Astronomy, an Observational Science

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Chapter 1: Astronomy,
an Observational Science

1. Introduction
- Astronomy is probably the oldest of all the sciences.

- It differs from virtually all other science disciplines in that it is not
possible to carry out experimental tests in the laboratory.

- Instead, the astronomer can only observe what he sees in the
Universe and see if his observations fi t the theories that have been
put forward.

1.2. Galileo Galilei’s proof of the Copernican theory of the solar system
One of the first triumphs of observational astronomy was Galileo’s series of
observations of Venus which showed that the Sun, not the Earth, was at the centre
of the Solar System so proving that the Copernican, rather than the Ptolemaic,
model was correct (Figure 1.1).

Figure 1.1 Galileo Galilei: a portrait
Wikipeda Commons.

In the Ptolemaic model of the Solar System (which is more subtle than is
often acknowledged), the planets move around circular ‘epicycles’
whose centres move around the Earth in larger circles, called deferents,
as shown in Figure 1.2.

Figure 1.2 The centre points of the
epicycles for Mercury and Venus
move round the Earth with the
same angular speed as the Sun.

Figure 1.3 shows a set of drawings of Venus made by
Galileo with his simple refracting telescope
In the Ptolemaic model, Venus
lies between the Earth and the
Sun and hence it must always
be lit from behind, so could only
show crescent phases.
In contrast, in the Copernican
model Venus orbits the Sun.
When on the nearside of the
Sun, it would show crescent
phases whilst, when on its far
side but still visible, it would
show almost full phases

1.3. The celestial sphere and stellar magnitudes
(Thiên cầu và độ sáng sao)
Looking up at the heavens on a clear night, we can imagine that the stars
are located on the inside of a sphere, called the celestial sphere, whose
centre is the centre of the Earth.
1.3.1 The constellations (chòm sao)
As an aid to remembering the stars in the night sky, the ancient astronomers
grouped them into constellations; representing men and women such as Orion, the
Hunter, and Cassiopeia, mother of Andromeda, animals and birds such as Taurus
the Bull and Cygnus the Swan.

The chart in Figure shows the brighter stars that make up the
constellation of Ursa Major.

1.3.2 Stellar magnitudes (độ sáng của sao)

The early astronomers recorded the positions of the stars on the
celestial sphere and their observed brightness.
The first known catalogue of sta...
Chapter 1: Astronomy,
an Observational Science
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