So many queries about the solar system, or the members of it, have come
recently to the attention of those in charge of this journal, from various
sources, that it is thought best to make a brief statement of the present
state of knowledge that astronomy has of the solar neighborhood in which
we live.
Naturally we begin with the sun, and the oldest and most important problem
which the study of this body offers is the determination of its distance
from the earth in terrestrial units of measure. This distance is important
because the knowledge of all the phenomena of all the heavenly bodies,
except those of the moon, depend directly or indirectly on its value. The
problem of the sun's distance is difficult because the data given for
determining it are insufficient to enable the astronomer to apply the
principles of trigonometry directly to it. He is, therefore, compelled to
use indirect methods of solution, which, at best, give only approximations
to the true distance, arising chiefly from small errors in observation,
which, at the present time, seem unavoidable. A familiar illustration will
make our meaning clear. The knowledge we have of the sun's distance
depends on the accurate measurement of a small angle formed by drawing two
lines from a point at the sun to the extremities of the earth's radius.
That angle is called the sun's parallax. Ptolemy thought that this angle
was 3' of arc, but we now know that its value is very near 8.80" of arc,
and that the error of this amount from the true angle probably is not more
than 0.02". To measure this small angle has been the astronomer's great
trouble since the time of Aristarchus, and he does not yet know its value
accurately. His problem is like that of a surveyor attempting to measure a
ball, whose real diameter is one foot, at the distance of 4.4 miles
nearly; and unless he can determine the diameter of the ball so that he
shall not be uncertain in his measure to the amount of 0.03 of an inch,
his work will not add anything useful to present knowledge.
If we suppose the angle of parallax to be known, the computation of the
distance of a celestial body is easy. Multiply earth's radius by 206,265
(seconds of arc in the unit radius), and divide the product by the angle
of parallax in seconds of arc. The mean equatorial radius of the earth, as
given in Clark's Geodesy, is 3963.3 English miles. The sun's distance for
a parallax of 8.78" would be
206,265" x 3963.3
----------------- = 93,108,000 miles.
8.78"
For parallax of 8.80" = 92,897,000 miles.
For parallax of 8.82" = 92,686,000 miles.
The range of error in parallax, as here given, is 0.04", and the change of
the distance of the sun in allowing for this error is nearly half a
million of miles. If 8.80" be the assumed parallax, with +- 0.02" as
probable error, then the uncertainty of the sun's distance is still nearly
a quarter of a million of miles.
So far astronomers are pretty generally agreed, unless it be in the value
of the earth's radius used above. In his excellent work, entitled "The
Sun," we notice that Professor Young gives 3,962.72 English miles as the
"latest and most reliable determination" (page 22), while he seems to use
Bessel's value of 3,962.80 in obtaining 92,885,000. This may be because
the last named value is still in most general use, though less accurate
undoubtedly than that of Clarke.
Since the transit of Venus, of 1874, the determination of the solar
parallax has not been very much improved. The transit of 1882, so far as
known, has given surprisingly discordant results, and probably they will
be of very little service in improving our knowledge of the distance of
the sun. In the midst of all this uncertainty of late work, in ordinary
methods two ways of studying the problem show results almost exactly
alike. They are obtained from late improved measures of the velocity of
light, and from measures by the heliometer. The parallax from these
sources is 8.794". The Brazilian results of transit of Venus for 1882, by
Wolf and Andre, recently published, make the parallax 8.808". The American
reductions for the last transit are not yet completed.
From the above brief statement of results, it seems that the value of the
solar parallax is likely to be a trifle under 8.80", rather than above it,
making the distance of the sun probably very near 93,000,000 miles.
The next most important problem pertaining to the sun is its constitution,
which is usually considered under four heads:
1. The central portion, thought to be made up chiefly of intensely heated
gases.
2. That part which is seen by the aid of the telescope, called the
photosphere, consisting of a "shell of luminous clouds formed by the
cooling and condensation of the condensible vapors at the surface where
exposed to the cold of outer space." (Young.)
3. Outside of the photosphere is a shallow stratum, called the
chromosphere, "composed mainly of uncondensible gases (conspicuously
hydrogen) left behind by the formation of the photospheric clouds, and
bearing something the same relation to them that the oxygen and nitrogen
of our own atmosphere do to our own clouds." (Young.) And--
4. The corona, which is the beautiful halo seen, with the naked eye,
outside of all, during the time of a total eclipse of the sun. This
curious halo with all its streamers and rifts is thought to be composed
chiefly of an incandescent material, in a far more attenuated state than
that of hydrogen, the rarest gas known, because it yields freely in the
spectroscope a certain line, 1474 K, which most agree can mean nothing
else, although no one knows what the gas or metallic vapor is. Hydrogen is
also found in the corona extending to the height of 600,000 miles above
the photosphere, and possibly 1,200,000 miles. Suspended in this mixture
of vapors, and "falling into, or projected from, the sun is a large
quantity of solid or liquid material, which is at such a temperature as to
be self-luminous. It is this which yields the continuous spectrum, free
from dark lines.
"Besides these components in the outer envelope, there is present matter
which reflects or diffuses light much as our own atmosphere does.
"To this is attributed the partial radial polarization of the corona. The
streamers and rifts indicate matter repelled, in various quantities, from
the sun by forces which may be electrical." (Hastings.)
These are the views advanced by astronomers and physicists, as theories or
working hypotheses, until something better or more certain can be known.
They are not held as facts by any, because of insufficient proof to
establish them as such, and because there are very grave objections to
some of them which are at present unanswerable.
For example, the spectroscope shows that the gaseous pressure at the limit
of the chromosphere is very small, although that is at the base of an
atmosphere from 600,000 to 1,200,000 miles deep, and under the influence
of a force of gravity more than twenty-seven times as great as that in
action at the surface of the earth.
Optically, the atmosphere of the earth ceases at a height of forty-five
miles, but bodies at twice that altitude, moving at the rate of
twenty-seven miles per second, meet resistance of air enough to render
them incandescent almost instantly. But the evidence seems clear that, far
within the corona, the resistance to moving bodies is much less than in
our atmosphere at a height of sixty miles. The great comet of 1882 passed
through the coronal atmosphere within 300,000 miles of the sun, with a
velocity one hundred and eighty times that of the earth in its orbit. The
comet was not stopped, nor destroyed, nor its orbit disturbed, as
subsequent observations showed. The same thing was true, so far as known,
of the comet of 1843, which passed still nearer the solar surface. These
facts are troublesome to explain on the hypothesis of a coronal
atmosphere.
Still further: if the sun be surrounded by a gaseous envelope, its
density, as aforesaid, ought to diminish from the solar surface outward to
its upper limits; but the fact is, the material of 1474 K line always
appears in the spectrum of chromosphere, which would seem to indicate, by
its place, that it is as much more dense than hydrogen as is magnesium
vapor, or even the vapor of iron. But the evidence of the spectroscope
makes this 1474 K material far less dense than that of hydrogen, and this
is a contradiction that is very troublesome to the student of solar
physics.
In studying the polarization of the light of the corona, it is clear that
the amount of polarized light reflected from a particle at the surface of
the sun is nothing, "because the luminous source there is a surface with
an angular subtense of 180 deg.;" hence polarization of the corona near the
limb of the moon ought to be small, farther away, larger. But observation
shows that the contrary is true, _i. e._, the percent. of polarized light
increases as the corona is observed nearer the limb of the moon during
totality.
These are a few of the difficult questions that stand in the way of
accepting the foregoing theories as facts pertaining to, or well grounded
knowledge of, the constitution of the sun. They are by no means all, or
possibly the most important ones. They are certainly among those that are
receiving very general attention at the hands of physicists at the present
time.--_Sidereal Messenger._