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Discovery of Comets and Meteors

Posted on July 23rd, 2007 by Admin.
Categories: Uncategorized.

Ever since Halley discovered that the comet of 1682 was a member of
the solar system, these wonderful objects have had a new interest for
astronomers; and a comparison of orbits has often identified the
return of a comet, and led to the detection of an elliptic orbit where
the difference from a parabola was imperceptible in the small portion
of the orbit visible to us. A remarkable case in point was the comet
of 1556, of whose identity with the comet of 1264 there could be
little doubt. Hind wanted to compute the orbit more exactly than
Halley had done. He knew that observations had been made, but they
were lost. Having expressed his desire for a search, all the
observations of Fabricius and of Heller, and also a map of the comet’s
path among the stars, were eventually unearthed in the most unlikely
manner, after being lost nearly three hundred years. Hind and others
were certain that this comet would return between 1844 and 1848, but
it never appeared.

When the spectroscope was first applied to finding the composition of
the heavenly bodies, there was a great desire to find out what comets
are made of. The first opportunity came in 1864, when Donati observed
the spectrum of a comet, and saw three bright bands, thus proving that
it was a gas and at least partly self-luminous. In 1868 Huggins
compared the spectrum of Winnecke’s comet with that of a Geissler tube
containing olefiant gas, and found exact agreement. Nearly all comets
have shown the same spectrum.[1] A very few comets have given bright
band spectra differing from the normal type. Also a certain kind of
continuous spectrum, as well as reflected solar light showing
Frauenhofer lines, have been seen.

[Illustration: COPY OF THE DRAWING MADE BY PAUL FABRICIUS. To define
the path of comet 1556. After being lost for 300 years, this drawing
was recovered by the prolonged efforts of Mr. Hind and Professor
Littrow in 1856.]

When Wells’s comet, in 1882, approached very close indeed to the sun,
the spectrum changed to a mono-chromatic yellow colour, due to sodium.

For a full account of the wonders of the cometary world the reader is
referred to books on descriptive astronomy, or to monographs on
comets.[2] Nor can the very uncertain speculations about the structure
of comets’ tails be given here. A new explanation has been proposed
almost every time that a great discovery has been made in the theory
of light, heat, chemistry, or electricity.

Halley’s comet remained the only one of which a prediction of the
return had been confirmed, until the orbit of the small, ill-defined
comet found by Pons in 1819 was computed by Encke, and found to have a
period of 3 1/3 years. It was predicted to return in 1822, and was
recognised by him as identical with many previous comets. This comet,
called after Encke, has showed in each of its returns an inexplicable
reduction of mean distance, which led to the assertion of a resisting
medium in space until a better explanation could be found.[3]

Since that date fourteen comets have been found with elliptic orbits,
whose aphelion distances are all about the same as Jupiter’s mean
distance; and six have an aphelion distance about ten per cent,
greater than Neptune’s mean distance. Other comets are similarly
associated with the planets Saturn and Uranus.

The physical transformations of comets are among the most wonderful of
unexplained phenomena in the heavens. But, for physical astronomers,
the greatest interest attaches to the reduction of radius vector of
Encke’s comet, the splitting of Biela’s comet into two comets in 1846,
and the somewhat similar behaviour of other comets. It must be noted,
however, that comets have a sensible size, that all their parts cannot
travel in exactly the same orbit under the sun’s gravitation, and that
their mass is not sufficient to retain the parts together very
forcibly; also that the inevitable collision of particles, or else
fluid friction, is absorbing energy, and so reducing the comet’s
velocity.

In 1770 Lexell discovered a comet which, as was afterwards proved by
investigations of Lexell, Burchardt, and Laplace, had in 1767 been
deflected by Jupiter out of an orbit in which it was invisible from
the earth into an orbit with a period of 5-1/2 years, enabling it to be
seen. In 1779 it again approached Jupiter closer than some of his
satellites, and was sent off in another orbit, never to be again
recognised.

But our interest in cometary orbits has been added to by the discovery
that, owing to the causes just cited, a comet, if it does not separate
into discrete parts like Biela’s, must in time have its parts spread
out so as to cover a sensible part of the orbit, and that, when the
earth passes through such part of a comet’s orbit, a meteor shower is
the result.

A magnificent meteor shower was seen in America on November 12th-13th,
1833, when the paths of the meteors all seemed to radiate from a point
in the constellation Leo. A similar display had been witnessed in
Mexico by Humboldt and Bonpland on November 12th, 1799. H. A. Newton
traced such records back to October 13th, A.D. 902. The orbital motion
of a cloud or stream of small particles was indicated. The period
favoured by H. A. Newton was 354-1/2 days; another suggestion was 375-1/2
days, and another 33-1/4 years. He noticed that the advance of the date
of the shower between 902 and 1833, at the rate of one day in seventy
years, meant a progression of the node of the orbit. Adams undertook
to calculate what the amount would be on all the five suppositions
that had been made about the period. After a laborious work, he found
that none gave one day in seventy years except the 33-1/4-year period,
which did so exactly. H. A. Newton predicted a return of the shower on
the night of November 13th-14th, 1866. He is now dead; but many of us
are alive to recall the wonder and enthusiasm with which we saw this
prediction being fulfilled by the grandest display of meteors ever
seen by anyone now alive.

The _progression_ of the nodes proved the path of the meteor
stream to be retrograde. The _radiant_ had almost the exact
longitude of the point towards which the earth was moving. This proved
that the meteor cluster was at perihelion. The period being known, the
eccentricity of the orbit was obtainable, also the orbital velocity of
the meteors in perihelion; and, by comparing this with the earth’s
velocity, the latitude of the radiant enabled the inclination to be
determined, while the longitude of the earth that night was the
longitude of the node. In such a way Schiaparelli was able to find
first the elements of the orbit of the August meteor shower
(Perseids), and to show its identity with the orbit of Tuttle’s comet
1862.iii. Then, in January 1867, Le Verrier gave the elements of the
November meteor shower (Leonids); and Peters, of Altona, identified
these with Oppolzer’s elements for Tempel’s comet 1866–Schiaparelli
having independently attained both of these results. Subsequently
Weiss, of Vienna, identified the meteor shower of April 20th (Lyrids)
with comet 1861. Finally, that indefatigable worker on meteors,
A. S. Herschel, added to the number, and in 1878 gave a list of
seventy-six coincidences between cometary and meteoric orbits.

Cometary astronomy is now largely indebted to photography, not merely
for accurate delineations of shape, but actually for the discovery of
most of them. The art has also been applied to the observation of
comets at distances from their perihelia so great as to prevent their
visual observation. Thus has Wolf, of Heidelburg, found upon old
plates the position of comet 1905.v., as a star of the 15.5 magnitude,
783 days before the date of its discovery. From the point of view of
the importance of finding out the divergence of a cometary orbit from
a parabola, its period, and its aphelion distance, this increase of
range attains the very highest value.

The present Astronomer Royal, appreciating this possibility, has been
searching by photography for Halley’s comet since November, 1907,
although its perihelion passage will not take place until April, 1910.

FOOTNOTES:

[1] In 1874, when the writer was crossing the Pacific Ocean in
H.M.S. “Scout,” Coggia’s comet unexpectedly appeared, and (while
Colonel Tupman got its positions with the sextant) he tried to use the
prism out of a portable direct-vision spectroscope, without success
until it was put in front of the object-glass of a binocular, when, to
his great joy, the three band images were clearly seen.

[2] Such as _The World of Comets_, by A. Guillemin; _History of
Comets_, by G. R. Hind, London, 1859; _Theatrum Cometicum_, by S. de
Lubienietz, 1667; _Cometographie_, by Pingre, Paris, 1783; _Donati’s
Comet_, by Bond.

[3] The investigations by Von Asten (of St. Petersburg) seem to
support, and later ones, especially those by Backlund (also of
St. Petersburg), seem to discredit, the idea of a resisting medium.