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The Night Sky
A. Astronomy vs. Astrology
Astronomy → scientific study of what is beyond Earth. This branch of science is concerned
with the motions, compositions and structure of celestial objects.
Astrology → the belief that the position of celestial objects can influence and predict human
behaviour.
B. Celestial Objects
Any natural object in space, such as the Sun or the Moon, is a celestial object. Most of the
celestial objects we see at night are stars.
Some celestial objects can be seen with the unaided eye and can be identified by their motion.
We can see the Sun and stars because they emit their own light, they are luminous. We can
see the planets and other celestial objects because they reflect light from the Sun.
C. The Solar System
The Solar System consists of the Sun and the celestial objects that orbit the Sun due to its
gravitational pull. This includes the eight planets: Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, and Neptune.
All of the planets orbit the Sun in the same plane but at different speeds.
The Solar System includes two regions populated by smaller celestial objects: the asteroid
belt and the Kuiper Belt.
i.
The asteroid belt lies between Mars and Jupiter and consists of celestial objects
composed mainly of rock and metal.
ii.
The Kuiper belt lies beyond the planets and is much larger than the asteroid
belt. It consists mainly of frozen “ices”, such as methane, ammonia, and water.
The Kuiper belt is home to at least three dwarf planets- Pluto, Haumea, and
Makemake.
D. The Planets
The first four planets, are known as the “inner” or “terrestrial” planets and have only three
moons between them.
The four outer planets, known as “gas giants” have more than 60 moons between them.