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Starting in the 17th century, early astronomers used steadily improving telescopes to observe the colorful bands and spots that appeared, disappeared, and moved across Jupiter's face. They deduced that the planet must have a substantial atmosphere. By timing how fast these features moved, they estimated that the rotation period of Jupiter--a Jovian day--was about 10 hours.
In the late 19th and 20th centuries, telescopes were powerful enough to enable astronomers to study Jupiter's atmosphere in more detail. They used spectrographs to measure its chemical composition, discovering methane and ammonia. Combined with the fact that Jupiter is low in density, the presence of these hydrogen-rich compounds suggested that Jupiter is mostly made of hydrogen. Juno, of course, will be able to make the very detailed measurements of Jupiter's composition.