Lecture 5 – The Night Sky

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

Lecture 5 – The Night Sky