1

Chapter 2Discovering the Universe for Yourself

Mark Twain (1835 – 1910)American author, from Huckleberry Finn

We had the sky, up there, all speckled withstars, and we used to lay on our backs andlook up at them, and discuss about whetherthey was made, or only just happened.

Constellations

In ancient times,constellations only

referred to the brighteststars that appeared to

form groups, representingmythological figures.

Today, constellations are well-defined regions of thesky, irrespective of the presence or absence of bright

stars in those regions.

88 constellations

The stars of aconstellation onlyappear to be close

to one another.

Usually, this isonly a projection

effect.

The stars of aconstellation maybe located at verydifferent distances

from us.

2

How do we locate objects in thesky?

(1) Know your reference points.(2) Locate an object by its altitude (above horizon) and direction (along horizon)

We measure the sky in angles...

Angle measurements:

• Full circle = 360º

• 1º = 60′ (arcminutes)

• 1′ = 60″ (arcseconds)

Review: Coordinates on the Earth• Latitude: position north or south of equator

– projection of latitude on to sky is Declination

• Longitude: position east or west of prime meridian (runsthrough Greenwich, England)– projection of longitude on to sky is Right Ascension

3

The Celestial Sphere• Zenith = Point on

the celestial spheredirectly overhead

• Nadir = Point on thec.s. directlyunderneath (notvisible!)

• Celestial equator =projection of Earth’sequator onto thec.s.

• North celestial pole= projection ofEarth’s north poleonto the c.s.

The North Starlies above North Celestial Pole, soit appears stationary as Earth spins

The Celestial Sphere The Celestial Sphere

• Positions of starsindicate location onearth - latitude

• Elevation of Northstar = geographiclatitude ℓ (northernhemisphere)

ℓ

90o - ℓ

4

Why do stars rise andset?

Earth rotates counter-clockwisewhen looking down at the NorthPole, so stars appear to rise inthe east and set in the west.

Stand and spin...

• Orientation– North = your head– East = left hand– West = right hand– South = feet

• Spin counter-clockwise as seen from above.• In which direction do you see objects first

(or rise)?• In which direction do you see objects last

(or set)?

SkyGazer

• We will learn more about the motions ofthe stars using SkyGazer

• Complete Motions of Stars lab