Few astronomical phenomena have excited as much attention and awe as the appearance of a comet in the sky. Unlike the motions of the Sun, Moon, and planets that are complicated but predictable, comets appear without warning, occasionally become very bright, move quickly over the sky when they are most noticeable, and sometimes occupy a large fraction of the twilight or evening sky. The Bayeaux Tapestry and numerous woodcuts testify to the awesome, even ominous, apparition this must have provided. Aristotle argued that comets were atmospheric phenomena, but Tycho Brahe's careful positional work on the comet of 1577 demonstrated that this comet was at least three times farther than was the Moon. Comets were thought to be completely unpredictable until the work of Edmund Halley, who was the first to identify the repeated apparitions of ~76 years apart as reappearances of the same comet—the one that now bears his name (see Figure 5.15).

Most comets, indeed, are still unpredictable. Comets are members of the solar system30 at very large average dis

29 Mercury's high eccentricity means that its distance from the Sun varies greatly; for a conjunction near aphelion, its high inclination means a smaller chance to transit the Sun.

30 This is known from the fact that the eccentricities of newly seen comets are less than or equal to 1. An orbital eccentricity of 1 means the object is in a parabolic orbit—it has no more than the bare minimum energy to escape the Sun's gravity. This means that at very large distances from the Sun, they are essentially at rest with respect to the Sun. However, the Solar system is in orbit about the center of the Milky Way galaxy, and therefore, external objects encountering the solar system will have larger than parabolic velocities. Very few such velocities have been measured for comets, and these may be due to perturbations by the giant planets as the comets pass through that part of the solar system (no comet has been detected, thus far, beyond this region because they are still too faint).

Table 5.6. Selected transits of Venus and Mercury.


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