This particular cycle, cycle 25, appears to be "significantly more active than what people predicted", says Müller, with the relative sunspot number – an index used to measure the activity across the visible surface of the Sun – eclipsing what was seen as the peak of the previous solar cycle. The National Oceanic and Atmospheric Administration (Noaa) in the US had predicted a maximum monthly average of 124 sunspots a day in May, but the actual number was 170 on average, with one day exceeding 240, according to Müller.