The Astronomy Book

60

was directly related to the ratio of
the focal length of the objective lens
to the focal length of the eyepiece.
A longer-focus convex lens for
the objective, or a shorter-focus
concave lens for the eyepiece
was required. Since these were not
available, Galileo taught himself to
grind and polish lenses and made
them for himself. Living in northern
Italy, the glassmaking center
of the world at the time, helped
him considerably. He eventually

developed a new telescope with

33 times magnification, and it was

with this improved instrument

that he discovered the Jovian

(“of Jupiter”) moons.

“Three little stars”

Galileo discovered the moons

of the planet Jupiter on the night of

January 7, 1610. At first, he thought

he was looking at distant stars,

but he quickly realized that the new

bodies were moving around Jupiter,

At the time, Galileo was a 45-year-

old professor of mathematics at the

University of Padua near Venice.

When he published his pioneering

telescopic observations, he wrote:

“Through a spyglass, Jupiter

presented himself. And since

I had prepared for myself a

superlative instrument, I saw

(which earlier had not happened

because of the weakness of other

instruments) that three little stars

were positioned near him—small

but yet very bright. Although

I believed them to be among

the number of fixed stars, they

nevertheless intrigued me because

GALILEO’S TELESCOPE

they appeared to be arranged

exactly along a straight line and

parallel to the ecliptic ....”

Repeated observations

Galileo’s unexpected discovery

fascinated him. As he observed

Jupiter night after night, it soon

became clear that the new stars

were not beyond Jupiter, in the

distant heavens. They not only

accompanied the planet as it

moved along its path across

the sky, but also moved around

the planet.

Just as the moon orbits Earth

every month, Galileo realized that

there were four moons in orbit

around Jupiter, staying with it as it

orbited the sun. The more distant

moons took longer to complete

their orbits than the closer ones.

The time to complete one orbit

from the inner to the outer moon

is 1.77, 3.55, 7.15, and 16.69 days,

respectively. The Jovian moon

system looked like a small model

of the sun’s planetary system.

It was proof that not everything in

the cosmos orbited Earth, as had

been thought in pre-Copernican

days. The observation of these four

moons was a boost to the theory

of the sun-centered cosmos.

Refracting telescopes suffer from a problem known

as chromatic aberration. The different wavelengths of

light are brought to slightly different foci, so the final

image is surrounded by a halo of color.

My dear Kepler, what

would you say of the learned

who ... have steadfastly

refused to cast a glance

through the telescope?

Galileo Galilei

Galileo had the

experience of beholding

the heavens as they

actually are for perhaps

the first time.

I Bernard Cohen

Blue light refracts

the most.

Red light refracts

the least.

Single

lens

Green light is

in between red

and blue.

Different

foci