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The article “This Month in Physics History” (APS News, August/September 2019) described the earliest observations of cosmic rays by Theodor Wulf and Victor Hess. The text states that “[Hess] determined that the intensity increased significantly with height, and his radiation measurements during a solar eclipse effectively ruled out the sun as the source of these cosmic rays.”
Those observations by Hess may qualify as the first example of multi-messenger astronomy, since they combined visual astronomy (the solar eclipse, and shielding of the Sun by Earth at night) and particle astronomy (using ionization detectors). Even if Hess did not understand the particle-like nature of the "cosmic rays" he was detecting, he still sensed it was some form of “penetrating radiation” and could conclude that some part of it did not come from the Sun but from elsewhere in the cosmos.
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