the different seasons is not very great, as in the United States, and
in districts in which the southern light is observed (about latitude
40° S.), the chief minimum falls in winter: on the northern hemisphere,
in December; on the southern hemisphere, in June or July. A less
pronounced minimum occurring in the summer. Twice in the course of the
year the earth passes through the plane of the solar equator. During
these periods a minimum of solar dust trickles down upon the earth, and
that period is characterized by a larger number of polar lights which
is distinguished by a higher elevation of the sun above the horizon.
We may expect this; for most solar dust will fall upon that portion
of the earth over which the sun is highest at noon. The two maxima of
March or April and of September or October, when the earth is at its
greatest distance from the plane of the solar equator, are strongly
marked in all the series, except in those for the polar districts
Iceland and Greenland. There the auroral frequency is solely dependent
upon the intensity of the twilight, so that we find a single maximum in
December and the corresponding minimum in June. More recent statistics
(1891-1903) indicate, however, a minimum in December. For the same
reason the summer minimum in countries of high latitudes, like Sweden
and Norway, is very much accentuated.
Similar reasons render it difficult for most localities to indicate the
daily periodicity of the polar lights. Most of the solar dust falls
about noon; and most polar lights should therefore be counted a few
hours after noon, just as the highest temperature of the day is reached
a little after noon. On account of the intense sunlight, however,
this maximum can only be established in the wintry night of the polar
regions, and even there only when a correction has been made for the
disturbing effect of the twilight. In this way Gyllenskiöld found a
northern-light maximum at 2.40 P.M. for Cape Thordsen, on Spitzbergen,
the corresponding minimum being at 7.40 A.M. In other localities we
can only ascertain that the polar lights are more intense and more
frequent before than after midnight. In central Europe the maximum
occurs at about 9 P.M.; in Sweden and Norway (in latitude 60° N.), half
an hour or an hour later.
A few other periods, approximately of the length of a month, have
been suggested with regard to polar lights. A period lasting 25.93
days predominates in the southern lights, where the maximum exceeds
the average by 44 per cent. For the northern lights in Norway the
corresponding excess percentage is 23; for Sweden, only 11.[9]
[Footnote 9: The reason is that in the southern district only
very few, and chiefly the most intense, auroras are recorded.
If we observe very assiduously in a large country, and conduct
the observations at different spots, we shall find polar light
almost every night. This consideration partly wipes out the
just-mentioned differences.]
The same period of nearly twenty-six days had already been pointed out
for a long series of other especially magnetic phenomena which, as we
shall see, are very closely connected with auroras, and it had also
been found in the frequency of thunder-storms and in the variations of
the barometer. This periodicity has often been thought to be connected
with the axial rotation of the sun. The Austrian scientist Hornstein
has even gone so far as to propose that the length of this period
should be carefully determined, “because it would give a more accurate
value for the rotation of the sun than the direct determinations.”
We know now that the length of the solar revolution is different for
different solar altitudes, a circumstance with which observations
of sun-spot movements at different latitudes had already made
Carrington and Spörer familiar, but which was not safely established
before Dunér’s spectroscopical determination of the movement of the
solar photosphere. Dunér found the following sidereal revolutions
for different latitudes of the sun to which the subjoined synodical
revolution would correspond. (By sidereal revolution of a point on the
sun we understand the time which elapses between the two moments when a
certain star passes, on two consecutive occasions, through the meridian
plane of the point—that is to say, through a plane laid through the
poles of the sun and the point in question. The synodical revolution
is determined by the passage of the earth through this meridian. On
account of the proper motion of the earth the synodical period is
longer than the sidereal period.)
Latitude on the sun (degrees) 0 15 30 45 60 75
Sidereal revolution (days) 25.4 26.4 27.6 30.0 33.9 38.5
Synodical revolution (days) 27.3 28.5 29.9 32.7 37.4 43.0
That the periods of rotation of the solar photosphere, and, in a
similar way, the periods of the spots, the faculæ, and the prominences,
should become so considerably longer with increasing latitudes is one
of the most mysterious problems of the physics of the sun. Something
similar applies to the clouds of Jupiter, but the difference in that
case is much smaller—only about one per cent. The clouds of our
atmosphere behave quite differently, a fact which is explained by our
atmospheric circulation.[10]
[Footnote 10: The very highest strata of our atmosphere (at
levels of from 20 to 80 km., 15 to 50 miles) may perhaps form
an exception. The luminous clouds which were observed in the
years 1883-1892 at Berlin (after the eruption of Krakatoa), and
which were floating at a very high level, showed a drift with
regard to the surface of the earth opposite to the drift of the
cirrus clouds, which are directed eastward.]
In our case, of course, the position of the sun with regard to
the earth—that is to say, the synodical period—can alone be of
importance. We recognize that the period of 25.93 days does not at all
agree with any period of the solar photosphere. The solar equatorial
zone differs least, and it would be appropriate to reckon with this
period, since the earth never moves very far from the plane of the
solar equator, and returns to that plane, at any rate, twice in the
course of a year.
But there is another peculiarity. The higher a point is situated
in the atmosphere of the sun, the shorter is its period. Thus the
synodical period of the faculæ near the equator is on an average 26.06,
the period of the spots 26.82, of the photosphere 27.3 days. Faculæ
situated at higher levels revolve still more rapidly, and we are thus
driven to the conclusion that the period to which we have alluded
agrees with the period of the faculæ situated at higher levels in the
equatorial zone of the sun, and is probably dependent upon them. That
would conform to our ideas concerning the physics of the sun. For the
faculæ are produced in the ascending currents of gas and at rather
lower levels than the spherules which are expelled by the radiation
pressure. This radiation pressure is strongest just in the neighborhood
of the faculæ.
For the same reason the repulsion of the solar dust becomes
particularly powerful when the faculæ are strongly developed—that is
to say, just in the time of pronounced eruptive activity of the sun
which is characterized by many sun-spots.
We must thus imagine that the radiation of the sun will be stronger
in times of strong eruptive activity than during the days of low
sun-spot frequencies. Direct observations of the intensity of the solar
radiation which have been made by Saveljeff in Kieff confirm this
assumption. It must be pointed out, however, that another phenomenon
investigated by Köppen seems to contradict this conclusion. Köppen
ascertained that in our tropics the temperature was by 0.32° Cent.
(nearly 0.6° F.) lower during sun-spot maxima than the average, and
that five years later, a year before the sun-spot minimum, it reached
its maximum value of 0.41° Cent. (0.7° F.) above the average. A similar
peculiarity can be traced to other zones, but on account of the less
steady climates it is much less marked there than in the tropics. A
French physicist, Nordmann, has fully confirmed the observations of
Köppen. On the other hand, Very, an American astronomer, has found that
the temperature in very dry (desert) districts of the tropics (near
Port Darwin, 12° 28´ S., and near Alice Springs, 23° 38´ S., both in
Australia) is higher at sun-spot maxima than at minima; but Very was
in this research guided merely by the records of maximum and minimum
thermometers. From Very’s investigation it would appear that the solar
radiation is really more intense with larger sun-spot numbers.[11]
This, it must be remarked, is only noticeable in exceedingly dry
districts in which there is no cloud formation worth mentioning. In
other districts the cloud formation which accompanies sun-spot maxima
interferes with the simplicity of the phenomena. The cooling effect of
the clouds seems in these cases by far to surpass the direct heating
effect of the solar rays, and in this manner Köppen’s conclusion
would become explicable. If we could observe the temperatures of the
atmospheric strata above the clouds, their variation would no doubt be
in the same degree as that in the desert.
[Footnote 11: According to Memery (_Bull. Soc. Astr._, March 7,
1906, p. 168) an instantaneous rise of temperature is observed
immediately when a sun-spot is first seen, and the temperature
sinks again when the sun-spot disappears.]
Finally, we have to note another period in the phenomena of the polar
lights—the so-called tropical month, whose length is 27.3 days. The
nature of this period is little understood. It is possibly connected
with the electric charge of the moon. The peculiarity of this period
is that it acts in an opposite way in the northern and southern
hemispheres. When the moon is above the horizon, it seems to prevent
the formation of polar lights; but for this case the difficulties of
observation caused by the moonlight must, of course, be taken into
consideration.
[Illustration: Fig. 42.—Curve of magnetic declination at
Kew, near London, on November 15 and 16, 1905. The violent
disturbance of November 15, 9 P.M., corresponds to the maximum
intensity of the aurora. Compare the following figure]
Celsius and Hiorter observed in 1741 that the polar lights exercise an
influence on the magnetic needle. From this circumstance we have drawn
the conclusion that the polar lights are in some way due to electric
discharges which act upon the magnetic needle. These magnetic effects,
the disturbances of the otherwise steady position of the magnetic
needle, are not influenced by the light of the sun and moon, and can
therefore be studied to greater advantage than auroras. We have
already pointed out that it is only the aurora of the radial, streamer
type which exerts this magnetic influence (compare Figs. 42 and 43).
[Illustration: Fig. 43.—Curve of horizontal intensity at Kew on
November 15 and 16, 1905. On November 15 a magnificent aurora
was observed in Galicia, Germany, France, Norway, England,
Ireland, and Nova Scotia, with a maximum at 9 P.M. The polar
light was unusually brilliant as early as 6 P.M.]
These magnetic variations show exactly the same periods as the northern
lights and the sun-spots. As regards, first, the long period of 11.1
years, our observations prove that the so-called magnetic disturbances
of the position of the magnetic needle faithfully reflect the
variations in the sun-spots. This connection was discovered in 1852 by
Sabine in England, by Wolf in Switzerland, and by Gautier in France.
Even the more irregular diurnal variations in the magnetic elements
are subjected to a solar period. The magnetic needle points in our
districts with its north end towards the north—not exactly, though,
being deflected towards the west. This western deviation or declination
is greatest soon after noon, about one o’clock, and this diurnal change
is greater in summer than in winter, and the fluctuation of the
position of the magnetic needle greater in daytime than at night-time.
It is, therefore, manifest that we have to deal with some solar
effect. This becomes still more distinct when we study the change with
reference to the daily variation in the number of sun-spots. In the
subjoined table the variation in the declination has been compiled for
Prague for the years 1856 to 1889. Only years with maxima and minima of
sun-spots and of magnetic variations have been noticed in this table:
1856 1860 1867 1871 1879 1884 1889
Sun-spot number 4.3 95.7 7.3 139.1 3.4 63.7 6.3
DAILY VARIATIONS IN DECLINATION
1856 1859 1867 1871 1878 1883 1889
Observed 5.98 10.36 6.95 11.43 5.65 8.34 5.99
Calculated 6.08 10.20 6.22 12.15 6.04 8.76 6.17
We see that the maxima and minima years of the two phenomena very
nearly coincide. The accord is so evident that we may calculate the
diurnal variation as proportional to the increase in the number of
sun-spots. This is shown by the two last lines of the table.
The yearly variation is again exactly the same as that of polar lights,
as the following table indicates, in which the disturbances of magnetic
declination, horizontal intensity, and vertical intensity are compiled
for Toronto, Canada; for comparison the means of these three magnitudes
are added for Greenwich. The unit of this table is the average annual
variation:
TORONTO
Jan. Feb. Mar. April May June July
Declination =0.57= 0.84 1.11 =1.42= 0.98 =0.53= 0.94
Horizontal =0.56= 0.94 0.94 =1.50= 0.90 =0.36= 0.61
Vertical =0.57= 0.74 1.08 =1.49= 1.12 =0.50= 0.71
Aug. Sept. Oct. Nov. Dec.
Declination 1.16 =1.62= 1.31 0.78 0.76
Horizontal 0.75 =1.71= 1.48 0.98 0.58
Vertical 1.08 =1.61= 1.29 0.75 0.61
GREENWICH
Jan. Feb. Mar. April May June July
Means 0.93 =1.23= 1.22 1.09 0.81 =0.71= 0.81
Aug. Sept. Oct. Nov. Dec.
Means 0.90 1.15 =1.18= 1.02 =0.83=
The daily variation of the disturbances has been analyzed by Van
Bemmelen for the period 1882-1893 and for the observatory of Batavia,
on Java. The maximum occurs there about 1 P.M., and is about 1.86 times
as great as the average value for the day. The minimum of 0.48 occurs
at 11 P.M. Between 8 P.M. and 3 A.M. the disturbances are almost as
rare as about 11 o’clock at night.
The variation is greatest with that declination which has its maximum
of 3.26 at 12 M., and its minimum of 0.14 at 11 P.M.
The period of almost 26 days first investigated by Hornstein has also
been refound in the magnetic variations and disturbances by Broun,
Liznár, and C. A. Müller. It must be added, however, that Schuster does
not consider these data as in any way conclusive.
The moon has also a slight influence upon the magnetic needle, as
Kreil proved as early as 1841. The effect is in a different sign in
the northern and southern hemispheres, and may be likened to a tidal
phenomenon.
The ultra-violet rays of the sun are strongly absorbed by the
atmosphere, and they cause an ionization of the molecules of the air.
This ionization is, on the whole, more marked at higher altitudes. The
ascending air currents carry with them water vapor which is condensed
on the ions, particularly on the negative ions. In this way most
clouds become negatively charged; this interesting fact—_i.e._, that
they are more frequently charged with negative than with positive
electricity—was first proved by Franklin in his kite experiments.
When the rain-drops have fallen, the air above remains positively
charged; this has been observed during balloon ascensions. The clouds
which are formed at high levels are most strongly charged; for this
reason thunder-storms over land occur mostly in the summer-time. The
thunder-storms also show the 26-day period, as Bezold has proved for
southern Germany, and Ekholm and myself have shown for Sweden.
A vast amount of material concerning these questions and magnetic
phenomena in particular has been collected by the various
meteorological observatories and is awaiting analysis.
Although some observers like Sidgreaves question the correlation of
sun-spots and polar lights or magnetic disturbances, because strong
spots have been seen on the disk of the sun without any magnetic
disturbances having been noticed, yet the view predominates that the
magnetic disturbances are caused by sun-spots when the sun-spots cross
the central meridian of the sun which is opposite the earth. Thus
Maunder observed a magnetic storm and a northern light succeeding the
passage of a large sun-spot through the central solar meridian on the
8th to the 10th of September, 1898. The magnetic effect attained its
maximum about twenty-one hours after the passage through the meridian.
Similarly Riccò established in ten instances, in which exact
determination was possible, a time interval of 45.5 hours on an
average between the meridian passage of a spot and the maximum
magnetic effect. Riccò also submitted to an analysis the data which
Ellis had collected and which Maunder had investigated. He found for
these instances, on an average, almost exactly the same numbers,
the time interval being 42.5 hours. That would correspond to a mean
velocity of the solar dust of from 910 to 980 km. per second. On the
other hand, we have no difficulty at all in calculating the time
which a spherule of a diameter of 0.00016 mm. (those particles travel
fastest) and of the specific gravity of water would need in order to
reach the earth, under the influence of solar gravitation and of a
mechanical radiation pressure 2.5 times as large from the outside of
the sun. The time found, 56.1 hours, corresponds to a mean velocity
of 740 km. per second. In order that the solar dust may move with the
velocity calculated by Riccò, its specific gravity should be less than
1—_viz._, 0.66 and 0.57. This value looks by no means improbable,
when we assume that the spherules consist of hydrocarbons saturated
with hydrogen, helium, and other noble gases. We should also arrive
at larger velocities for the solar dust, as has already been pointed
out with regard to the tails of comets, when we presume that the
particles consist of felted marguerites of carbon or silicates, or of
iron—materials which we regard as the main constituents of meteorites.
It should, perhaps, be mentioned that the most intense spectrum line
of the polar lights has been found to be characteristic of the noble
gas krypton. As this gas is found only in very small quantities in our
atmosphere, it does not appear improbable that it has been carried
to us together with the solar dust, and that its spectrum becomes
perceptible during the discharge phenomena. The other spectrum lines
of the polar lights belong to the spectra of nitrogen, argon, and of
the other noble gases. The volumes of noble gases which are brought
into our terrestrial atmosphere in this manner would in any case be
exceedingly small.
The electrical phenomena of our terrestrial atmosphere indirectly
possess considerable importance for organic life and, consequently,
for human beings. By the electrical discharges part of the nitrogen is
made to combine with the oxygen and hydrogen (liberated by the electric
decomposition of water vapor) of our air, and it thus forms the
ammonia compounds, as well as the nitrates and nitrites, which are so
essential to vegetable growth. The ammonia compounds which play a most
important part in the temperate zones appear especially to be formed
by the so-called silent discharges which we connect with auroras. The
oxygen compounds of nitrogen, on the other hand, seem to be chiefly the
products of the violent thunder-storms of the tropics. The rains carry
these compounds down into the soil, where they fertilize the plants.
The supply of nitrogen thus fixed amounts in the course of a year to
about 1.25 gramme per square metre in Europe, and to almost fourfold
that figure in the tropics. If we accept three grammes as the average
number for the whole firm land of the earth, that would mean 3 tons per
square kilometre, and about 400 million tons per year for the whole
firm land of 136 million square kilometres. A very small portion of
this fixed nitrogen, possibly one-twentieth, falls on cultivated soil;
a larger portion will help to stimulate plant growth in the forests
and on the steppes. We may mention, for comparison, that the nitrogen
contained in the saltpetre which the mines of Chili yield to us has
risen from 50,000 tons in 1880 to 120,000 tons in 1890, to 210,000 tons
in 1900, and to 260,000 tons in 1905. The nitrogen produced in the
shape of ammonium salts (sulphate) by the gas-works of Europe amounts
to about one-quarter of the last-mentioned figure. To this figure we
have, of course, to add the production of coal-gas ammonia in the
United States and elsewhere. Yet even allowing for this item, we shall
find that the artificial supply of combined nitrogen on the earth does
not represent more than about one-thousandth of the natural supply.
As the nitrogen contents of the air may be estimated at 3980 billion
tons, we recognize that only one part in three millions of the nitrogen
of the atmosphere is every year fixed by electric discharges, presuming
that the nitrogen supply to the sea is as great per square kilometre
as to the land. The nitrogen thus fixed benefits the plants of the sea
and of the land, and passes back into the atmosphere or into the water
during the life of the plants or during their decay. Water absorbs
some nitrogen, and equilibrium between the nitrogen contents of the
atmosphere and of the sea is thus maintained. Hence we need not fear
any noteworthy depletion of the nitrogen contents of the air. This
conclusion is in accord with the fact that no notable accumulation of
fixed nitrogen appears to have taken place in the solid and liquid
constituents of the earth.
The reader may remember (compare page 57) that during the annual cycle
of vegetation not less than one-fiftieth of the atmospheric contents
in the carbon dioxide is absorbed. Since oxygen is formed from this
carbon dioxide, and since the air contains about seven hundred times as
many parts per volume of oxygen as of carbon dioxide, the exchange of
atmospheric oxygen is about one part in thirty-five thousand. In other
words, the oxygen of the air participates about one hundred times more
energetically in the processes of vegetation than the nitrogen, and
this is in accordance with the general high chemical activity of oxygen.
Before we close this chapter we will briefly refer to the peculiar
phenomenon known as the Zodiacal Light, which can be seen in the
tropics almost any clear night for a few hours after or before sunset
or sunrise. In our latitudes the light is rarely visible, and is best
seen about the periods of the vernal and autumnal equinoxes. The
phenomenon is generally described as a luminous cone whose basis lies
on the horizon, and whose middle line coincides with the zodiac. Hence
the name. According to Wright and Liais, its spectrum is continuous. It
is stated that the Zodiacal Light is as strong in the tropics as that
of the Milky Way.
[Illustration: Fig. 44.—Zodiacal Light in the tropics]
There can be no doubt that this glow is due to dust particles
illuminated by the sun. It has therefore been assumed that this dust is
floating about the sun in a ring, and that it represents the rest of
that primeval nebula out of which the solar system has been condensed,
according to the theory of Kant and Laplace (compare Chapter VII.).
Sometimes a fairly luminous band seems to shoot out from the apex of
the cone of the Zodiacal Light and to cross the sky in the plane of
the ecliptic. In that part of the sky which is just opposite the sun
this band expands to a larger, diffused, not well-defined spot of
light covering about 12° of arc in latitude and 90° in longitude. This
luminescence is called the counter-glow (Gegenschein), and was first
described by Pezenas in 1780.
The most probable view concerning the nature of this counter-glow is
that it is caused by small particles of meteorites or dust which fall
towards the sun. Like the position of the corona of the aurora, the
position of this counter-glow seems to be an effect of perspective; the
orbits of the little particles are directed towards the sun, and they
therefore appear to radiate from a point opposite to it.
We know very little about this phenomenon. Even the position of the
Zodiacal Light along the zodiac which has given rise to its name
has been questioned, and it would appear from recent investigations
that the glow is situated in the plane of the solar equator. However
that may be, the view is very generally held that the glow is due to
particles which come from the sun or enter into it. We have already
adduced arguments to prove that the mass of solar dust cannot be
unimportant; this dust may therefore be the cause of the phenomenon
which we have just been discussing.
VI
END OF THE SUN—ORIGIN OF NEBULÆ