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 ConstellationsAndromeda
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 July
Looking south in January
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 SimulatorLook at Outline Ecliptic and
Celestial Equatorial Coordinates
There are four important dates in the motion of the
Sun
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.
The solstices occur when the Sun is directly over either the Tropic of Cancer (summer) or the Tropic of Capricorn (winter).
One more type of motionA 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 days1 Tropical Year = 365.2422 mean solar days slightly less due to the Precession of the Equinox
Pope Gregory XIII’s CalendarThe Gregorian Calendar (1582)
Most years have 365 daysYears evenly divisible by 4 have 366 days except century years. Only century years evenly divisible by 400 are leap yearsi.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
phasesBecause 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
CycleThe Saros Cycle is approximately 18 years 11 and 1/3 days