Chapter 2:Patterns in the Sky –

Motions of Earth

Looking around the sky:What are the

Constellations?

Every culture has their own constellations

So whose constellations do astronomers use?

88 Official ConstellationsAndromeda

Established by the IAU in 1922 with boundaries finalized in 1928

What you see at night depends on what time it is

If it’s clear, go out tonight sometime and note the locations of the bright stars. Do the same thing two or three hours later to see how things have

moved

What you see at night depends on what month you

look

Looking south in July

Looking south in January

What you see at night depends on where you are

Cerro Tololo Chile

Why does the sky change during the course of a

night?

The Earth rotates about its’ polar axis so the stars make circles around the celestial pole.

Why does the sky change over the course of a year?

As we orbit the Sun, the direction opposite the Sun changes and we only see the stars that are on the

opposite side of Earth from the Sun

Why does the sky change with your location?

As you move away from the pole your horizon moves with you but the locations of the celestial

poles and celestial equator remains the same

The Earth moves in two ways that affect what we see

Orbital motion around the Sun causes seasonal changes in the constellations

Rotational motion around the polar axis causes changes over the course of the night

The Celestial Sphere Model

We know the universe doesn’t rotate around us but it can be useful to imagine that it does

The Celestial Sphere can be used to explain the daily

motions and the changes due to location