At this speed, the Earth makes one full rotation. Definition: the circular movement of a body around a central point called an axis –A full rotation takes.

At this speed, the Earth makes one full At this speed, the Earth makes one full rotationrotation..DefinitionDefinition: the circular movement of a body around a : the circular movement of a body around a central point called an axiscentral point called an axis

– A full rotation takes 23 hours, 56 minutes, and 4.1 seconds – A full rotation takes 23 hours, 56 minutes, and 4.1 seconds – but we round it up to 24 hours, or 1 day.but we round it up to 24 hours, or 1 day.

The rotation of the Earth causes half the world to be The rotation of the Earth causes half the world to be exposed to sunlight while the other half is bathed in exposed to sunlight while the other half is bathed in darkness.darkness.– This is why we experience day and night – and helps define This is why we experience day and night – and helps define

the Earth’s weather and climate regions.the Earth’s weather and climate regions.

The Earth’s Tilted AxisThe Earth’s Tilted AxisThe axis of the Earth is tilted approximately 23.5 The axis of the Earth is tilted approximately 23.5 degrees.degrees.– The tilt of the Earth’s axis causes parts of the Earth to The tilt of the Earth’s axis causes parts of the Earth to

receive more sunlight than other parts receive more sunlight than other parts – – causing the four causing the four seasonseason. .

The first day of summer in the The first day of summer in the Northern HemisphereNorthern Hemisphere is called the is called the summer solstice summer solstice and the first day of and the first day of winter is called the winter is called the winter solsticewinter solstice..– The summer solstice usually falls around June 21The summer solstice usually falls around June 21stst and is and is

that day that has the longest period of daylight.that day that has the longest period of daylight.– The winter solstice usually falls around December 21The winter solstice usually falls around December 21stst and is and is

the day that has the shortest period of daylight.the day that has the shortest period of daylight.

Days in which periods of daylight and night are Days in which periods of daylight and night are equal equal lengthlength are referred to as equinoxes are referred to as equinoxes (the sun is directly (the sun is directly

overhead at the equator)overhead at the equator)..– Every year, two equinoxes occur Every year, two equinoxes occur – the – the vernal equinoxvernal equinox and and

the the autumnal equinoxautumnal equinox..– The The vernal (spring) equinox vernal (spring) equinox usually falls around March usually falls around March

2121stst, and the day and night are each 12 hours in length., and the day and night are each 12 hours in length.– The The autumnal (fall) equinox autumnal (fall) equinox usually falls around September usually falls around September

2323rdrd, and the day and night are each 12 hours in length., and the day and night are each 12 hours in length.

The Earth’s Coordinate System:The Earth’s Coordinate System:Latitude and LongitudeLatitude and Longitude

A coordinate system is a method of locating an exact A coordinate system is a method of locating an exact location on a two-dimensional surface.location on a two-dimensional surface.– The The equatorequator divides the planet into two hemispheres. divides the planet into two hemispheres.

DefinitionDefinition: the imaginary line, also known as zero degrees : the imaginary line, also known as zero degrees latitude, that divides the Earth in half into the Northern and latitude, that divides the Earth in half into the Northern and Southern HemisphereSouthern Hemisphere

– Each hemisphere is further divided by horizontal lines Each hemisphere is further divided by horizontal lines that mark the locations north and south of the equator that mark the locations north and south of the equator called lines of called lines of latitudelatitude..

DefinitionDefinition: parallel lines the run east and west across Earth’s : parallel lines the run east and west across Earth’s surface, measuring locations north and south of the equatorsurface, measuring locations north and south of the equator

– These lines are marked in degrees – These lines are marked in degrees – the equator being the equator being represented by 0represented by 0oo, and increasing in both directions as , and increasing in both directions as you move away from the equator lineyou move away from the equator line..

Lines of latitude are useful only for determining an Lines of latitude are useful only for determining an exact location either north or south of the equator – exact location either north or south of the equator – they become more helpful if used in combination with they become more helpful if used in combination with the lines of the lines of longitudelongitude..

DefinitionDefinition: coordinate lines used on the earth’s surface : coordinate lines used on the earth’s surface that run north and south from pole to pole and measure that run north and south from pole to pole and measure a location east and west of the prime meridiana location east and west of the prime meridian

– Degrees of longitude are also marked by degrees, only Degrees of longitude are also marked by degrees, only there is no natural halfway point that divided the Earth there is no natural halfway point that divided the Earth vertically vertically – – so astronomers have designated a place on so astronomers have designated a place on our planet our planet (called the prime meridian) (called the prime meridian) to represent 0to represent 0oo degrees.degrees.

Topographical MapsTopographical MapsMaps the we use to represent a three-dimensional Maps the we use to represent a three-dimensional surface of the Earth are called topographical maps.surface of the Earth are called topographical maps.

They represent changes in elevation on the Earth’s They represent changes in elevation on the Earth’s surface by using surface by using contour lines contour lines they are drawn on a they are drawn on a map to represent a specific elevation of the land surface map to represent a specific elevation of the land surface above sea level.above sea level.

DefinitionDefinition: isolines that mark areas of equal elevation on a : isolines that mark areas of equal elevation on a topographical map topographical map

– By adding contour lines to a map, it is possible to see By adding contour lines to a map, it is possible to see the true shape of the land.the true shape of the land.

Also found on a topographical map, along with contour Also found on a topographical map, along with contour lines, are lines, are contour intervalscontour intervals..

DefinitionDefinition: the specific change in elevation associated : the specific change in elevation associated with each contour line on a topographical mapwith each contour line on a topographical map

– Each contour interval is represented as the blank space Each contour interval is represented as the blank space between two lines – between two lines – on a map of this type, each contour on a map of this type, each contour line represents a predetermined increase in height and the line represents a predetermined increase in height and the space between represents the steepness of the land.space between represents the steepness of the land.

Chapter ThreeChapter ThreeThe Earth as a PlanetThe Earth as a Planet

Section One Section One – The Planet Earth– The Planet EarthSection TwoSection Two – Earth Motions– Earth MotionsSection Three Section Three -- Tools of Astronomy-- Tools of Astronomy

Apparent Motion of Celestial ObjectsApparent Motion of Celestial ObjectsBecause our Earth is spinning, objects in the Because our Earth is spinning, objects in the nighttime sky appear to move in regular motions.nighttime sky appear to move in regular motions.– These objects are called celestial objects, which These objects are called celestial objects, which

include planets, moons, stars, comets, asteroids, and include planets, moons, stars, comets, asteroids, and any other object located outside of the Earth’s any other object located outside of the Earth’s atmosphere.atmosphere.

The movement of celestial objects is called apparent The movement of celestial objects is called apparent motion – motion – this apparent motion travels from east to west this apparent motion travels from east to west

across the skyacross the sky..– The speed at which apparent motion travels is measured in The speed at which apparent motion travels is measured in

degrees, with the sky representing 180 degrees from degrees, with the sky representing 180 degrees from horizon to horizon horizon to horizon – – this is broken down so that the speed this is broken down so that the speed of the celestial objects travel is approximately 15 degrees of of the celestial objects travel is approximately 15 degrees of sky per hoursky per hour..

Early Models of the UniverseEarly Models of the UniverseAs a result of the apparent motion of celestial objects, As a result of the apparent motion of celestial objects, for thousands of years humans believed that the for thousands of years humans believed that the planets and stars revolved around the Earth.planets and stars revolved around the Earth.– Claudius Ptolemy, the great Romanian mathematician,

geographer, and astronomer, who lived almost 2,000 years ago, was the first scientist to formulate this idea.

In the Ptolemaic system, or geocentric view of the universe, Ptolemy described the planets and stars are revolving around the Earth in perfect circular orbits.

Definition: an early model of the universe which puts the Earth at the center of the solar system and universe

The Geocentric model was accepted for over one thousand years until Copernicus changed the way that humans looked at the heavens.

Nicolaus Copernicus observed the motions of stars and planets for decades and eventually published his theory called the Copernicus heliocentric system.

Definition: a model of the solar system that puts the Sun at its center with the Earth and other planets revolving around it

– Copernicus’ theory created much controversy, and lacked sufficient proof to verify it.

– Galileo Galilei took up Copernicus’ idea when he used improved telescopes of his own design to help support the heliocentric model.

Galileo first observed the Earth’s moon with his new telescopes revealing that its surface was much like the Earth, consisting of mountains, valleys, and craters.– He then started to observe the moons of other planets and

he noticed that each of those moons were revolving around their planets.

Galileo also observed the surface of the sun with his telescopes.– Although it made him nearly blind, his observations

discovered its unique sunspots.– After carefully plotting the location of these dark patches on

the Sun, he discovered that it was also most likely rotating on its axis, like the Earth.

While Galileo was trying to prove his theories, the work of German astronomer, Johannes Kepler, caused another breakthrough for modern astronomy.– Kepler used the work of another astronomer to reveal the

true nature of the orbits of the planets– It was previously thought that the orbits were circular, but he

recognized that the only way to accurately explain the positions of the planets was to describe their orbits as being elliptical.

His research is now known as Kepler’s laws of planetary motion:– The first law states that the planets all revolve around the Sun

in elliptical orbits.

– The second law explains that as the planets revolve around the Sun, their velocity changes in relationship to their distance to the Sun.

– The third law explains that the period of time it takes for a planet to orbit the Sun is related to the size of its elliptical path.

Finally in 1687, the English astronomer and mathematician Isaac Newton published his three laws of gravitation- these helped to explain how Kepler’s laws worked.

Getting to this point shows how the advancement of human knowledge progresses through time as scientists build upon the theories of others.

Orbital MotionOrbital MotionThe motions of all celestial objects are based on the concept of an ellipse.

Definition: the oval-like path of the orbit of a celestial object around two points known as foci, one of which is the Sun

– The ellipse can be generally described as the oval-like path of a celestial object.

The path of the ellipse, known as the orbit, is defined by two points, individually known as a focus, and together called foci.

The oval nature of elliptical paths can be mathematically described as their eccentricity.

Definition: the mathematical expression of how far as ellipse is from a perfect circle, which can be determined by dividing the distance between the foci by the length of the major axis

An ellipse with an eccentricity of 0 represents a perfect circle and an ellipse with an eccentricity of 1 is regarded as a flat line.– The closer the eccentricity is to 1, the more eccentric or

oval shaped the orbital path is.– Pluto’s orbit is the most eccentric, or furthest from

being a perfect circle.– Venus has the least eccentric orbit, which is closest to

being a perfect circle.

The time it takes for a planet to make one complete orbit around the Sun is called one revolution.

Definition: the movement of an object in an orbit around another object