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Lec. 26: The Night Sky - II
The Night Sky II
Lecture 26
Lec. 26: The Night Sky II
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Lecture Topics
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Astronomical coordinates
The changing sky
Using RA and Dec
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Finding objects using coordinates
What’s up tonight?
If time:
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Planetarium demo
See Planetarium S/W link in “interesting
astronomy sites” on class web site
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Lec. 26: The Night Sky - II
Equatorial Coordinates
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Astronomers use
equatorial coordinates
to locate objects on the
celestial sphere.
Right Ascension
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Notation: RA or 
Equivalent to longitude
Declination


Notation: Dec or 
Equivalent to latitude
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Defining RA and Dec.

RA is measured in hours
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The range is from 0 to 24 hours
increasing on sky towards the
east.
The “zero point” is towards the
constellation Pisces (Vernal
Equinox).
Dec is measured in degrees.


The zero is on the equator
North Pole = 90o
South Pole = -90o
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Lec. 26: The Night Sky - II
Equatorial Coordinates (Cont’d)
The equatorial (celestial) coordinate
system is fixed on the sky.
 The coordinates (, ) of the stars and
constellations do not change (ignoring
precession).
 Since it is an “earth-centered” system
the coordinates (, ) of the sun do
change.

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Equinoxes
North
23.5o
Sun here on
first day of fall
Autumnal
Equinox
Equatorial
Plane
1st day of
summer
12
6 hr
18
Ecliptic
Plane
0
1st day of
winter
Vernal Equinox
Sun here on first
day of spring
Equinoxes at the intersection of
the equatorial and ecliptic planes.
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Lec. 26: The Night Sky - II
The Changing Sky
At the same time each night, a different
RA will be on the meridian at different
times of the year.
 For instance at midnight, the RA’s on
the meridian are:

Sept. 21
 Dec. 21

0 hr ,
6 hr ,
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March 21 12 hr
June 21 18 hr
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Viewed looking
down from the
north.
6 hr
Winter
Sun
12 hr
Sun
0 hr
Fall
Midnight
Sep. 21
Midnight
Dec. 21
18 hr
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Lec. 26: The Night Sky - II
Simulation 1: Yearly variation


Earth moving around
the sun.
Why we see different
parts of the sky at
different times of the
year.
Here the observer is out
at midnight each night
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Simulation 2: Daily variation


Rotation of earth
as it moves around
the sun.
How we see
different portions of
the sky at night.
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Lec. 26: The Night Sky - II
Viewed looking down
from the north.
6 hr
Spring
midnight
12 hr on
meridian
12 hr
0 hr
Sun
4:00 AM
16 hr on
meridian
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18 hr
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The Changing Sky (continued)
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Lec. 26: The Night Sky II
to
sta
r
sta
r
mi
dn
igh
t
4 min.
to

Each night a given
object will pass over the
meridian 4 minutes
earlier.
This corresponds to 2
hours earlier each
month, or 24 hours in
one year.
Objects rise and set
earlier each day.
At a given time, the RA
crossing the meridian
increases by 4 min. per
day.
midnight

1 day
along orbit
Motion of earth along
orbit is exaggerated.
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Lec. 26: The Night Sky - II
In-class question

You are in Santiago, Chile (in the
southern hemisphere). Which direction
does the Sun move through the sky?
a)
b)
c)
d)
e)
north to south
east to west
south to north
west to east
It doesn’t move
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Using RA to find an object
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Given two of RA, meridian transit time, and date
Find remaining one
Method
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Always work with RA (RAmid) on the meridian at
midnight
Find closest reference date (via RA or date)
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Sep 21 (0), Dec 21 (6), Mar 21 (12), Jun 21 (18 hr)
Use fact sky changes by ~ 1 hour of RA per 2 weeks.
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Lec. 26: The Night Sky - II
On what date does Orion appear on
the meridian at midnight?

Orion Nebula

RA = 5.5 hr
Dec = -5.5 deg
RA = 6 hr transits at midnight on Dec 21.
 Orion transits at midnight on Dec 14.
Also
 Orion transits at 9:00 p.m. on Jan 28.
 Orion transits at 3:00 a.m. on Oct. 31.

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Example 1: What RA is on the
meridian for a given date and time?

What RA is on the meridian at 3:00 am
on Feb. 21?
Dec 21 -- 6 hr overhead at midnight
 Feb 21 -- 2 months later
=> add 4 hr
=> 10 hr overhead at midnight
 3:00 am
=> 3 hr later
=> 13 hr overhead

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Lec. 26: The Night Sky - II
Ex. 2: On what day does a given RA
cross the meridian at a specific time?

A constellation is at RA = 14 hr. When
will it cross the meridian at 9:00 pm?
When 14 hr crosses at 9:00 pm, 17 hr
crosses at midnight.
 Mar 21: 12 hr crosses at midnight
 17-12 = 5 hr
=> 2.5 months (10 weeks)
=> June 7

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RA/Meridian Question
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What RA is on the meridian at 9:00 pm tonight (October 29)?
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Where is Vega (RA ~ 18.5 hrs) at this time?
What is nearest reference date?
a) Mar 21
b) Jun 21
c) Sep 21
d) Dec 21

What is RA at midnight on that date?
a) 0 hr
b) 6 hr
c) 12 hr
d) 18 hr

There are 23 days between ref. date and today. What is RA at midnight
tonight?
a) 22 hr
b) 0 hr
c) 2 hr
d) 4 hr
0 + 2 hr = 2 hr

What RA is on meridian at 9:00 pm tonight?
a) 9 hr
b) 23 hr
c) 2 hr
d) 5 hr
2 - 3 hr = 23 hr

Is Vega east or west of this?
a) east
b) west

When did Vega transit?
23 - 18.5 = 4.5 hr
a) 3 pm
b) 4:30 pm
c) 6:30 pm
d) 9:00 pm
9:00 - 4:30 = 4:30 pm

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Lec. 26: The Night Sky - II
Finding objects (continued)

Circumpolar objects can be visible any
time of the year
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For example, Polaris, the pole star.
From Ithaca southerly objects are best
observed during transit.

Really, all objects are best observed when
they transit (you look through the least
atmosphere)
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What’s “up” tonight?
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The Moon is near full.
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Bad time for viewing planets -
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Not good for star gazing.
Mars may be visible at sunset
Jupiter will be visible ~ 2.5 hours after sunset
Notable constellations
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Cygnus is west of the meridian at sunset
The Big Dipper will be low in the north
Orion rises at ~ 10:00 AM
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