Observation
Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus); at times Mars appears brighter than Jupiter. Depending on Jupiter's position with respect to the Earth, it can vary in visual magnitude from as bright as −2.9 at opposition down to −1.6 during conjunction with the Sun. The angular diameter of Jupiter likewise varies from 50.1 to 29.8 arc seconds. Favorable oppositions occur when Jupiter is passing through perihelion, an event that occurs once per orbit. As Jupiter approached perihelion in March 2011, there was a favorable opposition in September 2010.
Earth overtakes Jupiter every 398.9 days as it orbits the Sun, a duration called the synodic period. As it does so, Jupiter appears to undergo retrograde motion with respect to the background stars. That is, for a period Jupiter seems to move backward in the night sky, performing a looping motion.
Jupiter's 12-year orbital period corresponds to the dozen astrological signs of the zodiac, and may have been the historical origin of the signs. That is, each time Jupiter reaches opposition it has advanced eastward by about 30°, the width of a zodiac sign.
Because the orbit of Jupiter is outside the Earth's, the phase angle of Jupiter as viewed from the Earth never exceeds 11.5°. That is, the planet always appears nearly fully illuminated when viewed through Earth-based telescopes. It was only during spacecraft missions to Jupiter that crescent views of the planet were obtained.
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Famous quotes containing the word observation:
“I know not anything more pleasant, or more instructive, than to compare experience with expectation, or to register from time to time the difference between idea and reality. It is by this kind of observation that we grow daily less liable to be disappointed.”
—Samuel Johnson (1709–1784)
“It is, in both cases, that a spiritual life has been imparted to nature; that the solid seeming block of matter has been pervaded and dissolved by a thought; that this feeble human being has penetrated the vast masses of nature with an informing soul, and recognised itself in their harmony, that is, seized their law. In physics, when this is attained, the memory disburthens itself of its cumbrous catalogues of particulars, and carries centuries of observation in a single formula.”
—Ralph Waldo Emerson (1803–1882)
“I should say that the most prominent scientific men of our country, and perhaps of this age, are either serving the arts and not pure science, or are performing faithful but quite subordinate labors in particular departments. They make no steady and systematic approaches to the central fact.... There is wanting constant and accurate observation with enough of theory to direct and discipline it. But, above all, there is wanting genius.”
—Henry David Thoreau (1817–1862)