The Sun-Earth-Moon System
(Chapter 3)
Student Learning Objectives
• Associate cycles in the sky with time
• Describe why Earth has seasons
• Determine the phase of the moon
• Compare and contrast all types of eclipses
What are the cycles that determine time keeping?
Many early calendars were based on the cycles of
Moon phases.
The first indications of time keeping are notched bones which may have
been used to record moon phases (Africa and Europe 20,500 BC)
We currently use the Gregorian calendar (1582).
o Leap years must be divisible by 4
o Leap years exclude years divisible by 100
Interesting Note: 10 days were dropped from the month of
October in 1582 to bring the dates for the seasons and
religious holidays back into alignment when the switch was
made from the Julian calendar to the Gregorian calendar.
Practice: What is
the path of the Sun
through the zodiac
called?
Synodic vs Sidereal
Synodic motion
relates to an Earth
rotation/revolution
which results in the
same view of the Sun
or Moon
Sidereal motion is the
process of returning to
the same position with
respect to the
background stars
http://www.skywise711.com/Skeptic/Sidereal/sidereal.html
 Sidereal year:
time for Sun
to appear to return a
particular place in
the sky
 Solar year:
time for one
complete revolution
(orbit) of Earth
around the Sun
Precession of Time
One sidereal year = (1 + 1/26,000) tropical years
Precession makes it take longer for the same
alignment between Earth and background stars
each year. (20 minutes/year)
The Month is based on the motion of the Moon.
 One synodic month = 29.53 days (complete cycle moon phases)
 One sidereal month = 27.3 days (one moon orbit)
A Day
The day is based on the rotation of Earth.
 A sidereal day = 23 h, 56m, 4.09 s
 A solar day = 24 hours
1→2
Earth Observer re-points
to distant star (sidereal)
Earth rotates 360°
1→3
Earth Observer re-points
to sun (synodic)
Practice
1) In general, what does sidereal time measure?
2) Why do we have leap year?
3) What is the change in position of the moon in
the sky, in 24 hours? (Number of degrees)
What causes Earth to have Seasons?
 The changing distance
between the Earth and the
Sun is not the cause of the
seasons.
 Our distance from the Sun
only varies by 2-3%
The reason we have
seasons is because
Earth’s axis is tilted 23½º
http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html
 The more direct sunlight a hemisphere receives, the more
it is heated.
 The beginning of each season is always on the same date
(± 1 or 2 days).
Season
Spring
Summer
Fall
Winter
Date
Mar 22
Jun 21
Sep 21
Dec 21
Location of Sun
Vernal Equinox
Summer Solstice
Autumnal Equinox
Winter Solstice
Only on the date of an equinox does the Sun
rise directly east and set directly west.
 The altitude of the Sun changes with the seasons.
Practice
1) Refer to your celestial
sphere map. Locate the
position of the Sun for
each season.
2) Will the seasons on
Earth ever change?
Explain.
3) At what latitude is the
Sun directly overhead, at
noon, on the first day of
our “summer”? What is
this latitude called?
4) At what latitude is the
Sun directly overhead, at
noon, on the first day of
our “winter”? What is
this latitude called?
Why do we see different shapes of the Moon?
As the amount of
reflected sunlight
that we can see
changes, we see
the phases of the
Moon.
 New Moon Phase – No illumination
 Full Moon Phase – All illumination
 1st and 3rd Quarter – Half
 Gibbous – More than half
 Crescent – Less than half
 Waxing –Increasing
 Waning – Decreasing
Practice
1) There is always half of
the Moon’s sphere
reflecting sunlight.
a. True
3) Identify the moon phase
decreasing illumination nightly
b. False
2) Is the moon ever visible
during the day?
http://astro.unl.edu/naap/lps/animations/lps.swf
What are the characteristics of solar and lunar eclipses?
 An eclipse is a shadow.
 Umbra: complete shadow
 Penumbra: partial shadow
 Eclipses only occur when
the Earth, Moon, and Sun
all lie in the same plane
(the ecliptic) and are in the
same location of space.
Total Solar Eclipse: umbra of the Moon's shadow
touches Earth’s surface. (Sun-Moon-Earth)
In 2017, for the first time since 1918, a total solar eclipse will
sweep completely across the US
 Annular Solar Eclipses: anti-umbra touches Earth
 Partial Solar Eclipses: penumbra touches Earth
Practice
1) What is the phase of the Moon during a total
solar eclipse?
2) The Moon is much smaller than the Sun. Why
then is it possible for a solar eclipse to occur?
Lunar Eclipses
Total Lunar eclipse: umbra of Earth's shadow
covers Moon. (Sun-Earth-Moon)
The fully eclipsed Moon appears red.
Refraction
Practice
1) What is the phase of the Moon during a total lunar
eclipse?
2) Why is the shadow dark and then red, during a total
lunar eclipse?
3) How often can an eclipse occur?
4) Which are more commonly seen, solar or lunar
eclipses? Why?