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”

(Arcturus)• Galaxies (M81) • Basic astronomical

phenomena known since antiquity

• 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

chart!

Go out again at 10PM in mid-September

• Scorpius (and Antares) will be due south, to south-west

• The constellation Aquila (bright star Altair) will be in southeast

• Bright star Vega straight overhead

• 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”?

Demonstration, more next time