Meteor

The Leonids.

The Leonids being the best known of all me teor streams, their history will be briefly traced. The following table gives the dates of appearances of bright showers: It is apparent that while the showers certainly come at inter vals of about 33 years, still there are long gaps. These may be explained in two ways ; first that no record survives due to mere chance, second that the meteor stream missed the earth in these years. A fine return was hoped for in 1899, but very few Leonids were seen. It was, however, proved that the main group of Leo nids, which normally would have met the earth in November 1899, had passed near the giant planet Jupiter en route towards us, and so had been switched aside somewhat by its attraction. This caused them to miss the earth. By the time the part of the stream that met us in 1901 passed Jupiter's orbit, the planet had passed on too far to disturb appreciably these meteors, so they appeared in considerable numbers. Indeed a few Leonids are met every November, so some must be scattered all around their orbit, but the dense part that can give us a really fine shower is condensed into a relatively small part of the whole circumference.

That the Leonids are not yet uniformly distributed, but are mostly included in a small group, seems to prove that they were comparatively recently turned into their present orbit, possibly by Uranus in A.D. 126. The predicted shower of 1932-34 fell on Nov. 16, 1932, though Jupiter's attraction again reduced the rate of fall, to approximately 3o per hour.

Biela's Comet suddenly divided into two parts in 1845. On its next return in 1852 the two parts were seen to be about equally bright and were a million miles apart. The comet was not found at the predicted return of 1859 or 1866, but on Nov. 27, 1872, a splendid display of meteors was seen. Calculation showed that these Andromedes, as they are now called, followed the same orbit as the lost comet which had had a period of 6f years. An ot'ler fine display, notable in that during it a piece of meteoric iron fell at Mazapil in Mexico, occurred on Nov. 27, 1885, and another lesser one on Nov. 23, 1892. On Nov. 24, 1899, a fair shower was seen, but, from then on, the earth has never encoun tered any considerable number of these meteors. Nor has Biela's Comet reappeared. Older records show showers that were prob ably due to this group on Dec. 6, 1741, Dec. 7, 1798 and Dec. 7, 1838. The main groups have either been broken up or switched so far from the earth's orbit that we no longer meet them.

Lyrids, Perseids and Others.

There is an annual shower of Lyrids, with a radiant near the star Vega, which in most years is inconspicuous, but occasionally abundant. For instance, on April 20, 1803, it furnished a very brilliant display, and a fair one on April 21, 1922. Ancient annals give us nine other great showers on corresponding dates, the earliest being in 687 B.C. The best known stream that is practically sure to give us a good annual display is the Perseid. These meteors, which are seen late in

July and through the first half of August, have been extensively observed for the past hundred years. Their maximum comes on Aug. 11 or 12, when as many as 120 per hour sometimes may be counted. Definite daily motion of a radiant point was first proved for the Perseids. Many of these meteors leave persistent trains, and owing to their appearing in the summer for northern observ ers, they may be specially recommended to the amateur.

The Orionids which appear during the last half of every Octo ber, and the Geminids which appear the first half of every De cember, complete the list of really conspicuous meteor streams.

The Orionids have been the subject of much controversy as to whether their radiant was in motion or stationary. The prepon derance of evidence now points to daily motion. For the Geminids motion of the radiant has been generally conceded. At the max ima of both streams, from 20 to 40 meteors per hour may fall when conditions are favourable. Others are the Quadrantids, Jan. 1-3 ; the Eta Aquarids, May r---r ; and the Delta Aquarids, July 27-31. The Eta Aquarids, connected with Halley's Comet, are the most important of the three. The great shower of Oct. 9, 1933, in Europe was associated with the Giacobini Comet.

Orbits and Perturbations.

Meteor showers cannot be ex pected to return an indefinite number of times. From their very nature they are not permanent, and we have already noted the case of the Andromedes, which, after furnishing several fine show ers, seem to have died out, as it were. There are several known reasons for this ; the most obvious that the stream misses the earth entirely, owing to its having been pulled aside by the at traction of some planet, the second that being at best a very loose aggregation of solid particles, their mutual gravitation is small. Hence when a group passes near a planet those nearest are more pulled aside by its attraction than those farther off, and as this must continually recur, eventually the group is widely scattered.

Meteor Radiants.—The possibility of a radiant remaining fixed in the same apparent place in the heavens has been seriously investigated. The existence of numerous so-called "stationary radiants" has been insisted upon by several experienced observers, and as firmly denied by many others. The reason why a radi ant should shift daily is, that even if all meteors from it for many days or weeks are moving in practically parallel paths, yet the earth in its orbital motion around the Sun changes its direction about I° daily. The position of the apparent radiant de pends upon the directions of motion of both earth and meteors. Hence if either changes, in general the radiant must shift. It has been proved, indeed, that a radiant, which lies near the ecliptic, can remain nearly stationary for some weeks.