![]() ![]() Reported Int Date Dist Date Dist Date Mag Name, Notes ![]() At its brightest, the planet Jupiter appears at magnitude -2.7.ġst Date Obs Perihelion Perigee Brightness Max. The brightest star in the sky (Sirius) has an apparent magnitude of -1.5. Compared to a comet whose magnitude is 4, a 3rd magnitude comet would appear 2.5 times brighter and a magnitude 2 comet would appear 2.5 x 2.5 = 6.3 times brighter still, etc. A diffuse cometary image becomes noticeable to the naked eye when it reaches a magnitude of approximately 3.4 in a dark sky. The following columns give the date and distance when the comet reached its closest point to the Earth (perigee), and the date and apparent magnitude when the comet reached its brightest in a dark sky. One astronomical unit is approximately the mean distance between the sun and Earth. The next two columns give the date and distance in astronomical units when the comet reached its closest point to the sun (perihelion). The following column gives the approximate observational interval (in days) during which the comet remained a naked eye object. The first tabular entry gives the approximate date when the comet was first reported as a naked-eye object. With the single exception of periodic comet Halley, all the tabulated comets have passed through the inner solar system either for the first time or the intervals between their returns are measured in thousands or millions of years. While applying the appellation "great comet" to a particular cometary return is a subjective process, the following Table is an attempt to list the great naked-eye comets that have been reported. In either case, great comets must be seen in a dark sky. An active comet can only become great by making a particularly close approach to the sun so that it produces enormous quantities of gas and dust or by making a close approach to the Earth so that its tail can be easily viewed. However, this activity by itself does not insure that a comet will become a great comet. It is the fluorescing of these gases, and particularly the reflection of sunlight from the minute dust particles in the comet's atmosphere and tail, that can make these objects so visually impressive. However, when near the sun, the icy cometary surfaces vaporize and throw off large quantities of gas and dust thus forming the enormous atmosphere and tails that make comets so visually striking. They are not large enough to be seen with the naked eye. Far from the sun, the solid portions of comets, which consist mostly of water ice and embedded dust particles, are inactive. Just the right set of circumstances must occur. A relative few comets are so visually impressive as to be termed "great comets". ![]()
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