Chapter 1:
The Cycles of
the Sky
Constellations
help
astronomers
know the
positions
of celestial
objects
Zodiac signs
These coordinates can be
used to describe the position
of stars locally
zenith
Altitude
Azimuth
Angular distances of stars are
measured in degrees
An
imaginary
Celestial
Sphere
surrounding
Earth aids in
navigating
the sky
What is wrong with this
cartoon?
Rotation vs. Revolution
• Rotation represents
the Earth’s spin on its
axis.
• Revolution represents
the Earth’s orbit
around the Sun.
• This defines the
length of 1 day.
• This defines the
length of 1 year
• 1 day= 24hours
• 1 year= 365.25 days
Earth’s
counterclockwise
rotation
causes the
Sun and
stars to
appear to
move from
East to West
Motion of Stars at the North
Pole
Motion of Stars at the Equator
Precession
causes the
position of the
North Celestial
Pole to change
over a period of
26,000 years.
UNLEARN
Common Misconception
The seasons occur due to the changing
distance of the Earth from the Sun.
When we are in our part of the orbit
that is closer to the Sun, it is Summer,
and when we are farther away from
the Sun, it is winter
This statement is NOT correct!
LEARN
Some Facts
• When the Earth is physically
closer to the Sun, it is winter
time in the Northern
Hemisphere, NOT summer
• The Northern hemisphere and
Southern hemisphere have
opposite seasons from each
other.
• When the Earth is physically
farther from the Sun, it is
summer time in the Northern
hemisphere, NOT winter
Thus, Earth’s seasons are NOT
caused by its changing
distance from the Sun!!!
Birds in the Northern Hemisphere
fly south for the Winter...
That is when it is winter in the
Northern Hemisphere, it is
summer in the Southern
Hemisphere!
...which means that the Earth’s
revolution around the Sun can
not be the only explanation for
seasons!
The correct reason for the
seasons:
1) the tilt of the Earth’s axis of
rotation and
2) Earth’s revolution around the
Sun
Earth is tilted!
Summer solstice
Winter solstice
Equinox position
The “altitude” of the Sun impacts how
much energy is received on Earth.
Concept Question
What would be different if the
Earth had no tilt?
UNLEARN
Common Misconception
The phases of the Moon occurs because
as the Moon orbits the Earth, the Earth
casts a shadow on the Moon and blocks
some or all of the sunlight from reaching the
the Moon causing the amount of sunlight
reflecting off the Moon to change.
This statement is NOT correct!
LEARN
The correct
reason for the
phases of the
Moon:
Although half of
the Moon is
always lit by the
Sun, we can only
see the side of the
Moon that is
facing the Earth
and only on that
side does
the amount of
illumination
change
as the Moon
orbits the Earth.
Concept Question
If the moon was between the Earth and the
Sun, what phase do you think it will be?
A) full
B) new
C) first quarter
D) third quarter
In other words ...
• The Earth’s rotation on its axis
determines the length of the day.
• The moon’s orbit around the Earth
determines the length of a month.
• The Earth’s orbit around the Sun
(revolution) determines the length of the
year.
Lunar eclipses occur only when the Moon
crosses the ecliptic during the full phase
Lunar Eclipses
Lunar eclipse only happens
when the moon travels through
the Earth’s shadow
Darkest part of the shadow =
Partial
Full
Partial
Intermediate part of the shadow =
Lunar Eclipses
The moon is not completely dark during a total lunar eclipse
Lunar Eclipses
The moon is not completely dark during a total lunar eclipse since
small amount of light is bent around the edge of the Earth’s atmosphere!!!
Solar eclipses occur only when the Moon
crosses the ecliptic during the new phase
Solar Eclipses
Moon = 400 X smaller than
the Sun, but 400X closer
The moon has the same
angular size as the Sun!
Solar Eclipse from Space
Solar
Eclipse
Annular Eclipse
Partial Eclipse
Total Eclipse
Solar Eclipses
Next annular
Last
total solar
eclipse
eclipse
visible
visible
in the
in the
U.S.
U.S.
was
is May
August
20, 2012!
21, 2017!