Chapter 2:
Patterns in the Sky –
Motions of Earth
Looking around the sky:
What are the Constellations?
Every culture has their own
constellations
So whose constellations do astronomers use?
88 Official Constellations
Andromeda
Established by the IAU in 1922 with
boundaries finalized in 1928
What you see at night
depends on what time it is
If it’s clear, go out tonight sometime and note the
locations of the bright stars. Do the same thing two
or three hours later to see how things have moved
What you see at night depends
on what month you look
Looking south
in January
Looking
south in July
What you see at night
depends on where you are
Cerro Tololo Chile
Why does the sky change
during the course of a night?
The Earth rotates about its’
polar axis so the stars make
circles around the celestial
pole.
Why does the sky change over
the course of a year?
As we orbit the Sun, the direction opposite the Sun
changes and we only see the stars that are on the opposite
side of Earth from the Sun
Why does the sky change with
your location?
As you move away from the pole your horizon moves
with you but the locations of the celestial poles and
celestial equator remains the same
The Earth moves in two ways
that affect what we see
Orbital motion around
the Sun causes
seasonal changes in
the constellations
Rotational motion around
the polar axis causes
changes over the course
of the night
The Celestial Sphere Model
We know the universe
doesn’t rotate around us
but it can be useful to
imagine that it does
The Celestial Sphere can be used
to explain the daily motions and
the changes due to location
The Seasons
The plane formed by the orbit of the
Earth around the Sun is called the ecliptic
The distance to the Sun only
changes slightly
The difference
in distance
from closest to
most distant is
only just over
3%. That is not
enough to
cause the
seasons, only
moderate them
slightly.
Because of
the tilt on
Earth’s axis
the Sun
moves
higher and
lower in the
sky during
the year
Watch ClassAction Coordinates
& Motions Module Animations
Paths of the Sun simulator
As the Sun moves higher and
lower, sunlight strikes the
ground at different angles
Watch ClassAction Coordinates & Motions module
Animations Seasons Simulator
Look at Outline Ecliptic and
Celestial Equatorial Coordinates
There are four important dates
in the motion of the Sun
The solstices
occur when the
Sun is directly
over either the
Tropic of
Cancer
(summer) or
the Tropic of
Capricorn
(winter).
The equinoxes are when the Sun crosses the celestial
equator. On the Vernal Equinox it is moving northward
and on the Autumnal Equinox it is moving southward.
One more type of motion
A spinning gyroscope doesn’t
stands upright. It “wobbles”. The
wobble is called precession
The Earth also “wobbles”
The Precession of the Equinoxes
is due to the pull of the Moon
Without our large moon the tilt would fluctuate from 0°
to 90° over tens of thousands of years. Mars suffers
from this since it doesn’t have a large moon
Precession slowly moves the
location of the celestial pole
I takes 25,765 years to complete one cycle of precession
The celestial
poles move
in big circles
because of
the
precession
The Year
1 orbit around the Sun = 365.2564 days
1 Tropical Year = 365.2422 mean solar days
slightly less due to the Precession of the Equinox
Pope Gregory XIII’s Calendar
The Gregorian Calendar (1582)
Most years have 365 days
Years evenly divisible by 4 have
366 days except century years.
Only century years evenly
divisible by 400 are leap years
i.e. 1600, 2000, 2400, 2800…
Why does the Moon do this?
The Moon does rotate.
The Moon takes the same amount of time to complete
one rotation as it does to complete one orbit
Look at ClassAction
Lunar Cycles
module,
Animations, Three
Views Simulator as
seen from Sun
The
Cycle of
Lunar
Phases
Look at
ClassAction
Lunar Cycles
module,
Animations,
Lunar Phase
Vocabulary
The orbital period is not the
same as the cycle of phases
Because the
Earth-Moon
system orbits the
Sun, the Moon
won’t be in the
same place with
respect to the
Sun when it has
completed one
orbit with respect
to the stars.
Lunar Phases are not due
to shadows but eclipses are
The Total Lunar Eclipse
Next one visible from North America is April
15, 2014 starting around 1am CDT
Lunar Eclipse: the Earth passes
directly between the Sun and Moon
The Moon is in the Earth’s shadow so it goes from
Full Moon to dark Moon over a few hours
A Solar Eclipse occurs when
the Moon passes directly
between the Sun and Earth
Check out the Eclipse Shadow Simulator in
ClassAction Lunar Cycle module animations
The Moons’ shadow is not
large enough to cover the
Earth
The next total solar eclipse visible from North America
is on August 21, 2017 and Clarksville is (almost) the
best place on Earth to view it
A solar eclipse develops more
rapidly than a lunar eclipse
Partial eclipses are more
common than total eclipses
Annular
Eclipses
occur
when the
Moon is at
or near
apogee
The Moon doesn’t orbit on the
Earth’s equator or on the ecliptic
Eclipses only occur when the
Earth, Sun and Moon all line up
Take a look at the Eclipse Table in the ClassAction
Lunar Cycles module animations
Eclipses
occur in an
18 year
cycle
known as
the Saros
Cycle
The Saros Cycle is
approximately 18
years 11 and 1/3 days