THE SUN
so; activity then dies down, until at minimum the disk may
be free of spots for many consecutive days or even weeks,
after which activity starts to build up once more towards
the next maximum. The cycle is not perfectly regular, but
11 years is a good average length, so that there were maxi-
ma in 1957–8, 1968–9, 1979–80, 1990–91 and 2000–2001.
The maxima are not equally energetic, and there seems
to have been a long spell, between 1645 and 1715, when
there were almost no spots at all, so that the cycle was
suspended. This is termed the Maunder Minimum, after
the British astronomer E. W. Maunder, who was one of the
first to draw attention to it. Obviously the records at that
time are not complete, but certainly there was a dearth of
spots for reasons which are not understood. There is also
evidence of earlier periods when spots were either rare or
absent, and it may well be that other prolonged minima
will occur in the future. Whether this has any effect upon
the Earth’s climate is a matter for debate, but it is true
that the Maunder Minimum was a ‘cold spell’; during the
1680s the River Thames froze over in most winters, and
frost-fairs were held upon it.
There is a further peculiarity, first noted by the
German amateur F. W. Spörer. At the start of a new cycle,
the spots break out at latitudes between 30 and 45 degrees
north or south of the solar equator. As the cycle progresses,
new spots appear closer and closer to the equator, until at
maximum the average latitude is only 15 degrees north or
south. After maximum new spots become less common,
but may break out at latitudes down to seven degrees.
They never appear on the equator itself, and before the last
spots of the old cycle die away the first spots of the new
cycle appear at higher latitudes.
According to the generally accepted theory, proposed
by H. Babcock in 1961, spots are due to the effects of
the Sun’s magnetic field lines, which run from one pole
to the other just below the bright surface. The rotationperiod at the equator is shorter than that at higher latitudes,
so that the field lines are dragged along more quickly,
and magnetic ‘tunnels’ or flux tubes, each about 500 kilo-
metres (300 miles) in diameter, are formed below the sur-
face. These float upwards and break through the surface,
producing pairs of spots with opposite polarities. At maxi-
mum the magnetic field lines are looped and tangled, but
then rejoin to make a more stable configuration, so at the
end of the cycle activity fades away and the field lines
revert to their original state.
The polarities of leader and follower are reversed in
the two hemispheres, and at the end of two cycles there is
a complete reversal, so there are grounds for suggesting
that the true length of a cycle is 22 years rather than 11.
Tracking sunspots is a fascinating pastime. A group
takes slightly less than two weeks to cross the disk from
one limb to the other, and after an equivalent period it will
reappear at the following limb if, of course, it still exists.
A spot is foreshortened when near the limb, and the
penumbra of a regular spot appears broadened to the limb-
ward side. This ‘Wilson effect’ indicates that the spot is a
depression rather than a hump, but not all spots show it.
Many spots are associated with faculae (Latin for
‘torches’) which may be described as bright, cloudlike
features at higher levels; they are often seen in regions
where spots are about to appear, and persist for some time
after the spots have died out. And even in non-spot zones,
the surface is not calm. The photosphere has a granular
structure; each granule is about 1000 kilometres (600
miles) in diameter with a lifetime of about eight minutes.
They represent currents, and it is estimated that the surface
includes about four million granules at any one time.
It would be idle to pretend that we have anything like a
complete understanding of the Sun. Many problems have
been solved, but we still have much to learn about our
‘daytime star’.▲ The Wilson Effect.
As shown in these three
pictures, many spots
behave as though they
were hollows; the penumbra
to the inward side appears
broadened when the spot is
foreshortened. The original
observations, by Scottish
astronomer A. Wilson, were
made in 1769. Sunspots, 26 May 1990;
I made this sketch by
projection with a 12.7-cm
(5-inch) refractor. Faculae
are shown to the upper left.E152-191 UNIVERSE UK 2003mb 7/4/03 5:41 pm Page 157