The Astronomy Book

59

Galileo demonstrates his telescope
to Leonardo Donato, the Doge of
Venice. Like other astronomers of his
time, Galileo relied on patronage to
fund and legitimize his work.

are visible. By turning his new
telescope to the night sky, Galileo
would become one of the very first
people to appreciate the true nature
of this band of stars across the sky.

Building a telescope
Galileo did not invent the telescope
himself. The idea of combining two
lenses—a large one at the front of
a tube to collect the light, and a
small one at the back to magnify
the image—had come from the
Dutchmen Hans Lipperhey, Jacob
Metius, and Sacharias Janssen
in around September 1608. (It had
taken over 300 years to progress
from the invention of reading
glasses to the invention of a

telescope.) After hearing about

this new instrument, Galileo had

resolved to make one for himself.

A telescope does two important

things. Its resolution (the detail a

telescope can detect) is proportional

to the diameter of the objective

lens—the large lens at the front

that collects the light. The larger

the objective lens, the better the

resolution. An eye that has fully

adapted to the dark has a pupil

that is about ¼ in (0.5 cm) across,

and a resolution of around^1 ⁄ 60 o. Put

the eye at the back of a telescope

with an objective lens of 1, 2, or 4

cm diameter, and the resolution

improves to^1 ⁄ 120 o,^1 ⁄ 240 o , and^1 ⁄ 480 o^

respectively. Details then spring into

view. Jupiter, for example, looks like

a disk and not just a point.

A telescope also acts as a “light

bucket.” Every time the diameter

of the objective lens is doubled, the

light gathered increases by a factor

of four, and objects of similar light

output can be detected if they are

twice as far away. Objective lenses

of 1, 2, and 4 cm enable the eye to

discern 20,000, 160,000, and

1,280,000 stars respectively.

Galileo was not satisfied with

his first instrument, which only

magnified three times. He realized

that a telescope’s magnification ❯❯

See also: The Copernican model 32–39 ■ The Tychonic model 44–47 ■ Elliptical orbits 50–55 ■
Barnard (Directory) 337

THE TELESCOPE REVOLUTION

Jupiter has at least

four moons.

To the naked eye,

Jupiter looks like

a bright star.

The telescope shows

a finer resolution than

the naked eye.

This reveals that

Jupiter is a disk with

four stars around it.

The four stars

can be seen to be

orbiting Jupiter.