescope might see 10 or 20. A powerful
space-based observatory could see even more -- say, 50 to 100. Which
is the correct sunspot number?
are two official sunspot numbers in common use. The first, the daily
"Boulder Sunspot Number," is computed by the NOAA Space
Environment Center using a formula devised by Rudolph Wolf in 1848:
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.). Scientists combine data from lots of observatories -- each
with its own k factor -- to arrive at a daily value.
Above: International sunspot numbers from 1745 to the
Boulder number (reported daily on SpaceWeather.com) is usually about
25% higher than the second official index, the "International
Sunspot Number," published daily by the Solar Influences Data
Center in Belgium. Both the Boulder and the International numbers
are calculated from the same basic formula, but they incorporate
data from different observatories.
a rule of thumb, if you divide either of the official sunspot numbers
by 15, you'll get the approximate number of individual sunspots
visible on the solar disk if you look at the Sun by projecting its
image on a paper plate with a small telescope.
-- Explore the sunspot cycle with this interactive tool
Data Center -- source of the official International Sunspot
Wolf -- inventor of the modern sunspot number
Space Weather--Social and Economic Impacts
Sun--the Blankest Year of the Space Age
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