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Lecture 4

AST 101 Lecture Notes - Lecture 4: Celestial Spheres


Department
Astronomy
Course Code
AST 101
Professor
Walter Freeman
Lecture
4

Page:
of 3
We#can#treat#the#stars#as#all#rotating#together,#on#an#invisible#sphere#far#away
We#expect#this#to#get#the#stars#"right"#(things#that#are#really#far#away)#and#the#
planets#and#Sun#"wrong"#(things#that#are#closer)
The#axis#of#rotation#ins#the#same#as#the#Earth's,#and#it#rotates#once#per#day
Only#half#of#the#sphere#is#visible#because#Earth#is#in#the#way
Summary#of#the#Celestial#Sphere
The#Earth#rotates#from#West#to#East
Stars/planets/Sun#rise#in#the#East#and#set#in#the#West
Remember,#the#celestial# sphere's#apparent#rotation#is#backwards#from#this
Celestial#Sphere#"rotates"#from#East#to#West
Rise:#becoming#visible#over#the#horizon
Star#is#always#visible
CIRCUMPOLAR:#rotating#around#a#celestial# pole#without#setting#(i.e.#Polaris)
The#celestial# sphere#turns#counterclockwise!#(Rising#in#east#and#setting#in#west)
Which#way#does#the#celestial# sphere#turn?
Over#the#course#of#one#day,#the#Earth#doesn't#move#very#much
This#means#the#celestial# sphere#model#can#show#us#how#the#sun#moves#each#day
It#rises#in#the#East#and#sets#in#the#West,#just#like#all#the#other#stars
How#about#the#Sun?
The#stars#move#a#lot#over#time#since#the#Earth#isn't#pointed#in#the#same#
direction
The#sun#moves#higher#or#lower#in#the#sky#a#little#bit
Exactly#24#Hours
Solar#Day:#Day#judged#by#when#the#sun#is#highest#in#the#sky#to#when#the#sun#is#
highest#in#the#sky
Sidereal#Day:#Day#judged#by#when#the#stars#are#highest#in#the#sky#to#when#those#
same#stars#are#highest#in#the#sky
Consequences#of#the#Earth's#Revolution
Consequences)of)the)Earth's)Rotation)&)Revolution
Tuesday,# September# 6,#2016
2:09#PM
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The#stars#don't#move#at#all
The#Sun#and#the#Planets#move#a#lot
23#hours#and#56#minutes
same#stars#are#highest#in#the#sky
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