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G
alileo Galilei’s effective
use of a telescope marked
a watershed in the history
of astronomy. There have been
other turning points—such as the
introduction of photography, the
discovery of cosmic radio waves,
and the invention of the electronic
computer—but the invention of the
telescope was fundamental to the
advancement of the subject.Limits of the naked eye
Before Galileo, the naked eye was
all that was available to observe
the sky. The naked eye is limited
in two main ways: it is unable to
record detail, and it can only detect
objects that are reasonably bright.
When looking at a full moon, the
lunar diameter subtends (spans) an
angle of^1 ⁄ 2 o at Earth’s surface. This
means that two lines extending
from opposite sides of the moon
meet at the eye to make an angle of(^1) ⁄ 2 o. However, the naked eye can only
detect separate objects that are more
than about^1 ⁄ 60 o apart. This is the
eye’s resolution, and determines
the level of detail it can detect.
Looking at the full moon with
the naked eye, the lunar diameter
is resolved into only 30 picture
elements, analogous to individual
pixels in a digital photograph (see
below). Dark lunar seas and lighter
lunar highland are discernible, but
individual mountains and their
shadows are beyond detection.
Looking up at the night sky
on a cloud-free, moonless night in
Galileo’s Italian countryside, 2,500
stars would be visible above the
horizon. The Milky Way—the disk
of the solar system seen side-on—
looks like a river of milk to the
naked eye. Only a telescope shows
that the Milky Way seems to be
made up of individual stars; the
bigger the telescope, the more stars
GALILEO’S TELESCOPE
IN CONTEXT
KEY ASTRONOMER
Galileo Galilei (1564 –1642)
BEFORE
1543 Nicolaus Copernicus
proposes a theory of a sun-
centered cosmos, but proof
is needed because Earth does
not seem to move.
1608 Dutch eyeglass-makers
develop the first telescopes.
AFTER
1656 Dutch scientist
Christiaan Huygens builds
ever-bigger telescopes that
are capable of detecting more
detail and fainter objects.
1668 Isaac Newton produces
the first reflecting telescope,
an instrument that is far less
affected by the distortion of
chromatic aberration (p.60).
1733 The first flint glass/crown
glass achromatic lens is made.
This greatly improves the
potential image quality of
refracting telescopes.
The Milky Way is nothing
else but a mass of
innumerable stars planted
together in clusters.
Galileo Galilei
The resolution of the naked eye is about^1 ⁄ 60 °.
The moon subtends an angle of^1 ⁄ 2 ° seen from Earth,
meaning that the lunar diameter can be resolved
into 30 picture elements.
Moon
1 picture element
½°
(^1) ⁄ 60 °
Eye