SCIENCE-GOS STP. 
CHAPTERS FORK YOUNG ASTRONOMERS. 
By Frank C, Denner. 
THE SUN. 
(Continued from page 187). 
THE group of sunspots of 1896 already referred to 
took no less than forty-four hours to cross the sun’s 
syparcat central meridian ; and covered an area of 
about 2,400,000,000 square miles. It is well to take 
the measurements of groups from day to day, because 
the extent of surface covered, often varies very con- 
siderably in the course of a few hours. 
In September, 1898, a remarkable group of spots 
crossed the disc. On the morning of September 9th 
it was measured on the scale as previously described. 
The principal spot had a total length of over 43,000 
miles, and a breadth of about 40,000 miles whilst its 
umbra alone was nearly 29,000 miles in length. The 
entire group had a length of about 130,000 miles. 
The greatest area of this group amounted to some 
2,700,000,000 square miles. The accompanying 
figure is a reproduction of the drawing made with the 
aid of a 3-inch telescope by an observer signing 
219 
spot which would give the impression that the observer 
was looking down into a whirlpool, or perhaps more 
correctly a cyclone, and others have in some measure 
confirmed this, but such spots are very rare. 
There are two explanations given as to what sun- 
spots really are. Wilson and Sir Wm. Herschel 
looked upon them as openings in the bright surface of 
the sun, or photosphere as we now call it. A duller 
surface beneath the photosphere revealed through this 
opening, forms what we know as the penumbra. The 
opening through this lower surface looks black and is 
called the umbra. Kirchoff considered rather that 
sunspots were due to the accumulation of dense matter 
over the surface, keeping the light of the photosphere 
from reaching us. The structure of the ordinary spot 
however is such that I cannot understand any practical 
observer holding such a theory. Very often, just as a 
considerable spot has come round the limb, and when 
it looks little more than a line, the umbra seems to 
rise above it, as if it were a projection beyond the 
general surface. I have seen this with a telescopic 
Two explanations of 
aperture of barely two inches. 
SeuNspors, AS SEEN THROUGH 3IN. REFRACTOR T ELESCOPE. 
himself ‘*‘ E.0.,” and is here given by the kind per- 
mission of the Editor of the ‘‘ English Mechanic,” 
where it first appeared. This group is remark- 
able as being apparently the cause of a great 
magnetic disturbance, and a brilliant display of 
Aurora Borealis, both of which occurred upon 
September 9th, 1898. On the afternoon of the 
15th of that month the large spot was visible as an 
indentation on the limb, and was thus photographed 
by Newbegin. According to the late Padre Secchi 
such an indentation, observed by him in 1873, had 
a depth of 8” or about 3,600 miles. La Hire 
and Cassini were the first to thus note spots on the 
limb. The phenomenon is usually seen at the western 
limb, but probably this is due to the fact that the 
observer then knows where to look for such appear- 
ance, but when coming round the eastern limb, not 
being expected, it is lost amid the constant ripple of 
light. 
There are decided motions amongst the spots, the 
distance between any given pair sometimes increasing, 
sometimes decreasing. Occasionally spots seem to 
have a motion around each other. Secchi has seen a 
this phenomenon have been given. One is, that the 
dark matter forming the umbra is by some internal 
force pushed up to a height greater than the general 
surface of the sun. The other theory considers that 
the cavity of the spot is filled with a dense vapour, 
which by refraction apparently raises the bottom of 
the spot when thus observed near the limb, just as 
water, looked at obliquely, seems to lift the bottom of 
the containing vessel. Observations’by our best phy- 
sicists appear to give contradictory results, sometimes 
favouring the one theory, then the other. For this 
reason dogmatic assertion is to be avoided. 
It may, however, be taken as a general rule, that 
spots of considerable size have the surface around 
them raised above the normal level. The inner edge 
of the penumbra also appears to be curved upward to- 
wards the observer, as if there were force pressing from 
below. The result of this is, that the surface imme- 
diately around spots near the limb seems to be much 
brighter than the general photosphere ; and the fila- 
ments of the penumbra to brighten as they reach 
towards the umbra. 
(To be continued.) 
