The Astronomy Book

61

Galileo quickly published his
discovery in his book Siderius
Nuncius (The Starry Messenger),
published on March 10, 1610. In
the hope of advancement, Galileo
dedicated the book to a former
pupil of his who later became the
Grand Duke of Tuscany, Cosimo II
de’ Medici. He named the moons
the Medicean Stars in honor of
the four royal Medici brothers. This
political thoughtfulness won him
the position of Chief Mathematician
and Philosopher to the Medici
at the University of Pisa. However,
the name did not catch on.

At first, many were sceptical,

suggesting that the moons were no

more than defects in the telescope

lens. However, other pioneering

telescopic astronomers such as

Thomas Harriot, Joseph Gaultier

de la Vatelle, and Nicolas-Claude

Fabri de Peiresc confirmed their

existence when Jupiter returned

to the night sky later in 1610, after

passing behind the sun.

Disputed priority

In 1614, German astronomer Simon

Marius published Mundus Iovialis,

in which he described Jupiter’s ❯❯

THE TELESCOPE REVOLUTION

Galileo’s telescope had a concave lens as an eyepiece.
When viewing a celestial object a great distance away, the
distance between the two lenses would equal the focal length
of the objective lens minus the focal length of the eyepiece.

Refracting telescopes

There were two kinds of

early refracting telescope: the

Galilean, and the Keplerian,

developed in 1611 by Johannes

Kepler (see left). They both had

a long-focus, large diameter

lens at the front, called the

objective. This collected the

light and brought it to a focus.

The image at the focus was

magnified using the smaller,

short-focus eyepiece lens.

The magnification of the

instrument is equal to the

focal length of the objective

lens divided by the focal

length of the eyepiece. A

flatter convex objective lens

reduced chromatic aberration

(see opposite), gave a longer

focal length, and, for a fixed

eyepiece, greater magnification.

For this reason, telescopes

became longer in the 17th

century. The minimum focal

length of eyepieces at the

time of Galileo and Kepler was

about 1–1½ in (2–4 cm). This

meant that, for a magnification

of x30, an objective lens with

a focal length of 24–48 in

(60–120 cm) was needed. Built

in 1888, the huge James Lick

Telescope on Mount Hamilton,

California (above), has a 36-in

(90-cm) lens and a focal length

of 57 ft (17.37 m).

Kepler’s telescope, developed soon after, had a convex lens
as an eyepiece. The length of the telescope was equal to the
objective focal length plus the focal length of the eyepiece.

Focal point of

objective lens

Concave

Objective eyepiece lens

lens

Parallel
light
rays
from
star

Objective focal length

Eyepiece focal length

Eye

Eye

Focal point of objective

and eyepiece lenses

Convex

eyepiece

lens

Objective lens

Objective focal length