Chapter 1
Discovering the
Night Sky
What do you think?
•
•
•
•
Are constellations just mythic figures in the sky?
What causes the seasons?
How many zodiac constellations are there?
When, if ever, is the Moon visible during the
daytime?
• Does the Moon have a dark side that we never
see from Earth?
THE SCALES OF
THE UNIVERSE
The range of objects
we study are from the
extremely small
subatomic particles, to
objects which are
gigantic, such as a
galaxy or the size of
the known universe
itself.
Each division up the
line indicates an
increase in size by
100,000.
Constellations make locating stars easy
Constellations and Asterisms
• Constellations are areas in the daytime and
nighttime sky.
• They originated from myths, but today they
help star gazers to organize and describe
what they see
• Asterisms are the mythical figures in the
sky.
• The asterism Orion is in the Constellation
Orion
Use the Big
Dipper in
the
northern
sky as a
way to find
other
groups of
stars
Use the
winter
triangle to
find southern
constellations
during winter
evenings
Use the Summer Triangle to find southern
constellations during summer evenings
Angular distances between stars in
the sky are measured in degrees
Angular distances between stars in
the sky are measured in degrees
Night time Sky
• The sky in and around Lisle isn’t very dark
• I seems a lot darker than Chicago, and it is,
but it is still not very dark.
• This is a map that will help to compare the
light pollution at different places in the U.S.
• Dark Sky finder
Using Angular Size to Solve a
Problem
• How does the Angular size of the Moon on
the horizon compare to the angular size of
the moon when it is high in the sky?
Using Angular Size to Solve a
Problem
• How does the Angular size of the Moon on
the horizon compare to the angular size of
the moon when it is high in the sky?
• How does the Angular size of the Sun on
the horizon compare to the angular size of
the Sun when it is high in the sky?
• How does the Angular size of the Sun
compare to the angular size of the Moon?
Solar Eclipse
Solar Elclipse
Hyperlink to Sidereal vs. synodic month
Solar
Eclipse
Annular Eclipse-moon
within sun
Partial Eclipse
Total Eclipse-totality
An anular eclipse is when a small ring of the Sun is visible
Here is some cool information on
Lunar Eclipses
http://www.mreclipse.com/Special/
LEprimer.html
Eclipses occur only when the Moon crosses
the ecliptic during the new or full phase
Power of the Mayans
The serpent comes only on
the days of the equinoxes
Video of serpent
• The Greek philosopher Aristotle (died
324 B.C.) proposed that the heavens
were literally composed of 55
concentric, crystalline spheres to
which the celestial objects were
attached and which rotated at
different velocities with the Earth at
the center. The following figure
illustrates the ordering of the spheres
to which the Sun, Moon, and visible
planets were attached.
• With the help of the Catholic Church,
this view predominated until 1611
when Galileo was able to show
otherwise.
• The church believed that the holy
scripture had clearly indicated that
the Earth is at the center of the
Universe, anything else would be
heresy
An
imaginary
Celestial
Sphere
surrounding
Earth aids in
navigating
the sky
Ophiuchus
A neat trick for when you
become a Sailor
• Polaris is the North Star
• It is directly above the North Pole on the Celestial
Sphere
• The North Star is always the same degrees of
declination as you position in latitude
• This is one of the modes of
maritime navigation before GPS
(Beware of technology it can
make you stupid/helpless if you
are not careful!)
What we see in the Sky
• Here is a simulation that will help to
visualize our perspective on the celestial
sphere
Earth’s
rotation
causes the
Sun and
stars to
appear to
move
Motion of Stars at the North Pole
Motion of Stars at the Equator
What about the Sun? It seems to move across
the celestial sphere on a path called the ecliptic.
Signs of the zodiac
• Before the family guy, people used to spend
much more time gazing at the night sky.
• Over periods of 1000s of years, human kind
began to share stories about the night sky and
spread mythologies about how the stars “got into
the sky”
• Humans had “seen” figures in the sky and
named them.
• The most important of those are the asterisms
that make up the asterisms of the zodiac
Astronomy and Astrology don’t
always agree
Constellation
Astrology Date
True Date
Aries
March 21 - April 20
April 19 - May 13
Taurus
April 21 - May 21
May 14 - June 19
Gemini
May 22 - June 21
June 20 - July 20
Cancer
June 22 - July 22
July 21 - August 9
Leo
July 23 - August 21
August 10 - September 15
Virgo
August 22 - September 23
September 16 - October 30
Libra
September 24 - October 23
October 31 - November 22
Scorpio
October 24 - November 22
November 23 - November 29
Ophiuchus
Not Included
November 30 - December 17
Sagittarius
November 23 - December 22
December 18 - January 18
Capricorn
December 23 - January 20
January 19 - February 15
Aquarius
January 21 - February 19
February 16 - March 11
Pisces
February 20 to March 20
March 12 - April 18
Notice how the Sun spends more time in Ophiuchus (OH-fee-U-kus) than in Scorpio yet
Ophiuchus is not included in the Astrology
Summer solstice
Winter solstice
Equinox position
The Sun rises at different points along the horizon at different
times of the year and also peaks at different heights.
Position of the “noon-time” Sun when
photographed every two weeks over a year
The “altitude” of the Sun impacts how much
energy is received on Earth resulting in seasons.
The seasons are caused by the tilt
of the Earth’s axis of rotation
In other words ...
• The Earth’s rotation on its axis determines
the length of the day
• The Earth’s orbit around the Sun
(revolution) determines the length of the
year
• The Earth’s tilt with respect to the ecliptic
causes the seasons.
One common misconception is
that the Moon is only visible at
night. However, the time of day in
which the Moon is in our sky
varies depending on its phase.
This picture clearly displays the
Moon, visible during the day.
The changing phases of the Moon originally
inspired the concept of the month
What did you think?
• Are constellations just mythic figures in the sky?
A constellation is an entire region of the celestial sphere and all that
stars in it.
• What causes the seasons?
The tilt of Earth’s rotational axis causes the seasons.
• How many zodiac constellations are there?
There are 13 zodiac constellations, the “unknown one is Ophiuchus.
• When, if ever, is the Moon visible during the daytime?
The Moon is visible during the day for half of the lunar cycle of phases.
Self-Check
1. Describe the nature and value of constellations and
asterysms.
2. Define the elements of the celestial sphere.
3. Describe how the appearance of the sky changes
over time because of the motion on Earth.
3. Define two solstices and two equinoxes; explain
the orientation of the ecliptic on the celestial
sphere and how it produces seasons on the Earth.
Self-Check
5. Explain by drawing a diagram showing how lunar
phases are controlled by the relative positions of
the Sun and the Moon.
6. Explain when and why solar and lunar eclipses
occur and why there are not such eclipses every
month.
7. Be able to describe the parts of an eclipse.