# Chapter 2: Patterns in the Sky – Motions of Earth.

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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 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 Simulator Look 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 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

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