Download Lecture 02

The night sky?
With the naked
eye, we can see
more than 2000
stars as well as
the Milky Way.
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Constellations
A constellation is a
region of the sky.
Eighty-eight
constellations fill
the entire sky.
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The Celestial Sphere
Stars at different
distances all appear to
lie on the celestial
sphere.
The ecliptic is the
Sun’s apparent path
through the celestial
sphere.
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The Celestial Sphere
The 88 official
constellations
cover the celestial
sphere.
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Question : The celestial equator is ?
A The path of the Sun compared to the stars.
B The path of the Moon compared to the stars.
C Always directly overhead at the Earth's equator.
D The average path of planets on a star chart.
E Always along the horizon for people on Earth's
equator.
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The Milky Way
A band of light that
makes a circle
around the celestial
sphere
What is it?
Our view into the
plane of our galaxy
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The Milky Way
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The Local Sky
An object’s altitude (above horizon) and direction
(along horizon) specify its location in your local sky.
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The Local Sky
Zenith: The point
directly overhead
Horizon: All points
90° away from zenith
Meridian: Line
passing through zenith
and connecting N and
S points on the horizon
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Angular Size
An object’s angular size
appears smaller if it is
farther away.
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Apparent Brightness
• A bright star in the sky.
– Is really very bright?
– Or is it very dim but very close?
• A dim star in the sky
– Is it really very dim?
– Or is it very bright but very far away?
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Why do stars rise and set?
Earth rotates west to east,
so stars appear to circle
from east to west.
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Question : In the northern hemisphere, the stars rise in the
East, set in the West and revolve counter-clockwise around
the North celestial pole. In the southern hemisphere the
stars?
A. Rise in the East, set in the West and revolve anti-clockwise
around the South celestial pole.
B. Rise in the East, set in the West and revolve clockwise
around the South celestial pole.
C. Rise in the West, set in the East and revolve clockwise
around the South celestial pole.
D. Rise in the West, set in the East and revolve anti-clockwise
around the South celestial pole.
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Our view from Earth:
• Stars near the north celestial pole are circumpolar and
never set.
• We cannot see stars near the south celestial pole.
• All other stars (and Sun, Moon, planets) rise in east and
set in west.
A circumpolar
star never sets.
celestial equator
This star
never rises.
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your horizon
Thought Question
What is the arrow pointing to?
A. the zenith
B. the north celestial pole
C. the celestial equator
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Question : You are standing at the north pole and you look
at the stars. Which statement below describes what you
would see?
A
B.
C.
D.
There are no circumpolar stars.
There are only circumpolar stars just after sunset.
Some stars will be circumpolar and some will not.
Every star you see is circumpolar.
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Question : You are standing on the Equator and you look at
northern pole star on the horizon. Which statement below
describes what you would see?
A
B.
C.
D.
There are no circumpolar stars.
There are only circumpolar stars just after sunset.
Some stars will be circumpolar and some will not.
Every star you see is circumpolar.
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Why do the constellations we see depend
on latitude and time of year?
•
•
They depend on latitude because your position on
Earth determines which constellations remain
below the horizon.
They depend on time of year because Earth’s orbit
changes the apparent location of the Sun among
the stars.
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Review: Coordinates on the Earth
• Latitude: position north or south of equator
• Longitude: position east or west of prime meridian
(runs through Greenwich, England)
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The sky varies with latitude but not longitude.
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Altitude of the celestial pole = your latitude
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Thought Question
The North Star (Polaris) is 50° above your horizon,
due north. Where are you?
A. You are on the equator.
B. You are at the North Pole.
C. You are at latitude 50°N.
D. You are at longitude 50°E.
E. You are at latitude 50°N and longitude 50°E.
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The sky varies as Earth orbits the Sun
• As Earth orbits the Sun, the Sun appears to move eastward along
the ecliptic.
• At midnight, the stars on our meridian are opposite the Sun in
the sky.
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Special Topic: How Long Is a Day?
• Solar day = 24 hours
• Sidereal day (Earth’s rotation period) = 23 hours,
56 minutes
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Summary Of The Night Sky
• Now work on Page 1-6 of the tutorial book.
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