It is time for Starwatch to check once again on the star we know best, our Sun, and its current level of sunspot activity. I have warned repeatedly, though, about the dangers of solar observation. To look directly at the Sun through an unprotected telescope or binoculars is to guarantee serious eye damage.
The traditional method of solar observation is to project the Sun’s image through binoculars or a small telescope onto a white card held away from the eyepiece. However, I now recommend the use of inexpensive but accredited solar filters that cover the objective (Sun-facing) end of the telescope and block 99.999% of the Sun’s radiation. Provided these are fitted securely, with no light-gaps, holes or scratches, they provide a safe viewing experience and a better image than is usual by projection.
Sunspots, the most obvious features we see, are regions that form when the Sun’s magnetic flux inhibits the heat flowing from below so that they cool below the 5,780K (5,507C) of the remainder of the Sun’s visible surface, the photosphere. A spot can last for a day or two, or up to a month or more, and they often occur in groups, drifting east to west across the Sun as it rotates every 26 days as seen from the Earth.
One useful index of the level of solar activity is provided by the so-called sunspot number. It is based on the sum of the number of separate spots and ten times the number of groups and, after making allowance for different observers and their telescopes, it lets us trace activity back almost continuously through several centuries.
Our graph plots a rolling average of the mean sunspot numbers since 1955 and illustrates the solar cycle of about 11 years. The highest sunspot numbers on record occurred around the sunspot maximum of 1958 while the latest cycle began in 2008 and is the lowest since one that peaked in 1906.
As with several recent cycles, the current one shows a double peak, in 2012 and 2014, but is now certainly in decline towards the next sunspot minimum in perhaps 2020. Our image is from 21 July and shows the most recent sizeable sunspots before they disappeared around the Sun’s edge three days later. There has been at least one spotless day since, and already more this year than since 2010.
