Before the introduction of the photographic plate as an astronomical observational technique, cometary research in the 19th century had to be confined to the determination of cometary orbits, to visual photometry, and to the study of cometary tails (see Jaegermann 1903). Bessel (1836) showed that the shape of cometary tails could be explained if one assumed that as well as gravity, a force of repulsion that also declined as the square of the distance from the Sun, acted on the material in the tail. This repulsive force, or rather the ratio K between this repulsive force and gravity, depends on the nature of the particles in the tail, and in addition each particle may be described in terms of the time τ at which it leaves the cometary nucleus. Within the framework of this “mechanical theory”, curves of equal K were described as syndynes, and curves of equal τ as synchrones. If we know the position of a tail particle on the plane of the comet’s orbit, then mechanical theory allows the corresponding parameters K and τ to be determined (although unfortunately not always unambiguously). Bredikhin (see Jaegermann 1903) established that the tail particles may be divided into two types, Type I with K > 10 and Type II with K ≤ 3. He also introduced Type III for very small values of K, but this division is no longer considered to be valid and so plays no part in our discussion. Bessel and Bredikhin assumed that the force of repulsion was electrical in nature. Arrhenius (1900) recognized radiation pressure as being the source of the force of repulsion, and this explanation is still valid for Type-II tails.
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