Hi;

That is, in fact, a much harder task than it sounds.

Ok, so you say "I'm going to count the number of sunspots I see on the disk." Well, let's suppose you're using the projection method, using a pair of binoculars projecting an image of the sun onto a piece of paper and counting what you see on the paper, and your friend is using an image from the Solar Dynamics Obervatory (see, for example, http://www.spaceweather.com/images2011/18oct11/hmi4096_blank.jpg). Who's going to see more? The results depend on whether you can easily see small stuff or not for example. And then - do you count groups of spots or individual? What about those little dark things that aren't big spots yet, but you can see them (sometimes called proto-spots or pores)? There are sometimes hundreds of those, but only 3 or 5 big sunspots with well-developed umbral and penumbral structures.

So - it seems like an easy question, but it is challenging and depends on what question is being asked. How do we compare the number of spots we see now to observations that Galileo was making hundreds of years ago? So - the main task scientists have to do is to define a set of 'rules' to follow, and then stick to them, and then (for example, once you have some results) to tell other people what those rules were. So we will often use a computer algorithm (a computer program) that takes an image and finds locations where (for example) the brightness is less than 80% of the average, and call that a sunspot. But when we tell other scientists we'll also describe which data were used, describe the computer program, and give examples. So someone can reproduce what we did, or decide to do something different.

Cheers,

-KD