Concerning the identification of comet Skjellerup-Maristany

By MAUD W. MAKEMSON
(Research Assistant in Astronomy)

Soon after comet Skjelleru p-Maristany was discovered, elements were computed which made its identity with comet de Vico 1846 IV seem almost certain. Not only were the planes of the orbits similar, but the perihelion distance q was found to be the same, 0.6 of an astronomical unit. Later computations, based on observations extending over a longer period, have produced values for q between 0.3 and 0.14 A.U., and it is now definitely established that the true value lies below 0.2.

Under the direction of Professor Leuschner, an orbit determination for comet Skjellerup-Maristany was made at the Students' Observatory by Messrs. Mayall, Miles, and Whipple, all graduate students, while to me was given the opportunity of investigating the comet's identity.

Fortunately, the elements of comet de Vico were well determined by several noted astronomers, including Hind, Bond, Bren, Peirce, and von Hepperger, and the agreement between their independent computa­tions leaves no doubt as to the orbit constants. If the two comets are identical, the de Vico comet must have a period of 81.6 years instead of 75.6 years as given by the elements. Assuming an average daily motion (43".3874) to correspond with this period and using the elements by von Hepperger, I found that the orbit of comet de Vico could not be made to fit the observation of 1927 Dec. 19 unless the perihelion distance were reduced to one-third its original value. By taking q equal to 0.173 — computed from the observations of Dec. 19-22 — the mean motion as before, and the semi-major axis and eccentricity to correspond, and combining these with the constants to the equator for 1927.0 from the 1846 elements, I found that the observation of December 19 could be represented closely for a time of perihelion passage be­tween December 17.0 and 18.0. This result merely states that the two comets are moving in the same plane, as is confirmed by the elements subsequently computed at the Students' Observatory and elsewhere.

The important thing was to try to ascertain what might have hap­pened to comet de Vico during the 81.6 years to lengthen its period five years and to bring it closer to the sun at perihelion.

Two graphs, the first tracing the path taken by the de Vico comet in 1846 out to aphelion in 1884 and back to perihelion in 1921, the second following comet Skjellerup-Maristany back from 1927, imposing on it a period of 81.6 years, failed to reveal a close approach to a known planet.

As a check on this computation, I found the times when the hypo­thetical comet would have crossed the orbits of the major planets out­ward and inward bound. The results are tabulated here with the angu­lar distance between comet and planet as seen from the sun:

Figure 1.

Figure 1 illustrates the path which the comet would have traveled between 1846 and 1927 and the relative positions of the major planets at dates corresponding to those in the table. Since the inclination of the orbit to the plane of the ecliptic is approximately 80°, its projection on the planet orbits is here represented by a straight line. The line of nodes is so situated that only a small portion of the path is north of the ecliptic. The ascending node is close to perihelion.

Figure 2.

Figure 2 shows the tremendous sweep of the comet's path about the sun. The heliocentric arc of 197°, covered in 26 days, was the basis for the widely-varying preliminary orbits derived at different observatories. The difficulty of computing an orbit for a rapidly moving comet was increased by inaccuracies of observation due to indefiniteness in the set­tings. The drawing of this object by Graff in Astronomische Nachrichten, 232, No.5549, illustrates this difficulty.

During the observed interval represented in this figure, the comet changed its heliocentric latitude from —70° to +70° approximately.

Neptune, which might have caused all the disturbance, was on the opposite side of the sun during the entire episode. Furthermore, there is very little circumstantial evidence that a trans-Neptunian planet is the disturbing body, since the comet only went about eight astronomical units beyond Neptune's orbit and was in latitude —12.8 degrees at aphelion.

One may then conclude only that the comets de Vice and Skjellerup-Maristany move in practically the same plane, which is inclined almost at right angles to the plane in which the planets move.

More interesting is the unanswered question: Why was not the dc Vico comet observed if it returned to the sun in 1921?

Until more observations are obtained and the elements of the orbit of comet Skjellerup-Maristany are definitely known, speculation as to its identity may be postponed. At present, owing to the approximate nature of the observations, computers do not agree as to the perihelion distance or longitude, and even the inclination seems to be in doubt.

With definitive elements, it should be possible to trace backward the path along which the comet Skjellerup-Maristany made its swift flight into the central region of the solar system and find whether or not it intersects the orbit of de Vico's comet. For the present, the computer can only await further observations.

Students' Observatory, Berkeley, California.

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