Chapter 2: Patterns in the Sky – Motions of Earth

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