Small Meteors Research Articles

The Lunar Surface as an Impact Counter

The dust cover of the Moon sticks to mountain slopes and cannot possess any degree of fluidity. It is probable that only a limited migration of the dust from the slopes into adjacent valleys takes place, caused by meteor bombardment. The material of the micrometeors cannot escape from the Moon and is expected to form a layer of the order of 40 gr/cm 2 , accumulating over the maria and protecting them from further erosion by small meteors. Large meteorites are not stopped by the dust and produce meteor craters; at a velocity of 20 km/sec the ratio of crater to projectile diameter is estimated to be about 20. 812 crater diameters, measured over an area of 465,000 km 2 in Western Mare Imbrium, show a frequency distribution which is close to that predicted from the present population of interplanetary space and a time interval of 4.5 × 10 9 years. It is concluded that since the formation of Mare Imbrium the population of interplanetary space has not changed appreciably; a tentative explanation is given.

An Antenna Array for Studies in Meteor and Radio Astronomy at 13 Meters

The design of a high-frequency radar antenna making possible the detection of echoes from the ionized trails produced by very small meteors is described. Preliminary results indicate that the radar system, when fully completed, will be capable of detecting meteors down to the 15th visual magnitude, thus making possible radio studies of meteors five visual magnitudes smaller than the smallest which have heretofore been systematically investigated at back reflection. The results of these studies are expected to provide new information on the amount and distribution of interplanetary matter, the contribution of meteoric ionization to the lower ionosphere, and the radio propagation characteristics of vhf ionospheric scatter and meteor burst communication systems. The results of some preliminary measurements of meteors down to the 14th magnitude are presented. Other possible applications of the antenna are discussed.

Meteoric Ionisation in the E Region of the Ionosphere

IN a letter which recently appeared in these columns1, T. L. Eckersley described the discovery of great numbers of short-lived ionic clouds in the upper atmosphere. He suggests that the responsible agents might be small meteors or high-speed particles of cosmic origin. His data support the assumption of a meteoric origin so strikingly as to warrant special emphasis.

Our Astronomical Column

THE APRIL METEORS.—These meteors are due on April 20 or 21, but the moon, being full on April 23, ‘will be a bright object at the time and obscure small meteors. However, the shower occasionally ex hibits brilliant objects, so that it may be well worth looking for though the character of its display this year cannot be definitely foretold. The period of revolution of its supposed parent comet was com puted to be more than 400 years, but rich showers of Lyrids were witnessed in 1803, 1851, 1863, and other years, so that a short period apparently corresponds with some of the most abundant returns of the meteors. It is important to note the strength of the annual displays, so that the time of revolution of its more active returns may be determined. Its radiant is at 271°+ 33° on the night of maximum, but the centre of radiation travels eastwards one degree per day.

Our Astronomical Column

METEORS AND METEORITES.—The Nineteenth Century for September contains an interesting article by Mr. A. R. Hinkg on meteors and meteorites, suggested, as he says, by Prof. Olivier's recent book on the subject. The book regards large and small meteors as members of the same class, but Mr. Hinks gives reasons for his dissent from this view. He recalls with approval Sir Robert Ball's suggestion that the larger meteors may have been expelled from terrestrial volcanoes in long past ages. Their orbits would continue to lie near that of the earth, so that an eventual return to it would not be improbable. He also regards as inconceivable the idea that meteors of the complicated ‘plum-pudding’ structure could have originated either in the sun or in interplanetary space; they must have been formed on some planetary body, and the earth is the most obvious suggestion. His argument might also be used to support Proctor's view that the comets of short period had been expelled from Jupiter and the other giant planets.

The Perseids of 1909

THE weather, fortunately, took a very favourable turn on August 3, and observations have been obtained nearly every night since, though moonlight has rather seriously interfered with the work and obliterated many small meteors. Up to August 9, and including that date, the display of Perseids was decidedly poor, and quite disappointed expectation. On August 7 and 9, particularly, there appeared to be very few meteors, but the moon was shining rather strongly in the east.

On the Statistical Dynamics of Gas Theory as Illustrated by Meteor Swarms and Optical Rays 1

IMAGINE a cloud of meteors pursuing an orbit in space under outside attraction—in fact, in any conservative field of force. Let us consider a group of the meteors around a given central one. As they keep together their velocities are nearly the same. When the central meteor has passed into another part of the orbit, the surrounding region containing these same meteors will have altered in shape; it will in fact usually have become much elongated. If we merely count large and small meteors alike, we can define the density of their distribution in space in the neighbourhood of this group; it will be inversely as the volume occupied by them. Now consider their deviations from a mean velocity, say that of the central meteor of the group; we can draw from an origin a vector representing the velocity of each meteor, and the ends of these vectors will mark out a region in the velocity diagram whose shape and volume will represent the character and range of deviation. It results from a very general proposition in dynamics that as the central meteor moves along its path the region occupied by the group of its neighbours multiplied by the corresponding region in their velocity diagram remains constant. Or we may say that the density at the group considered, estimated by mere numbers, not by size, varies during its motion proportionally to the extent of the region on the velocity diagram which corresponds to it.

An Extraordinary Meteor

I BEG to send you the following, in case you consider it worth inserting:—At about 1.10 a.m. on the night between November 18 and 19, whilst going in the s.s. Bokhara in the Red Sea, about midway from Aden to Suez, the quarter-master on duty called me, saying he had just seen a new comet, or shooting star, which was still visible many minutes after its first appearance. He said that whilst he was looking out ahead, or in a northerly direction, he suddenly noticed the effect of a bright light shining from astern, and on turning round saw a very bright shooting star still moving from left to right, and slightly downwards, in the south, at an altitude of about 40°. The star speedily disappeared, but left a bright train of light behind it, which continued so long (from five to ten minutes he gue sed) that he thought I might like to see it. I came on deck a little before a quarter past one by the ship's clock, and found a streak of light which I estimated as 8° or 10° in length, and rather less than half a degree in width, apparently stationary, midway between Sirius and Canopus, and nearly as bright as the comet, the head of which must have risen half an hour or more previously. I watched the streak till half-past one o'clock, when it seemed sensibly fainter, though still a conspicuous object, notwithstanding the presence of the moon, the comet, and a number of bright stars. Whilst watching I noticed two small meteors shooting from left to right across the southern sky, which struck me as probably belonging to the same group as the large one whose train I was watching.

The Aurora and its Spectrum

You have already illustrated in NATURE the aurora and electric storm of January 31. Though somewhat tardily, another drawing with a description of this fine display has found its way into my hands, of sufficient interest, I think, to be added to those already published. It comes from Mr. C. L. Prince, the well-known meteorologist of Crowborough Beacon, Tunbridge Wells, who says, “I inclose a photo of my sketch on a slate.1 I had a splendid sight of the aurora at about 9h. 15m. It soon became very hazy, but I saw it again at 11h. 15m., when I made the sketch (see drawing). The arch was exceedingly well defined, and extended about 80° along the horizon. At 11h. 20m. some more brilliant streamers shot up along the whole convexity of the arch, and the two patches of light became very tremulous, almost shifting a little from right to left; but I particularly noticed that they did not vibrate simultaneously, i.e. if one indicated motion the other was quiescent until the first had ceased to show excitement, and this action was alternating for nearly an hour. At 11h. 40m. the arch had much contracted, and by midnight had nearly faded away. The whole phenomenon was free from colour. I noticed a few small meteors. The night was quite calm; wind south-west.”

The Aurora and its Spectrum

You have already illustrated in NATURE the aurora and electric storm of January 31. Though somewhat tardily, another drawing with a description of this fine display has found its way into my hands, of sufficient interest, I think, to be added to those already published. It comes from Mr. C. L. Prince, the well-known meteorologist of Crowborough Beacon, Tunbridge Wells, who says, “I inclose a photo of my sketch on a slate.1 I had a splendid sight of the aurora at about 9h. 15m. It soon became very hazy, but I saw it again at 11h. 15m., when I made the sketch (see drawing). The arch was exceedingly well defined, and extended about 80° along the horizon. At 11h. 20m. some more brilliant streamers shot up along the whole convexity of the arch, and the two patches of light became very tremulous, almost shifting a little from right to left; but I particularly noticed that they did not vibrate simultaneously, i.e. if one indicated motion the other was quiescent until the first had ceased to show excitement, and this action was alternating for nearly an hour. At 11h. 40m. the arch had much contracted, and by midnight had nearly faded away. The whole phenomenon was free from colour. I noticed a few small meteors. The night was quite calm; wind south-west.”

Auroral Phenomena

IT is perhaps worth a note that my daughter saw at Folkestone a very unusual phenomenon on the evening of January 25, a little before 6.30. Some distance to the left of Orion (for the night was clear and starry) she observed a small cloud of a bright golden hue, from which streamers of great brilliancy darted in various directions, the cloud alternately paling anc brightening. She describes the streamers as like small meteors, leaving trails of light behind them.

Meteors in New Caledonia

DURING the last few nights we have seen numerous flights of small meteors; indeed, so frequent have they been, that they have attracted the notice of the most casual observers. I first observed them on the night of the 9th inst. No fixed direction seems to be followed; in fact, I saw one display such as I have never seen before, which will illustrate my meaning. Two fair-sized meteors proceeded severally from the neighbourhood of Castor and Pollux, and crossed mid-way between those two stars. To me it was a very interesting sight.

On the aurora borealis which occurred on the evening of Friday, the 17th of November, 1848

The author states that the 17th of November was a fine day with a clear sky and bright sunshine: towards evening the sky became cloudy and a few drops of rain fell, but it soon again became clear, with the exception of a few fleecy clouds that here and there dimmed its brightness. At 6 h 45 m a soft and gentle light began to illumine the northern region of the sky; and at 7 o’clock a considerable portion of it was covered with dark-red streams of light towards the east; while streamers moving to and fro, arrayed in colours of golden and silvery hues, overspread the south and north. About 8 o’clock there appeared near the zenith, and upon the magnetic meridian, a ring of an elliptical form, from which proceeded in all directions towards the horizon, beams or columns of light, giving to the heavens the appearance of a splendid vault, with its top adorned with a crown or wreath; while around and within the vault were to be seen clouds of brilliant light flashing towards and from the crown or central circle of the aurora, sometimes tinged with prismatic rays, at other times intensely white and lucid. About half-past nine nearly the whole of the aerial canopy was clad with clouds of a bright red colour, casting a curious reddened hue over the objects on the surface of the earth. After a short period of time had elapsed, the red colour began to diminish in intensity, and was again replaced by the white dome. However, in various parts of the sky the red colour still remained, principally in the north-west, southwest, and north-east. Between the hours of twelve and one beams of brilliant white light commenced shooting up in the south from the horizon to the central ring or pole. The beams appeared to be at nearly equal distances from each other, the entire column of them stretching over a space equal to about one-fifth part of the visible horizon, in the form of a fan. The whole figure rapidly changed from a pure white light into a glow of brilliant colours of every tint, variegating the undulating waves as they rolled on their way to the pole of the aurora. In the course of three minutes these gave place to the white flashing radiations. During the time of the aurora there were a great number of small meteors, the direction of whose motion was from east to west, and which appeared to be considerably below the sphere of the aurora.