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Several times each century, a comet shines brightly enough to be visible even when the Sun is above the horizon. From the records of the past couple of centuries, it would appear that this beautiful phenomenon is not as rare as a reading of historical records might suggest, and that many daytime comets of earlier years have gone unnoticed and unrecorded.
It may seem ironic that many of the most brilliant comets of history were probably missed when at their peak brightness, but there are actually very good reasons why this might have happened. Because the intrinsic brightness of a comet is so strongly dependent on its distance from the Sun (as explained in Chapter 1), most comets capable of reaching daylight brilliance come to perihelion well within Earth’s orbit and, for the brief time that they shine at their brightest, appear at very small elongations, i.e., very close to the Sun in our skies. The very brightest ever recorded were seen at elongations of less than 3 or 4 degrees. Such...