1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<html> 3<head> 4<title>SpaceWeather.com: The Sunspot Number</title> 5<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 6<link href="reference_subpage_styles.css" rel="stylesheet" type="text/css"> 7</head> 8 9<body> 10<table width="1058" height="1000" border="0" cellpadding="0" cellspacing="0"> 11 <tr> 12 <td width="31"> </td> 13 <td width="1027" valign="top"><table width="100%" border="0" cellspacing="0" cellpadding="0"> 14 <tr> 15 <td width="9%"> </td> 16 <td width="91%"><img src="images/ref_subpage_menu.jpg" width="558" height="56" border="0" usemap="#Map2"></td> 17 </tr> 18 </table> 19 <table width="100%" border="0" cellspacing="0" cellpadding="0"> 20 <tr> 21 <td><img src="images/spacer.gif" width="10" height="17"></td> 22 </tr> 23 </table> 24 <a href="http://spaceweather.com"><img src="images/sunspots_title.jpg" alt="Spaceweather.com" width="402" height="106" border="0"></a> 25 <table width="100%" height="800" border="0" cellpadding="0" cellspacing="0"> 26 <tr> 27 <td width="5%"> </td> 28 <td width="72%" valign="top" class="subpageStandardText"> 29<table width="100%" border="0" cellspacing="0" cellpadding="0"> 30 <tr> 31 <td><img src="images/spacer.gif" width="10" height="19"></td> 32 </tr> 33 </table> 34 <p><font size="5">The Sunspot Number</font></p> 35 <p><B><FONT SIZE="2" FACE="Verdana, Arial, Helvetica, sans-serif">S</FONT></B><FONT size="2" FACE="Verdana, Arial, Helvetica, sans-serif">cientists 36 track solar cycles by counting sunspots -- cool planet-sized areas 37 on the Sun where intense magnetic loops poke through the star's 38 visible surface.</FONT></p> 39 <P><FONT size="2" FACE="Verdana, Arial, Helvetica, sans-serif">Counting 40 sunspots is not as straightforward as it sounds. Suppose you looked 41 at the Sun through a pair of (properly filtered) low power binoculars 42 -- you might be able to see two or three large spots. An observer 43 peering through a high-powered telescope might see 10 or 20. A powerful 44 space-based observatory could see even more -- say, 50 to 100. Which 45 is the correct sunspot number?</FONT></P> 46 <P><FONT size="2" FACE="Verdana, Arial, Helvetica, sans-serif">There 47 are two official sunspot numbers in common use. The first, the daily 48 "Boulder Sunspot Number," is computed by the NOAA Space 49 Environment Center using a formula devised by Rudolph Wolf in 1848: 50 </FONT></P> 51 <P align="center"><FONT size="3" FACE="Verdana, Arial, Helvetica, sans-serif"><I>R</I>=<I>k</I> 52 (10<I>g</I>+<I>s</I>), </FONT></P> 53 <P><FONT size="2" FACE="Verdana, Arial, Helvetica, sans-serif">where 54 <I><strong>R</strong></I> is the sunspot number; <I> <strong>g</strong></I></FONT><FONT size="2" FACE="Verdana, Arial, Helvetica, sans-serif"> 55 is the number of sunspot groups on the solar disk; <I> <strong>s</strong></I> 56 is the total number of individual spots in all the groups; and <I><strong>k</strong></I> 57 is a variable scaling factor (usually <1) that accounts for observing 58 conditions and the type of telescope (binoculars, space telescopes, 59 etc.). Scientists combine data from lots of observatories -- each 60 with its own <I>k</I> factor -- to arrive at a daily value.</FONT></P> 61 <P align="center"><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><A HREF="images2009/zurich.gif"><img src="images2009/zurich_strip.gif" width="600" height="397" border="0"><BR> 62 </A><B>Above</B>: International sunspot numbers from 1745 to the 63 present. [<A HREF="http://science.msfc.nasa.gov/ssl/
63pad/solar/sunspots.htm">more</A>]</font></P> 64 <P><font size="2" face="Verdana, Arial, Helvetica, sans-serif">The 65 Boulder number (reported daily on SpaceWeather.com) is usually about 66 25% higher than the second official index, the "International 67 Sunspot Number," published daily by the Solar Influences Data 68 Center in Belgium. Both the Boulder and the International numbers 69 are calculated from the same basic formula, but they incorporate 70 data from different observatories.</font></P> 71 <P><FONT size="2" FACE="Verdana, Arial, Helvetica, sans-serif">As 72 a rule of thumb, if you divide either of the official sunspot numbers 73 by 15, you'll get the approximate number of individual sunspots 74 visible on the solar disk if you look at the Sun by projecting its 75 image on a paper plate with a small telescope.</FONT></P> 76 77 <p><font size="2"><strong>RELATED LINKS:</strong></font></p> 78 <p><font size="2"><a href="sunspotplotter.htm">Sunspot Plotter</a> 79 -- Explore the sunspot cycle with this interactive tool</font></p> 80 <p><font size="2"><a href="http://sidc.oma.be/index.php3">Solar Influences 81 Data Center</a> -- source of the official International Sunspot 82 Number.</font></p> 83 <p><font size="2"><a href="http://www.ngdc.noaa.gov/stp/SOLAR/SSN/ssn.html">Rudolf 84 Wolf</a> -- inventor of the modern sunspot number</font></p> 85 <p><font size="2"><a href="http://science.nasa.gov/headlines/y2009/21jan_severespaceweather.htm">Severe 86 Space Weather--Social and Economic Impacts</a></font></p> 87 <p><font size="2"><a href="http://science.nasa.gov/headlines/y2008/30sep_blankyear.htm">Spotless 88 Sun--the Blankest Year of the Space Age</a></font></p> 89 <p> </p> 90 <p align="center">
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