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 23 hours, 56 minutes, and 4.1 seconds – but we round it up to 24 hours, or 1 day. The rotation of the Earth causes half the world to be exposed to sunlight while the other half is bathed in darkness. –This is why we experience day and night – and helps define the Earth’s weather and climate regions. The Earth’s Tilted Axis The axis of the Earth is tilted approximately 23.5 degrees. –The tilt of the Earth’s axis causes parts of the Earth to receive more sunlight than other parts – causing the four season.

The first day of summer in the Northern Hemisphere is called the summer solstice and the first day of winter is called the winter solstice. –The summer solstice usually falls around June 21 st and is that day that has the longest period of daylight. –The winter solstice usually falls around December 21 st and is the day that has the shortest period of daylight. Days in which periods of daylight and night are equal length are referred to as equinoxes (the sun is directly overhead at the equator). –Every year, two equinoxes occur – the vernal equinox and the autumnal equinox. –The vernal (spring) equinox usually falls around March 21 st, and the day and night are each 12 hours in length. –The autumnal (fall) equinox usually falls around September 23 rd, and the day and night are each 12 hours in length.

The Earth’s Coordinate System: Latitude and Longitude A coordinate system is a method of locating an exact location on a two-dimensional surface. –The equator divides the planet into two hemispheres. Definition: the imaginary line, also known as zero degrees latitude, that divides the Earth in half into the Northern and Southern Hemisphere –Each hemisphere is further divided by horizontal lines that mark the locations north and south of the equator called lines of latitude. Definition: parallel lines the run east and west across Earth’s surface, measuring locations north and south of the equator –These lines are marked in degrees – the equator being represented by 0 o, and increasing in both directions as you move away from the equator line.

Lines of latitude are useful only for determining an exact location either north or south of the equator – they become more helpful if used in combination with the lines of longitude. Definition: coordinate lines used on the earth’s surface that run north and south from pole to pole and measure a location east and west of the prime meridian –Degrees of longitude are also marked by degrees, only there is no natural halfway point that divided the Earth vertically – so astronomers have designated a place on our planet (called the prime meridian) to represent 0 o degrees. Topographical Maps Maps the we use to represent a three-dimensional surface of the Earth are called topographical maps.

They represent changes in elevation on the Earth’s surface by using contour lines  they are drawn on a map to represent a specific elevation of the land surface above sea level. Definition: isolines that mark areas of equal elevation on a topographical map –By adding contour lines to a map, it is possible to see the true shape of the land. Also found on a topographical map, along with contour lines, are contour intervals. Definition: the specific change in elevation associated with each contour line on a topographical map –Each contour interval is represented as the blank space between two lines – on a map of this type, each contour line represents a predetermined increase in height and the space between represents the steepness of the land.

Chapter Three The Earth as a Planet Section One – The Planet Earth Section Two– Earth Motions Section Three -- Tools of Astronomy

Apparent Motion of Celestial Objects Because our Earth is spinning, objects in the nighttime sky appear to move in regular motions. –These objects are called celestial objects, which include planets, moons, stars, comets, asteroids, and any other object located outside of the Earth’s atmosphere. The movement of celestial objects is called apparent motion – this apparent motion travels from east to west across the sky. –The speed at which apparent motion travels is measured in degrees, with the sky representing 180 degrees from horizon to horizon – this is broken down so that the speed of the celestial objects travel is approximately 15 degrees of sky per hour.

Early Models of the Universe As a result of the apparent motion of celestial objects, for thousands of years humans believed that the 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 Motion The 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