The moon Io is the innermost of the four moons of Jupiter discovered by Galileo in January 1610. It orbits Jupiter once every 42½ hours, and the plane of its orbit is very close to plane of Jupiter’s orbit around the sun – an eclipse. This means that it passes some of each orbit in the shadow of Jupiter – an Eclipse of Io – which is called an immersions of Io. At the point of opposition, both the immersion and the emergence would be hidden by Jupiter. For about four months after the opposition of Jupiter, it is possible to view emergences of Io from its eclipses.
About Rømer’s determination of the speed of light in brief
The discovery of the speed of light was made in 1676 by Danish astronomer Ole Rømer. He used the eclipses of the Jupiter moon Io to prove that light has a finite speed and so does not travel instantaneously. His theory was controversial at the time he announced it, and he never convinced the director of the Paris Observatory, Giovanni Domenico Cassini, to fully accept it. But it quickly gained support among other natural philosophers of the period, such as Christiaan Huygens and Isaac Newton. It was finally confirmed nearly two decades later, with the explanation in 1729 of stellar aberration by the English astronomer James Bradley. Viewed from the Earth, an eclipse of Io is seen in one of two ways. At the point of opposition, both the immersion and the emergence would be hidden by Jupiter. For about four months after the opposition of Jupiter, it is possible to view emergences of Io from its eclipses. But for about five or six months of the year, it is impossible to observe the eclipsed of Io at all because Jupiter is too close to the sun. Even during the periods before and after opposition, not all of the eclipsees can be observed from a given location on the Earth’s surface: some eclipses will occur during a given daytime for a given given location, while other will occur while Jupiter is below the horizon. The key was that the time elapsed between eclipses was not constant, but varied slightly at different times of year, at times when it was slightly different at the same place on the planet.
It is not possible to see both the eclipse and emergence for the same eclipse on the same day, because one or the other will be hidden by Jupiter itself. The moon Io is the innermost of the four moons of Jupiter discovered by Galileo in January 1610. It orbits Jupiter once every 42½ hours, and the plane of its orbit is very close to plane of Jupiter’s orbit around the sun – an eclipse. This means that it passes some of each orbit in the shadow of Jupiter – an Eclipse of Io – which is called an immersions of Io. The Io eclipse can be seen in the night sky in the early hours of the morning or in the afternoon in the late afternoon and early evening. It can also be seen at night in the daytime in the evening, when Jupiter is at its closest approach to the Sun. This is known as the Transit of Jupiter, or the transit of Jupiter or the Transit of Jupiter in the early evening, or night time in the middle of the day. The Eclipse of Io can be viewed in the dark by using a special type of telescope known as a scintillator, which can be used to track the position of the moon in the sky at night. This can be done by using the time of day, and thus longitude, based on the times of the eclipses of the Galilean moons.
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