sentences:  Before the launch of X-ray-observing satellites, such as the Chandra X-ray Observatory, X-ray observations generally had quite low angular resolution, but when the Moon passes in front of the nebula, its position is very accurately known, and so the variations in the nebula's brightness can be used to create maps of X-ray emission.When X-rays were first observed from the Crab Nebula, a lunar occultation was used to determine the exact location of their source.The Sun's corona passes in front of the Crab Nebula every June.