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Lecture 5 – The Night Sky
What is the connection between what’s in
the book and what’s in the sky?
We will discuss many types of
objects; let’s see them too
• Examples:
• Red Supergiants (Antares)
• “Population II Stars”
• Galaxies (M81)
• Basic astronomical
phenomena known since
• First, we need a system to
describe the sky
First reference system: the
Horizon System
Two coordinates: altitude and azimuth
In Horizon System, we see
motions in the sky
• The Sun rises in the east, reaches highest altitude
angle due south, sets in the west
• When the Sun sets, it gets dark and we see the
stars and planets
• The Moon “ “ “ “ “
• The Moon rises at a different time each night and
is seen against a different constellation
• The constellations in the evening sky are different
in different seasons
Question for the audience:
What is going on to
cause this east-to-west
motion of all objects,
rising in east and
setting in west?
Seasonal differences in the night
sky: go out tonight at 10PM
Constellations Bootes almost overhead
Constellation Leo in the west
Bright star Vega low in the east
Constellation of Virgo in the south
Constellation of Scorpius (with bright
star Antares) low in southeast.
• Check it out with the help of the SC1
Go out again at 10PM in midSeptember
• Scorpius (and Antares) will
be due south, to south-west
• The constellation Aquila
(bright star Altair) will be in
• Bright star Vega straight
• Arcturus and Bootes way
over in the west
• Come back in February,
more changes still
Question: What causes this
phenomenon of the “Parade of
the Constellations”?
more next time