Reply To: Measuring R number for the Sun

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Peter Meadows

Hi Duncan,

Thanks for your questions about how to determine the sunspot number R from your observations. The general guidance about counting the number of groups is, as you say, based on the longitude and latitude separation of sunspots: 10 deg in longitude and about 5 deg in latitude. In the case of AR3105 and 3107, the center of these groups in longitude are more than 10deg apart but less in latitude. Certainly using the annotations on SDO HMI image from can be useful to gain some experience of group separation but with experience this can be done oneself.

For sunspots with penumbra, it is the number of umbrae that should be counted towards the number of sunspots S. So if there is a penumbral sunspot with 3 umbrae, then this sunspot will contribute 3 to S. Individual sunspots, however small, also count towards S.

Yes, the observing equipment & conditions, the observer all contribute to each observers R value. It will also depend on whether an observers uses a low eyepiece for the full disk and then a high magnification eyepiece for the smaller sunspots. The key is to be consistent with your observing procedure from observation to observation, otherwise your daily R values will differ depending on how you observe. Each observer will have their own R values which will be different from other observers even from the same day – this is the advantage of combining observations from many observers as is done within the solar section.

You are welcome to add your daily observations to the BAA Solar Database which are collated monthly and published in the monthly newsletter (see attachment) and in the Journal. See for more details.

BAA Solar Section Assistant Director