Space exploration began with the launch of Sputnik and Astrophysics was born as the application of physics to the phenomena observed by Astronomy, which etymologically means laws of the stars.
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| Activity Index |
Any one of a number of indicators of the level of solar activity at a given time. Indices used to measure solar activity include the Wolf sunspot number, the total area covered by sunspots on the visible hemisphere, and a K-plage index derived from the areas and brightness of plages. In addition, the total radio and X-radiation from the Sun are also regarded as indices of solar activity.
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