The Great Comet of 1947

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Nature Magazine, March 1948

In the last month of 1947 there suddenly appeared, close to the setting sun, a truly great comet, the brightest to be observed since Halley's comet made its latest spectacular return to perihelion in 1910. So many people saw the new comet at the same time that no one observer could he credited with the discovery. Following the practice of naming comets alphabetically in the order of discovery in the current year, the new comet soon received the name of 1947n. It was, as its name indicated, the fourteenth, and, as it turned out, the last comet to be discovered in 1947. This sets a record, it is believed, for number of comets discovered in anyone year. Some of these were periodic comets returning to perihelion in 1947, including among them Encke's famous short-period comet (1947i), which attained maximum brightness late in November, when it was just at the limit of visibility without telescopic aid. Most of the comets of 1947 were visible only in the greatest telescopes.

At the Boyden Station of the Harvard College Observatory at Bloemfontein, South Africa, comet 1947n was first seen on the evening of December 8, and was described as having a bright nucleus and a tail extending to a distance of 30 or 35 degrees. To the Harvard College Observatory, which is a clearing house for astronomical news in this country, came numerous reports of observations of the comet from all over the southern hemisphere. Observatories in South Africa, South America, and Australia sent accurately determined positions. On December 12, a ship in the Pacific, northeast of New Guinea, reported that a large comet had been sighted, clearly visible to the naked eye, about 8 degrees above the horizon with a tail about 5 degrees long. It was then in the constellation of Sagittarius. Telegrams and announcement cards were sent from the Harvard College Ob-servatory to observatories, and to individuals who have arranged for this service, giving latest and most accurate positions of the comet, as is customary when any new object is discovered.

Three observations are all that are required to obtain an approximate orbit for the newly discovered object. It is assumed, at first, that it is moving in a parabolic orbit and making but one visit to the sun. Not until the object has been observed over an arc of its orbit of considerable length is it possible to determine whether it is moving in a parabolic orbit or an elongated ellipse. In the latter case the comet will return again to the sun, although the elliptical orbit may be so elongated that its period may be thousands of years long.

Within a few days of its discovery a number of independently computed orbits of the comet were available, and the results of the calculations were published on Harvard Announcement Cards. These were in good agreement and showed that the comet had passed perihelion on December 2, before it had been observed, and its approximate distance from the sun on that date was about ten million miles. Its motion was retrograde so it was moving in a west to east direction, which is contrary to the usual direction of motion of the members of the solar system. Comets, however, do not conform to the habits of the better regulated members of the sun's family, and; as often as not, move contrary to the usual direction of orbital motion. The orbit may be both greatly elongated and inclined at an angle of more than 90 degrees to the ecliptic, in which case the motion is retrograde.

It is at this time too early to say whether the great Comet of 1947 will return to the sun eventually, or is now headed toward interstellar space. The comet will be followed by large telescopes long after it has disappeared from view in small telescopes. Between December 15 and 20 observations were obtained at the Lowell Observatory at Flagstaff, Arizona, and the McDonald Observatory, a branch of the Yerkes Observatory on Mount Locke, in Texas. By this time the comet had lost much of its early splendor. So far south was it, in the days immediately following its discovery, that observers in the mid-latitudes of this hemisphere awaited in vain for a glimpse of it, in the southwest following sunset. When it did finally appear above the southwestern horizon it had already faded to 4th magnitude, and before the end of the year was visible only telescopically. This does not mean, however, that much information may not be obtained from spectroscopic observation of the changes that may take place in the body and its physical composition. Spectrograms taken at McDonald already show bands due to the presence of ammonia. Early observations showed a split in the bright nucleus of the body, as if it were splitting into two parts physically, or the head was composed of more than one mass. The early observations were confirmed by later observations made in this country, and at observatories in the southern hemisphere as well. So rapidly has the comet been moving since it first appeared spectacularly in the immediate vicinity of the sun, that by January 15 it was passing from Capricornus into Aquarius and was more than 130 million miles from the earth. Before long, it may be expected, there will be published better deter-mined orbits of the comet, and an ephemeris, or day by day positions, of the object in the heavens for the benefit of those who wish to follow its path through the heavens as it moves away from the sun and the earth. For a great comet the splendor of 1947n has been of extremely short duration, and its unfavorable position with respect to northern observers when it was at its best has been exceptionally disappointing to them.

Another comet of 1947, known as Bester IV, since it is the fourth comet to be discovered recently by this observer, or also as Comet 1947k, promises to be visible to the naked eye in this hemisphere, when it comes to perihelion on February 16, and to remain visible possibly well into April before it again becomes a telescopic object. It is difficult to predict so far in advance, with any certainty, how bright a comet will be, since these objects at times change suddenly and unaccountably in brightness. This change may arise from an encounter with some resisting medium in space, it has been suggested, or to some physical change taking place in the comet itself.

Comet Bester IV, at the time of discovery late in September, was of about the 11th magnitude, and was in Eridanus to the southwest of Orion and moving southward. For some time it has been visible only in the southern hemisphere, but when last seen in northern latitudes, early in November, it was of about the 9th magnitude and had a broad tail a quarter of a degree in length. At the time of its perihelion passage on February 16 it will be lost to view in the rays of the sun, but will appear shortly after in the morning sky in Capricornus, and may at that time he as bright as a star of the second magnitude. During March it should remain visible to the naked eye and will pass during the month from Capricornus through Aquila into Cygnus. It will continue to move in a northwesterly course during the spring, and by May 1 will be above the bowl of the Big Dipper in circumpolar regions, having passed during April from Cygnus through Lyra into Draco and Ursa Minor. It will gradually decrease in brightness during March, and may become visible only telescopically early in April.

Mercury will be a morning star throughout March, and will be at greatest western elongation on March 17. This is a very unfavorable elongation, however, for the planet will be less than ten degrees above the horizon at sunrise. Venus is now a magnificent object in the evening sky, selling about four hours after the sun in the middle of March. It is steadily increasing in brightness and drawing farther away from the sun. The planet Mars is now well up in the eastern sky at sunset a few degrees northwest of Regulus. It is now decreasing in brightness, and by the end of March will be less brilliant than Regulus. Jupiter now rises shortly before midnight in Sagittarius, and will be seen low in the southern sky for the remainder of the month. Saturn is in Leo, a few degrees northwest of Regulus and a little southwest of Mars, and is fully a degree fainter than Mars. It has been moving westward among the stars, or retrograding, since the beginning of the year.

Spring will commence in the northern hemisphere on March 20, at 11:57 A.M., Eastern Standard Time, when the sun crosses the equator coming north. The sun is then in the constellation of Pisces, but the sign of Aries.