They have been able to apply the tree-ring method to petrifiedtrunks from a nearby fossil forest.
This forest (imagined in an artist’s impression above) was buried by a volcanic eruption 290m years ago, during the Permian period.
And, as they report in Geology, Mr Luthardt and Dr Rossler have found that the sunspot cycle was little different then from what it is now.
The Chemnitz fossil trees, mostly conifers and ferns, are particularly well preserved.
Volcanic minerals seeped intothem soon after the eruptionand petrified them before bacteria and fungi could rot their tissues away.
Mr Luthardt and Dr Rossler selected 43 of the largest specimensand looked at their growth rings.
They found 1,917 rings which were in a good enough state to be measured under a microscope.
They knew that the trees had died simultaneously, giving them a baseline to work from, and so were able to compare the rings from different plants.
They were stunned by how clearly they could see the cycles.
About three-quarters of their specimens showed synchronousgrowth peaks like those caused by modern sunspot activity.
In total, the rings they measured let them study 79 years of forest growth before the eruption.
During this period, the solar cycle averaged 10.6 years.
That compares with 11.2 years in the modern era, although this figure conceals wide variation in the lengths of individual cycles.
Within statistical limits, then, it seems that the sunspot cycle was the same in the early Permian as it is now,
suggesting that the sun’s magneticoscillationswere the same then as they are at present.
Whether that is a coincidence has yet to be determined,
but there is no reason why the method Mr Luthardt and Dr Rossler have developed should not be applied to other petrifiedforests, from different periods, to find out.