Smithsonian

Contributions to Astrophysics

VOLUME 3, NUMBER 2

CATALOGS OF METEOR RADIANTS

by GERALD S. HAWKINS

SMITHSONIAN INSTITUTION

Washington, D. C.

1958
Publications of the Astrophysical Observatory

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Another series is Annals of the Astrophysical Observatory. It was started

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Among these are Smithsonian Physical Tables, Smithsonian Meteorological

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Catalogs of Meteor Radiants

By GERALD S. HAWKINS1

A large percentage of the meteors that

fall on the earth do not belong to the major

showers and we may class them as "sporadic"

meteors. A recent study (Hawkins, 1956a,

1956b) of sporadic meteors has shown that

their individual radiant points cluster towards

the ecliptic and that their orbits tend to lie in

the plane of the solar system. This study was

carried out by radar methods in which pulses

of radio energy were reflected from the ionized

trail of the meteor. By reinterpreting the

visual observations which have accumulated in

the past 150 years or so, it is possible to find

out more about the distribution of sporadic

orbits.

Early observers, prior to 1900, recorded the

direction of flight of meteors and attached

great significance to the intersection of a group

of paths when they were drawn on a star chart.

In the case of a meteor stream, of course, an

intersection defined the direction of the tangent

to the orbit of the stream and was known as a

"group" radiant. A radiant determined in

this way, however, becomes extremely unreliable in the case of weak streams and in most

cases would represent only a meaningless,

chance intersection of unrelated meteor paths.

For this reason, almost every radiant recorded

in the great catalogs of the early meteor

astronomers is spurious. But although the

radiants are fictitious the data are not entirely

worthless because they do indicate in a statistical sense the general distribution of sporadic

radiants.

A total of 3,035 radiant points is recorded

in the catalogs of Denning, Backhouse,

Bartfay, Corder, Denza, Gregg, Gruber, Herschel, Heis, Kobold, Konkoly, Konvesligethy,

Mftggi, Neumayer, Sawyer, Serpieni, Schiap,irelli, Schmidt, Tupman, Weiss and Zesioli

1

 Harvard College Observatory and Boston University.

(see Denning, 1886). The number of radiants

lying in 10° intervals of declination S is given

in figure 1. Since these observations were

made in mid-northern latitudes the visibility

of the radiants becomes progressively worse as

8 decreases from 90° at the celestial pole.

The table gives a correction factor which takes

account of the time a given part of the sky

is above the horizon and also allows for the

variation of meteor rate with elevation of the

radiant point (Hawkins, 1952).

Declination

—20°

- 1 0

0

10

20

30

40

50

60

80

Correction

factor

8.4

4.5

3. 1

2.3

1.9

1.6

1.4

1.2

1. 1

1.0

This correction factor has been applied to

the radiant distribution in the great catalog

of Denning. Figure 1 shows the resulting

curve, which indicates that the radiants tend

to concentrate at declinations +20° and +45°.

The first peak is considered to be a true

concentration of radiant points. Since a major

part of the observing was carried out in the

long winter nights when the ecliptic was high

in the sky (at 6~ + 20°) this peak represents

the clustering of radiant points towards the

plane of the solar system.

The second peak corresponds to the declination which passes through the zenith of the

observers. Most meteors have a downward

component in their angular motion across the

sky so that there is always a tendency for a

spurious group radiant to appear in the region

of the zenith. This second peak, therefore, is

7
8 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. S

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FIOUEI* 1-3.—Distribution in declination of apparent radiants

of sporadic meteors, a* found by Denning, Opik, and Mclntosh.

an indication of the errors in the catalog and

may be neglected.

Opik (1934) attempted to deduce a statistical

method for rejecting spurious group radiants.

From the 2,000 radiants observed by the Ari-

•sona meteor expedition, 223 were selected as

probably true. These, however, still did not

represent permanent streams as they did not

recur in successive years. The distribution in

declination from Opik's catalog is shown in

figure 2. No visibility correction has been applied, as the observations were made at latitude 35° N, where the ecliptic passes close to

the zenith. The distribution shows clearly the

concentration in the plane of the solar system

and a subsidiary spurious peak corresponding

to the declination which passes through the

zenith.

Observations made in the Southern Hemisphere are not numerous, but Mclntosh (1935)

has compiled a catalog containing 323 radiant

positions, in which rigorous selection rules were

applied to minimize the radiants produced by

spurious intersections. The latitude of the observations was 35° S, where the ecliptic passes

close to the zenith, so that again no visibility

correction has been applied. An analysis of

this catalog produces the distribution shown

in figure 3. It can be seen that the concentration towards the ecliptic is confirmed in the

Southern Hemisphere. This catalog probably represents the best observations of weak

meteor streams yet made, because the results

show no signs of the spurious grouping of radiants that one may expect at the zenith of the

observer.

References

DENNING, W. F.

1886. Monthly Notices Roy. Astron. Soc. London,

vol. 47, p. 35.

HAWKINS, G. S.

1952. Thesis, Univ. Manchester, England.

1956a. Monthly Notices Roy. Astron. Soc. London, vol. 116, p. 92.

1956b. Astron. Journ., voL 61, p. 386.

MCINTOSH, R. A.

1935. Monthly Notices Roy. Astron. Soc. London,

vol. 95, p. 601.

OPIK, E.

1934. Harvard Coll. Obs. Circ, No. 388.

Abstract

Denning'e historic catalog of 3,035 radiant points has been examined in the light of modern knowledge The

cataloged radiant points show the expected concentration towards the plane of the ecliptic and in addition show

a concentration at the declination corresponding to the zenith of the observer. This second concentration is spurious and indicates the percentage of fictitious radiants that have been included, in error, in the catalog. The visual

work of Opik and Mclntosh shows more clearly the concentration in the plane of the ecliptic, and the percentage

of spurious radiants is considerably reduced.