American Association of Variable Star Observers (AAVSO)
Sun, 04/14/2019 - 01:42

Hi All, 

I had a question regarding AR2738 currently crossing the face of the sun. I've noticed that the umbra appears to have split in two (light bridge?), and was wondering if it should be counted as two spots?

My observations were made in white light with a 6" F/8 Newtonian equipped with a Baader solar filter and continuum filter at the eyepiece  magnification was 50X.  

Thanks in advance. 


Active Region 2738

Hi Steve, 

Good question. There could be a debate about that. I would count them as two spots, but other folks might disagree.  Here are a few guide lines from Frederic Cletter (SILSO). 

  • Umbral splitting:
    • –  Each umbra in common penumbra is counted as a separate spot (Wolfer rule)
    • –  Two umbrae considered as split only if separated by a complete light bridge
    • –  Prone to interpretation
    • –  Can lead to a net bias in the Wolf number



American Association of Variable Star Observers (AAVSO)


Thanks for the response. I'll follow your lead and count it as two spots.