THE SUN AND OBSERVING IT
As lord of day, king of the heavens, mankind in the ancient world adored
the sun. By their researches into the epoch of the Assyrians, Hittites,
Phoenicians and other early peoples now passed from earth, archaeologists
have unearthed many monuments that evidence the veneration in which the
early peoples who inhabited Egypt and Asia Minor many thousand years ago
held the sun. A striking example is found in the architecture of early
Egyptian temples, on the lintels of which are carved representations of
the winged globe or the winged solar disk, and there is a bare
possibility that the wings of the globe were suggested by a type of the
solar corona as glimpsed by the ancients.
Little knew they about the distance and size of the sun; but the effects
of his light and heat upon all vegetal and animal life were obvious to
them. Doubtless this formed the basis for their worship of the sun.
Occasional huge spots must have been visible to the naked eye, and the
sun's corona was seen at rare intervals. Plutarch and Philostratus
describe it very much as we see it to-day.
How completely dependent mankind is upon the sun and its powerful
radiations, only the science of the present day can tell us. By means of
the sun's heat the forests of early geologic ages were enabled to wrest
carbon from the atmosphere and store it in forms later converted by
nature's chemistry into peat and coal. Through processes but imperfectly
understood, the varying forms of vegetable life are empowered to
conserve, from air and soil, nitrogen and other substances suitable for
and essential to the life maintenance of animal creatures. Breezes that
bring rain and purify the air; the energy of water held under storage in
stream and dam and fall; trade winds facilitating commerce between the
continents; oceanic currents modifying coastal climates; the violence of
tornado, typhoon and water-spout, together with other manifestations of
natural forces--all can be traced back to their origin in the tremendous
heating power of the solar rays. In everything material the sun is our
constant and bountiful benefactor. If his light and heat were withdrawn,
practically every form of human activity on this planet would come to an
early end.
How far away is the sun? What is the size of the sun? These are
questions that astronomers of the present day can answer with accuracy.
So closely do they know the sun's distance that it is employed as their
yardstick of the sky, or unit of celestial measurement. Many methods
have been utilized in ascertaining the distance of the sun, and the
remarkable agreement among them all is very extraordinary. Some of them
depend upon pure geometry, and the basic measure which we make from the
earth is not the distance of the sun directly; but we find out how far
away Venus is during a transit of Venus, for example, or how far away
Mars is or some of the asteroids are at their closer oppositions. Then
it is possible to calculate how far away the sun is, because one
measurement of distance in the solar system affords us the scale on
which the whole structure is built. But perhaps the simplest method of
getting the sun's distance is by the velocity of light, 186,300 miles a
second. From eclipses of Jupiter's moons we know that light takes 8
minutes 20 seconds to pass from sun to earth. So that the sun's distance
is the simple product of the two, or 93 millions of miles.
Once this fundamental unit is established, we have a firm basis on which
to build up our knowledge of the distances, the sizes and motions of the
heavenly bodies, especially those that comprise the solar system. We can
at once ascertain the size of the sun, which we do by measuring the
angle which it fills, that is, the sun's apparent diameter. Finding this
to be something over a half a degree in arc, the processes of elementary
trigonometry tell us that the sun's globe is 865,000 miles in diameter.
For nearly a century this has been accurately measured with the greatest
care, and diameters taken in every direction are found to be equal and
invariably the same. So we conclude that the sun is a perfect sphere,
and so far as our instruments can inform us, its actual diameter is not
subject to appreciable change.
The vastness of the sun's volume commands our attention. As his diameter
is 110 times that of the earth, his mere size or volume is 110x110x110
or 1,300 thousand times that of the earth, because the volumes of
spheres are in proportion as the cubes of their diameters. If the
materials that compose the sun were as heavy as those that make up the
earth, it would take 1,300 thousand earths to weigh as much as the sun
does. But by a method which we need not detail here, the sun's actual
weight or mass is found to be only 300 thousand (more nearly 330,000),
times greater than the earth's. So we must infer that, bulk for bulk,
the component materials of the sun are about one-fourth lighter than
those of the earth, that is, about one and one-half times as dense as
water.
To look at this in another way: it is known that a body falling freely
toward the earth from outer space would acquire a speed of seven miles a
second, whereas if it were to fall toward the sun instead, the velocity
would be 383 miles a second on reaching his surface. If all the other
bodies of the solar system, that is, the earth and moon, all the planets
and their satellites, the comets and all were to be fused together in a
single globe, it would weigh only one-seven hundred and fiftieth as much
as the sun does.
At the surface, however, the disproportion of gravity is not so great,
because of the sun's vast size: it is only about twenty-eight times
greater on the sun than on the earth; and instead of a body falling 16
feet the first second as here, it would fall 444 feet there. Pendulums
of clocks on the sun would swing five times for every tick here, and an
athlete's running high jump would be scaled down to three inches.
Let us next inquire into the amount of the sun's light and heat, and the
enormously high temperature of a body whose heat is so intense even at
the vast distance at which we are from it. The intensity of its
brightness is such that we have no artificial source of light that we
can readily compare it with. In the sky the next object in brightness is
the full moon, but that gives less than the half-millionth part as much
light as the sun. The standard candle used in physics gives so little
light in comparison that we have to use an enormous number to express
the quantity of light that the sun gives.
A sperm candle burning 120 grains hourly is the standard, and if we
compare this with the sun when overhead, and allow for the light
absorbed by the atmosphere, we get the number 1575 with twenty-four
ciphers following it, to express the candlepower of the sun's light. If
we interpose the intense calcium light or an electric arc light between
the eye and the sun, these artificial sources will look like black spots
on the disk. Indeed, the sun is nearly four times brighter than the
"crater," or brightest part of the electric arc. The late Professor
Langley at a steel works in Pennsylvania once compared direct sunlight
with the dazzling stream of molten metal from a Bessemer converter; but
bright as it was, sunlight was found to be five thousand times brighter.
Equally enormous is the heat of the sun. Our intensest sources of
artificial heat do not exceed 4,000 degrees Fahrenheit, but the
temperature at the sun's surface is probably not less than 16,000
degrees F. One square meter of his surface radiates enough heat to
generate 100,000 horsepower continuously. At our vast distance of 93
millions of miles, the sun's heat received by the earth is still
powerful enough to melt annually a layer of ice on the earth more than a
hundred feet in thickness. If the solar heat that strikes the deck of a
tropical steamship could be fully utilized in propelling it, the speed
would reach at least ten knots.
Many attempts have been made in tropical and sub-tropical climates to
utilize the sun's heat directly for power, and Ericsson in Sweden,
Mouchot in France, and Shuman in Egypt have built successful and
efficient solar engines. Necessary intermission of their power at
night, as well as on cloudy days, will preclude their industrial
introduction until present fuels have advanced very greatly in cost. All
regions of the sun's disk radiate heat uniformly, and the sun's own
atmosphere absorbs so much that we should receive 1.7 times more heat if
it were removed. So far as is known, solar light and heat are radiated
equally in all directions, so that only a very minute fraction of the
total amount ever reaches the earth, that is, 1 2200 millionth part of
the whole. Indeed all the planets and other bodies of the solar system
together receive only one one hundred millionth part; the vast remainder
is, so far as we know, effectively wasted. It is transformed, but what
becomes of it, and whether it ever reappears in any other form, we
cannot say.
How is this inconceivably vast output of energy maintained practically
invariable throughout the centuries? Many theories have been advanced,
but only one has received nearly universal assent, that of secular
contraction of the sun's huge mass upon itself. Shrinkage means
evolution of heat; and it is found by calculation that if the sun were
to contract its diameter by shrinking only two-hundred and fifty feet
per year, the entire output of solar heat might thus be accounted for.
So distant is the sun and so slow this rate of contraction that
centuries must elapse before we could verify the theory by actual
measurements. Meanwhile, the progress of physical research on the
structure and elemental properties of matter has brought to light the
existence of highly active internal forces which are doubtless
intimately concerned in the enormous output of radiant energy, though
the mechanism of its maintenance is as yet known only in part.
Abbot, from many years' observations of the solar constant, at
Washington, on Mount Wilson, and in Algeria, finds certain evidence of
fluctuation in the solar heat received by the earth. It cannot be a
local phenomenon due to disturbances in our atmosphere, but must
originate in causes entirely extraneous to the earth. Interposition of
meteoric dust might conceivably account for it, but there is sufficient
evidence to show that the changes must be attributed to the sun itself.
The sun, then, is a variable star; and it has not only a period
connected with the periodicity of the sun spots, but also an irregular,
nonperiodic variation during a cycle of a week or ten days, though
sometimes longer, and occasioning irregular fluctuations of two to ten
per cent of the total radiation. Radiation is found to increase with the
spottedness.
Attempts have been made on the basis of the contraction theory to find
out the past history of the sun and to predict its future. Probably 20
to 50 millions of years in the past represents the life of the sun much
as it is at present; and if solar radiation in the future is maintained
substantially as now, the sun will have shrunk to one-half its present
diameter in the next five million years.
So far then as heat and light from the sun are concerned, the sun may
continue to support life on the earth not to exceed ten million years in
the future. But the sun's own existence, independently of the orbs of
the system dependent upon it, might continue for indefinite millions of
aeons before it would ever become a cold dead globe; indeed, in the
present state of science, we cannot be sure that it is destined to reach
that condition within calculable time.
A few words on observing the sun, an object much neglected by amateurs.
On account of the intense light, a very slight degree of optical power
is sufficient. Indeed a piece of window glass, smoked in a candle flame
with uniform graduation from end to end, will be found worth while in a
beginner's daily observation of the sun. The glass should be smoked
densely enough at one end so that the sunlight as seen through it will
not dazzle the eye on the clearest days. At the other end of the glass,
the degree of smoke film should not be quite so dense, so that the sun
can be examined on hazy, foggy or partly cloudy days. An occasional
naked-eye spot will reward the patient observer.
If a small spyglass, opera glass or field glass is at hand, excellent
views of the sun may be had by mounting the glass so that it can be held
steadily pointed on the sun, and then viewing the disk by projection on
a white card or sheet of paper. Care must be taken to get a good focus
on the projected image, and then the faculae, or whitish spots, or
mottling nearer the sun's edge will usually be well seen. By moving the
card farther away from the eyepiece, a larger disk may be obtained, in
effect a higher degree of magnification. But care must be used not to
increase it too much. Keep direct sunlight outside the tube from falling
on the card where the image is being examined. This is conveniently done
by cutting a large hole, the size of the brass cell of the object glass,
through a sheet of corrugated strawboard, and slipping this on over the
cell. In this way the spots on the sun can be examined with ease and
safety to the eye.
For large instruments a special type of eyepiece is provided known as a
helioscope, which disposes of the intense heat rays that are harmful to
the eye. Frequent examination of the eyepiece should be made and the
eyepiece cooled if necessary. That part of the sun's surface under
observation is known as the photosphere, that is, the part which
radiates light. If the atmosphere admits the use of high magnifying
powers, the structure of the photosphere will be found more and more
interesting the higher the power employed. It is an irregularly mottled
surface showing a species of rice-grain structure under fairly high
magnification. These grains are grouped irregularly and are about 500
miles across. Under fine conditions of vision they may be subdivided
into granules. The faculae, or white spots, are sometimes elevations
above the general solar level; they have occasionally been seen
projecting outside the limb, or edge of the disk.