C. T. Russell Originally published in the journal Space Science Reviews, Volume 136, Nos 1–4. DOI: 10. 1007/s11214-008-9344-1 © Springer Science+Business Media B. V. 2008 The Sun-Earth Connection is now an accepted fact. It has a signi cant impact on our daily lives, and its underpinnings are being pursued vigorously with missions such as the Solar TErrestrial RElations Observatory, commonly known as STEREO. This was not always so. It was not until the middle of the nineteenth century that Edward Sabine connected the 11-year geomagnetic cycle with Heinrich Schwabe’s deduction of a like periodicity in the sunspot record. The clincher for many was Richard Carrington’s sighting of a great whi- light are on the Sun, on September 1, 1859, followed by a great geomagnetic storm 18 hours later. But was the Sun-Earth Connection signi cant to terrestrial denizens? Perhaps in 1859 it was not, but a century later it became so. Beginning in the 1930’s, as electrical powergrids grew in size, powercompanies began to realize that they occasionally had power blackouts during periods of intense geomagnetic activity. This correlation did not appear to be suf ciently signi cant to bring to the attention of the public but during the International Geophysical Year (IGY), when geomagnetic activity was being scrutinized intensely, the occurrence of a large North American power blackout during a great magnetic storm was impossible to ignore.