Counting Sunspots: How hard can it be?

NASA Image on Sunspots in 2001. How many sunspots can you see?

NASA Image of sunspots in 2001. How many sunspots can you see?

Warning: Never look directly at the Sun

The Sun is ever present – but rarely seen because we – very sensibly – never look at it directly.

And because of this, most people have never seen a sunspot – a blemish on the observable disc of the Sun.

However, since approximately 1610, people have made careful observations of the Sun’s observable disc, and counted the number of Sunspots. At least one observation has been made on almost every single day for the last 400 years.

People noticed that sunspots were formed near the poles of Sun and then, over the course of a few days or weeks, they travelled towards the Solar equator, where they faded away.

Over many years it became apparent that the number of Sunspots varied periodically. The periodicity ranged between 8 and 14 Earth years, but on average was approximately, 11 years in length.

The average number of sunspots visible on the Sun each year since 1700.

The average number of sunspots visible on the Sun each year since 1700. (Click for version: data location at end of this article)

In the modern scientific era, we have learned – from satellite measurements amongst other techniques – that the intensity of sunlight reaching the top of the Earth’s atmosphere (on average approximately 1366 watts per square metre) varies by up to 1.3 watts per square metre – or 0.1% of the total – with exactly the same cycle as the sunspot count.

This correlation allows us to infer the amount of energy emitted by the Sun back to 1610. And this is where things become interesting

While one can understand that measuring the radiant power of the Sun might be fraught with difficulty, not many people have considered the other end of the correlation: counting sunspots. After all: how hard can it be?

How hard can it be?

Reading an 80 page report by an international team, the answer to this question is straightforward: counting sunspots is very hard.

I can’t summarise the entire paper – it is in itself abridged in many ways! But here are four things that have struck me since reading the report: The main thing I learned; a surprising response; a lesson; and a treat.

1. The main thing I learned

Looking at a close-up image of sunspots, it is kind of obvious, but nonetheless surprising, that the idea that there is a particular number of discrete sunspots is nonsense.

Early observers had poor observing facilities and could perhaps see discrete spots. But as observational techniques improved it became clear that that there is in fact no unique answer to the question ‘How many sunspots are there?”

The first people to collate sunspot data from different observatories, found that in addition to technological factors, there were ‘personal factors’ varying by up to 50%.

Additionally, it is clear that groups of sunspots are more significant than individual sunspots, and yet one cannot resolve the sunspots within a group. And so the number of groups is also counted.

And what we actually plot is a number called the sunspot index that collates the sunspot number (SN) data from different observatories, count’s the number of groups (GN) as being  as equivalent to 10 sunspots, and applies a personal factor for each observatory based on ‘How they do it there!’

In short, the Sunspot Index, is not ‘data’, but what is known in modern parlance as a ‘data product’.

The purpose of the report was to look back at the procedures applied to the raw data, and to make sure that the comparable procedures were applied fairly to data over hundreds of years.

The final result is a revised Sunspot Index that removes historical biases and more fairly reflects ‘what really happened’. The differences are not very great, but they subtly alter the conclusions that can be drawn from the data.

And because of the link between sunspot number and solar output, these conclusions have implications for our understanding of climate history in the last few hundred years.

2. A surprising response

One conclusion that can be drawn from the revised dataset is that there has been essentially no upward trend in sunspot numbers – and by inference solar output – over the last few hundred years.

So I was surprised that the uber-denialist site Watt’s up with that (WUWT) accepted the revision with barely a comment.

The reason for my surprise is that WUWT routinely rejects the analogous revision of historical temperature data known as homogenisation. This is truly bizarre, because the temperature dataset is vastly more complex and subject to many more systematic biases than the sunspot index.

3. A lesson

The sunspot index provides us with a insight into the activity of the Sun in exceptional detail. However, many times the collation of the data has come close to collapse as observatories asked ‘What’s the point of that?’.

As I wrote last week, at first anyone involved in this kind of ‘pointless’ research is tolerated. But eventually they are seen to be ‘not pulling their weight’ in whatever the latest fashionable institutional activity is. And eventually, they are closed down.

But somehow, the sunspot record survived to an era – hundreds of years after its first records – in which people managed to correlate it with more objective measures of solar activity. And finally its true value was revealed.

So the lesson is this: although scientific activities do need to change over time, it may be hundreds of years before the impact of continuing or ceasing some activity is discovered.

4. A treat

And finally a breathtaking close-up movie of some sunspots as they they traverse the disk of Sun


Sunspot data can be downloaded from the Sunspot Index and Long-term Solar Observations SILSO web site

4 Responses to “Counting Sunspots: How hard can it be?”

  1. Philip Andrews Says:

    Can we have a piece on ” in/out of the EU what would the impact on British science be?

    • protonsforbreakfast Says:

      Mmmm. That’s a good question, and I will think about it. A fair fraction of my work is funded through the EU and involves collaboration with other European laboratories. And in general that is a very positive thing.

      Anyway, I will reflect on whether I have 500 words of wisdom on the subject!

  2. David Edwards Says:

    Piers Corbyn’s weather predictions are based partly on solar observations.
    He doesn’t believe in global warming.

    • protonsforbreakfast Says:

      Piers Corbyn does have rather strange views. He is very critical of the Met Office’s forecasts but – frankly – he has no credibility at all.

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