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Night Sky
Chapter 4
Topics
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The celestial sphere
Motion of the Sun
Phases of the Moon
Motion of the stars
Constellations and the changing view of the
night sky
Remember!
• Although we see very nice pictures and diagrams of
planets circling the Sun, the Moon circling the Earth, etc.,
we only observe things from the Earth (or near the Earth in
the case of space-based telescopes).
• Our only reference frame for observation is that of the
Earth (except for the few space probes, telescopes, etc.)
• The positions of stars (because they are so distant) are
essentially fixed. We are the ones who are revolving
(around the Sun) and rotating (about our axis). Thus, in our
reference frame, stars appear to move.
Celestial coordinates
• How do we specify the position of something in
the sky?
• “over yonder” is not technical enough.
• consider how we specify objects on the earth
– longitude (gives E/W location relative to the Prime
Meridian)
– latitude (gives N/S location relative to the Equator)
• use a similar coordinate system on a Celestial
sphere
Celestial sphere
NCP
• Aligned with the Earth
VE
• The Celestial Poles
• Celestial Equator
• The Ecliptic
SS
• Vernal Equinox
• Autumnal Equinox
• Winter Solstice
• Summer Solstice
WS
AE
SCP
Location of stars
NCP
• Right ascension is
measured ccw from
the vernal equinox in
units of hr:min:sec SS
• Declination is
measured from the
celestial equator in
deg:min:sec
VE
WS
AE
SCP
Question
• What is the radius of the celestial sphere?
A.
B.
C.
D.
E.
infinity
at about the Moon’s distance from Earth
at about the Sun’s distance from Earth
at about the nearest star’s distance from Earth
none of the above
• Answer:
E
Example
• star
location
• RA: 10 hrs
• Dec: 75
degrees
NCP , 90 degrees
VE 0 hours
SS
6 hrs
0 deg
AE
12 hrs
WS
18 hrs
Question
•
Does a star’s right ascension and declination depend on
where you are located on the Earth when you observe
the star?
A.
B.
C.
D.
•
yes
no
yes and no because it depends on your latitude, but not your
longitude
yes and no because it depends on your longitude, but not your
latitutude
Answer:
B
Precession
• Axis wobbles
• Period is 23,000
years
• “North Star” is not
always Polaris
• Locations of all stars
as measured by right
ascension and
declination will
change
Local sky coordinates
• Azimuth is the
angle around from
north and Altitude
is the angle up
from the horizon.
• Bean-boy sees the
E
90 deg
object at about
azimuth 200 deg
and altitude 60 deg
above horizon.
• Zenith is the point
directly overhead
90 deg, Zenith
S
180 deg
W
270 deg
N
0 deg
Motion of the Sun
• The Sun’s path (as viewed from Earth) defines the
plane of the ecliptic.
• The Moon, planets and most asteroids all lie close
to the plane of the ecliptic b/c the solar system is
nearly flat (like a pancake).
• The Sun’s declination changes during the year
depending on where it is in its path--this is the
cause of seasons.
Tilt of Ecliptic
NCP
• The ecliptic is tilted
23.5º from the
celestial equator.
VE
SS
WS
AE
SCP
Sun’s path in the sky
• The sun’s apparent path
in the sky (rising in the
east and setting in the
west) is “caused” by
Earth’s rotation
• During one day, the Sun
only moves 1/365 of the
way around the ecliptic.
• Therefore, it moves very
little in one day.
• The location along the
horizon where the Sun
rises and sets depends
on where the Sun is
located on the ecliptic.
Location of the Sun’s path
east
west
The actual path depends on time of year (i.e. where the Sun
is located on the ecliptic). Sometimes it rises higher, and
sometimes it rises lower in the sky.
Question
•
When is the Sun directly overhead (at the zenith)
at noon in High Point?
A.
B.
C.
D.
E.
F.
•
Every day of the year
At the vernal equinox
At the spring equinox
At the summer solstice
At the winter solstice
It will never appear directly overhead as viewed in
High Point
Answer:
F
Seasons
• Seasons are determined by
the location of the Sun on
the ecliptic.
• Near the summer solstice,
the Sun has a greater SS
declination above the
equator.
• At greater declinations,
our latitude experiences
the most direct sunlight
(summer).
NCP
VE
WS
AE
SCP
Question
•
The average temperature (at a certain location on
the Earth) when the Sun is at the autumnal
equinox is ________ the average temperature at
the same location when the Sun is at the vernal
equinox.
A. greater than
B. less than
C. equal to
•
Answer:
equal to
Practice
• A friend asks, “Why is it hot in the summer and
cold in the Winter?” Write an answer to your
friend.
• Your grandma asks “Why is it summer in the
United States when it is winter in Australia? Write
an answer to your grandma.
• Compare your response to someone else’s
response in the class.
The Moon
• The Moon orbits in 29.5 Earth days (one
lunar month)
• The Moon exhibits a cycle of phases as the
illuminated surface seen from earth changes
• The Moon’s orbital plane is tilted 5 degrees
to the ecliptic.
Phases of the Moon
Cycle of Phases -- one month
animation
day 1 - new
day 10.5
waxing gibbous
day 14 - full
day 3.5
day 7
waxing crescent first quarter
day 17.5
waning gibbous
day 21
last quarter
day 24.5
waning crescent
Eclipses
• The plane of the Moon’s orbit is tilted 5º from the
ecliptic, so eclipses are rare.
• Lunar eclipse--when the sun is between the Earth
and Moon at a time when the moon would
normally be full. As a result, the Earth shades the
moon and it “disappears.”
• Solar eclipse--when the Moon is between the
Earth and Sun (new moon) and its shadow is cast
on the Earth.
Question
•
Does everyone in a certain hemisphere see a
solar or lunar eclipse at the same time or at least
on the same day (assume it’s daytime or
nighttime in all of those locations)?
A.
B.
C.
D.
E.
•
yes
no
for solar eclipses, yes, but for lunar eclipses, no.
for lunar eclipses, yes, but for solar eclipses no.
It depends on the date the eclipse occurs, so
sometimes yes and sometimes no.
Answer:
B
Question
•
Can a total solar eclipse occur when the
moon is not full or new?
A. yes
B. no
C. it depends...
•
Answer:
B
Question
•
What is the dark side of the moon?
A. The side of the Moon that nobody on Earth
can see.
B. The side of the Moon that we in High Point
can’t see, although others on Earth can see it.
C. The side of the Moon that never faces the
Sun.
D. none of the above
•
Answer:
A
Question
•
In the movie, Apollo 13, the actors view the dark side of
the moon and comment on the fact that they would have
landed there. Why would this have been a bad idea?
A.
B.
C.
D.
E.
•
It would have been too cold and dark.
They could have only communicated with the southern
hemisphere.
They could have only communicated with the northern
hemisphere.
They could not have communicated with anyone at all.
We would not be able to see them with our television cameras.
Answer:
D
The night sky
• “Groups of stars” are called constellations.
• Constellations were officially defined by
IAU in 1930.
• There are 88 constellations.
• These groupings appear together in the sky,
although the stars may be very different
distances.
Zodiac
• Twelve constellations
• Significant in astrology
• Aligned with the plane of the ecliptic; thus,
the Sun passes through these
constellations.
Leo
Scorpius
Aquarius
Taurus
Virgo
Sagittarius
Pices
Gemini
Libra
Capricornus
Aries
Cancer
Zodiac
Scorpius
Libra
Virgo
Sagittarius
Leo
3/21
6/21
12/21
9/21
Aquarius
Gemini
Pices
Aries
Taurus
Zodiac
6/21
• In mid June the sun is “in” Cancer
• Overhead at night is Capricorn
Summary
• Our reference frame for observation is the Earth.
Although, it’s in motion about the Sun, from our
reference frame (i.e. perspective) it appears that
everything else is in motion with respect to the
Earth.
• The Celestial Sphere provides a coordinate system
and a useful model for understanding the
perceived motion of the Sun, Moon, and Stars.
• Using this model, we can explain the seasons and
phases of the moon