OUR SUN:  The "Human" Star

by:  P. Clay Sherrod

Introduction -
The first telescopic views of our sun - hitherto thought to be a "perfect sphere" of light and life - by Galileo almost 350 years ago revealed a turbulent and blemished world that would take centuries to fully understand.

Today, when amateur astronomers and those learning the components of our solar system, the first view of the sun is much as Galileo saw - yet today the sight frequently yields signs of disappointment rather than gasps of wonderment. All that is visible of the sun in the spectral sensitivity of our eyes is known as the PHOTOSPHERE, a bright yellow-white layer of explosively-hot gases that are occasionally interrupted by small features appearing not unlike craters on the moon - the SUNSPOTS.

Even though the sun may look a bit "boring" for dedicated observing, there is much to do on the sun - even on a daily basis - but two things are very needed before setting out:

1) a thorough understanding of the limitations of your telescope equipment when observing the sun; and,

2) special filters and precautions to protect your delicate eyesight from the intense burning rays of sunlight as well as the delicate interior of your precious telescope!

Observing the sun is an ideal activity for many enthusiasts, particularly those who:
* work evening and late night shifts and are unable to use their telescopes during nighttime hours;
* observers who are often frustrated because of bright sky conditions in their primary observing area;
* hosting a weekend cookout or other activity which can include a visit to our nearest star!


Of all objects we observe in space through our telescopes, only the sun poses any real physical threat to us; the very orb that provides the heat and light necessary and responsible for life on Earth is also potentially dangerous to your eyes, your skin and in some cases your general health.

Long ago, when I was a kid telescopes came with "sun filters," small little attachments that you would put on top of your eyepiece that reduced the intensity of the sun so that you could bear to look at it.  These did not protect anything.....they merely allowed you to get close enough to the eyepiece without setting your eyeball on fire. Nothing but a small piece of common welder's glass; many times after several minutes of collecting the heat focused from a small telescope, these "filters" suddenly would crack and pass intense light into the unsuspecting viewer's eye.

If you ever have a chance to use one of these jewels....don't. Throw it away.

Today we have specialized filters of all types for our telescopes that are entirely safe for both the observer and the equipment. Regarding equipment, you should NEVER allow sunlight to pass into a closed tube such as a Maksutov, Schmidt-Cass or refractor telescope; the heat will accumulate until eventually something is going to melt or pop. The lens of your telescope is less effective at "starting a fire on a leaf" than is the common magnifying glass focusing the sun's rays to a tiny and fiery point.

Full-aperture, optical glass mirrors are desirable for solar observing; these provide excellent "true" color (yellowish-orangish-yellow-white and all combinations). The "Mylar" foil filters are safe, but they have a false blue coloration and provide very poor contrast when compared the the better (and more expensive) glass types. Good optical glass filters reflect MOST of the light coming from the sun and REJECT the harmful ultraviolet and infrared radiations that most severely burn the eye's retina.
sherrod etxsun1

Although there are many brands available, I have actually tested two - from Orion and from Thousand Oaks - for optical quality, contrast and protective transmission/rejection that I would highly recommend to anyone; they are made in sizes to fit your telescope without any adaptation.

These filters fit like a cap covering the FRONT of your telescope; NOTE: even though these filters are provided with a metal cell and mounted, be sure to shim within the edge of the filter cell to assure a snug fit against the outer perimeter of your telescope tube. Some filters, if not properly snugged, will loosen and fall off, resulting in sudden surges of unwanted and unhealthy light to the unsuspecting observer.

As Figure 1 demonstrates, a good solar filter will show the visible layer of the sun's hot gases, the photosphere. Among the features that you might see are: 1) white-light flares (rare, white streaks usually near sunspot groups); 2) sunspots and sunspot groups (can be seen in Figure 1); and oftentimes "granulation," considerable mottled areas that are tops of turbulent mixings of the sun's gases.  In addition, there are NOW very high resolution "Solar Telescopes" on the market for non-professional astronomers which will indeed show PROMINENCES, solar flares, and extensive activity not seen with white light filters.  Your dealer can demonstrate these remarkable telescopes.


The sun, like all other stars and like Galileo pointed out for the first time, is NOT a perfect object. It turns on its axis (a point which eventually resulted in Galileo's house arrest!) as can be attested by observing the sunspots (following in Guide) as they slowly move east-to-west across the visible disk of the sun; the sun makes ONE complete rotation in about 26.7 days at the equator and slightly slower nearer the poles.

Our sun goes through periodic cycles, just like any variable star, every 11 years. The cycles are fairly predictable from one to the other. From MINIMUM sunspot activity it takes only 4.5 years to surge to MAXIMUM sunspot activity, then longer to subside or about 6.5 years. Over 300 sunspots can be observed on a single presentation of the sun during maximum and there are times during minimum that NO spots can be found even with quality larger-aperture telescopes.

In addition to the major 11-year cycle, the sun also exhibits another major cycle of 22 years, and another that consists of minor changes within a 200-year span. Even one estimated at near 10,000 or more has been determined based on climatic changes (tree ring analysis) through the centuries.


This guide is focused on some interesting and rewarding observing that you might undertake with your filtered telescope to monitor the sunspots, their growth and demise, their rotations with the spinning of the sun and how to determine the ZURICH DAILY NUMBER, or a simple "sunspot count."

SUNSPOTS: Every day will bring a new look to the sun; the main objects you will see with good solar filters and your smaller telescope will be "sunspots" and sunspot "groups." The changes in these spots and groups happens unpredictably and intense solar activity (i.e., when a lot of auroral activity is forecast) usually is a precursor of good observing for sunspot changes.

sherrod etxsun2

Figure 2 clearly shows the nice detail that is visible in sunspots; note that in the two excellent photographs in Figure 2 that we are actually looking at a "sunspot group," comprised of many individual sunspot "cells." Typically, a sunspot - or group - will have a very dark (black-appearing) UMBRA near the center axis surrounded by a lighter and often-detailed PENUMBRA, or lesser dark halo. Both features are clearly visible in the photographs.

Note that Peter Vasey's closeup photograph at lower right corresponds to Mike Weasner's full-disk image of the sun, both taken on September 23, 2000; the closeup shows the sunspot that Mike captured with his ETX 90 as seen just above the middle on the extremed right edge of the photograph.

Sunspots will perhaps take on a new meaning for you when you realize that they are actually some of the BRIGHTEST objects in our solar system! They only "appear" dark or black because they are slightly cooler (several thousands of degrees) than the surrounding hot photosphere, and thus appear less luminous than the yellow surrounding gases. If you could (thank goodness this is hypothetical) take a single small sunspot and put it in a dark room, it would be blindingly bright and of course very deadly!

Long before Galileo and the advent of the telescope, ancient Chinese astronomers were recording major sunspots with the naked eye; still today - particularly during sunset and sunrise when the Earth's air filters most of the light from the intense orb - we can see major sunspots to the unaided eye from times of great solar activity.

For sun observers, it is important to learn both the method to determine the ZURICH DAILY NUMBER (of sunspots and groups - this published regularly in astronomy journals and magazines) as well as "Sunspot Classifications" based on sunspots and their characteristics both as individual spots and in groups.
sherrod etxsun3

Figure 3, (as well as Figure 4) following, is taken from my book "A Complete Manual of Amateur Astronomy," [Prentice-Hall, 1981] and demonstrates clearly the major categories of sunspots and groups (no, there is is "no [i]" group!) as based on their distinguishing characteristics.

I will quickly and concisely describe each of these categories:

CLASS A - a small individual spot, or a spot group that shows NO penumbra;
CLASS B - similar to "A" but spots suggest some association to one-another, or they show "symmetry" - (BIPOLAR SPOTS); still NO penumbra is present;
CLASS C - "bipolar" (see above) groups in which only the largest spots have penumbras;
CLASS D - like "C" except ALL major spots show penumbra;
CLASS E - a very large bipolar group (very much like those of Figure 2), larger than 10 degrees across, in which the major spots show lots of detail within the penumbra; there may be many very small spots between the major spots;
CLASS F - largest of all bipolar groups, sometimes as much as 20 degrees, surrounded by very detailed penumbra and many random smaller surrounding spots;
CLASS G - just like "F" above, but NO smaller random spots around;
CLASS H - one large spot with penumbra surrounded by small random umbral spots;
CLASS J - one large single spot (POLAR) with a penumbra....nothing more.

Test your ability to differentiate the spots that you can see. Remember....the image of the sun that you observer will LIKELY exhibit many different categories of sunspots and groups all at the same time. Look around! Using medium magnification (about 30x per inch aperture) will reveal great detail within the spot groups and even some granulation that may surround nearby spot groups.

You would think that "counting sunspots" would be like counting sheep: 1) it seems like a pretty straightforward "1, 2, 3..." process; and, 2) it would put you to sleep.

Neither is the case.

The basis of the "Zurich Daily Number" is to use sunspots as a gauge to accurately monitor solar activity on any given day. In this procedure a SUNSPOT GROUP is considered "more active" than is an individual SPOT, so it is factored "higher" than would be an individual spot.

Thus, when counting sunspots for the Relative Spot Number (RSN), spots count as "1" and groups count as 10 times the activity of a single spot. IN ADDITION, like so much in today's world, there must be some standard for the SIZE AND TYPE of telescope used to monitor the counts. Obviously we all know that a larger telescope is capable of seeing MORE sunspots because of its greater resolution, so it stands to reason that the "home" of worldwide sun monitoring, the Zurich Observatory in Switzerland, would be using as BIG a telescope as possible to watch the sun....right?

Wrong. The world standard is set from a 2.4" refracting telescope. Thus, a small scope is an ideal telescope to match the world leader in sunspot observing! Anything larger will reveal TOO MANY sunspots that will skew the actual total, so we must FACTOR those larger instruments, even the 90mm ETX 90. More on that in a minute.

The RSN Formula - there is a simple formula that takes all this into account and we will apply it to some actual sunspot counts as a test. First, we know the rules:

1) spots are counted as "1" even if they are contained in a "group;"
2) groups (the entire mess of them) count as 10 times the value of a spot - each group is "10" no matter how many spots;
3) a telescope of 60mm is a factor of "1" (multiply the count by this factor); other scopes must factor differently.

Your Telescope Factor (the "k" factor) - if you are using a telescope of 60mm or less, you use a factor of "1" which matches the Zurich world standard. If not, try the values I have determined below:

        Scope Size (mm)    Factor (multiply by)
                60mm             1
                90              0.8
                125             0.6
                200             0.4

The Formula - don't be too stressed about is easy and we'll put it to the test for you in a moment.

RSN = [(g x 10) + n] x k,

RSN - "Relative sunspot number"
"g" is the number of groups you can see
"n" is the total number of sunspots, even those counted INSIDE of groups;
"k" is your telescope factor as provided above.

It is really very simple to use this formula (always attempt to use low power so that the entire sun's disk (Figure 1) can be positioned in the field of view:

1) count the number of GROUPS ("g") that you can see in a low-power field of view (do not change to higher magnifications at any point during your daily count!) and MULTIPLY that number by "10"...if you see three (3) groups, then you will have that value as "30" (the "g x 10" above in the formula);

2) now simply ADD to that number (in this example, "30") ALL of the individual sunspots ("n") that you can see AT THE SAME MAGNIFICATION; do not raise the magnification until AFTER you have can see more sunspots at higher magnifications; even though you have already counted the "groups", you still must COUNT THE SUNSPOTS within the groups as part of the "n" total;

3) if you have counted 70 individual spots, merely add that to the ("g x 10") total of 30 that you have obtained for the groups, and you get a count of "100" for that day;

4) if you have used an 3: scope, for example, you must NOW multiply that "100" by its "k" factor of "0.8" provided above to adjust your telescope to that of Zurich; so, very simply you multiply the "100" by 0.8 and obtain "80" as your RSN number for that day.

That's it....simple as that. Since an 8-inch scope will likely see more individual spots than the 3-inch, the factor number SHOULD result in a final number for both telescope that is fairly equal.

Okay, you've stood out there and counted black spots on the sun and used remedial math to figure out the RSN....what for? Is it worth anything? Who do you report this to?

Yes, is very important and as they say...."somebody's gotta' do it!" The RSN as mentioned is an indicator of relative solar activity on any given day; the sun's unpredictable nature makes it necessary for us to monitor this activity to better predict and understand the results of solar outbursts to us here on Earth. The sun's activity likely affects our weather and climate, and certainly affects the ionosphere through aurora, radio and microwave disruption, satellite transmission and other adverse affects.

We DO NOT totally understand the nature of the sun no more than we do that of any other star in space; but YOU, with some degree of diligence as you record the RSN can contribute to the body of knowledge of our closest star.

For example, if tomorrow you heard on the news about a huge solar flare that is causing all sorts of communications on Earth, you would immediately take notice. Now, how about if you HAPPENED TO HAVE BEEN OUT the day before that happened and recorded the sun's RSN?? It would really mean something to you then, wouldn't it? As a matter of fact, you may well likely be able to go back over a week's previous sunspot counts and realize that "....something was up!"

Your counts are of no useful purpose if you merely leave them in a notebook in the closet. You can log your RSN as frequently as you record it by contacting the American Association of Variable Star Observers (AAVSO) at:

where you can obtain important and very detailed information about solar observing programs, forms and the necessary framework to build a good solar observing program and the correct procedures for reporting your valued observations.

Figure 4 shows three rough sketches of "the sun" on different days with three different combinations of imaginary sunspots and groups. Using the method described above, do INDIVIDUAL counts of each Day 1, Day 2 and Day 3. First, count the "groups" and multiply by "10"; then ADD to that number the number of total spots you see (including those in the groups) and then multiply by your telescope factor. What do you get?

sherrod etxsun4

Here are the actual totals (I am using the "k" factor of "1"); go ahead and see what it would be with YOUR "k" factor as well:

Day 1 - four (4) groups ("g"); nine (9) individual spots ("n") / RSN = [(4 x 10) + 9] x "1" / RSN Day 1 = 49
Day 2 - two (2) groups ("g"); 12 spots ("n") / RSN = [(2 x 10) + 12] x "1" / RSN Day 2 = 32
Day 3 - three (3) groups ("g"); three (3) spots ("n") / RSN = [(3 x 10) + 3] x "1" / RSN Day 3 = 33

How did YOU do?

You really haven't "stargazed" until you have a chance to "gaze" at your own star...the sun.  Too many amateurs neglect the sun because it seems so static and counting sheep. but payday always comes around when a major solar outburst is recorded and you have the thrill of going back over your OWN PERSONAL records of the sun's recent life.

Only then, can you turn to your family after hearing about solar shock waves on the nightly news and say:

"Yeah....but let me tell you what was going on BEFORE that happened..."

P. Clay Sherrod
Arkansas Sky Observatory
Conway / Petit Jean Mountain
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