Relive the Discoveries of Galileo September 2014 | Page 14

Relive the Discoveries of Galileo

Discovery No. 5

WARNING! DO NOT LOOK AT THE SUN.

Sunspots
Galileo was stumped when he projected the
sun’s image through his telescope and onto a wall,
and saw imperfections.
In a letter to German astronomer Marcus Welser,
Galileo wrote, “I do not assert...that the spots are
clouds of the same material as ours, or aqueous
vapors raised from the earth and attracted by the Sun.
I merely say that we have no knowledge of anything
that more closely resemble them. Let them be vapors
or exhalations then, or clouds, or fumes sent out from
the Sun’s globe or attracted there from other places; I
do not decide on this - and they may be any of a
thousand other things not perceived by us.”
But Galileo continued, offering a guess that was,
well, spot on! “If I may give my own opinion...the
solar spots are produced and dissolve upon the
surface of the Sun and are contiguous to it, while the
Sun, rotating upon its axis in about one lunar month,
carries them along, perhaps bringing back some of
those that are of longer duration than a month, but so
changed in shape and pattern that it is not easy for us
to recognize them.”
Galileo’s speculation that the spots were actually on
the Sun’s surface dealt another challenge to the
Aristotle/Ptolemy assertion that the heavenly bodies
were perfect spheres, without any imperfections.
By observing the Sun on consecutive days over the
course of several weeks, Galileo was able to watch

June 23, 1613

- 14 -

Never aim binoculars or a telescope at the sun.
Looking at the sun through unfiltered binoculars or a telescope, even a one-second peek,
will burn your retina and cause permanent
blindness.
Special sun filters are available from astronomy retailers. See a retailer that specializes in
astronomical equipment if you are interested in
obtaining a filter for safe viewing of the sun.
Do not attempt to invent your own solar filter.
Without a proper solar filter, the only safe
way to view the sun is to display its image with
a pin-hole projector as described on page 15.
sunspot complexes move in tandem across the solar
disk, thereby discovering that the Sun rotates. It was
already known that the Moon rotates - allowing for
the same side to point towards Earth while it orbits
us. By discovering that the Sun also rotates, Galileo
was raising the possibility that maybe other globes,
including Earth, spin about their axis, a requirement
for a Sun-centred system that Copernicus proposed in
his book De revolutionibus.
In order for the Sun to appear to “rise in the east” and
“set in the west” without actually racing around the
Earth on