Unit 12 Chapter 26

Models of the Solar System

1. the first astronomers who studied the sky thought that the stars, planets, and sun revolved around Earth. 2. this idea led to the first model of the solar system. 3. but, this model changed as scientists learned more about how the solar system works.

Planets of our Solar System Section Two: Models of the Solar System

Planets of our Solar System Section Two: Models of the Solar System

Formation of the Calendar

1. A calendar is a system created for measuring long intervals of time by dividing time into periods of days, weeks, months, and years. 2. because the year is about 365 1/4 days long, the extra 1/4 day is usually ignored to make the number of days on the calendar a whole number. 3. but, we must account or the extra time. 4. so, every four years, one day is added to the month of February called a leap year.

Daylight Savings Time

1. Because of the tilt of Earth's axis, the duration of daylight is shorter in the winter months than the summer months. 2. During the summer months, days are longer so that the sun rises earlier in the morning when any people are sleeping. 3. To take advantage of that daylight, the United States uses daylight savings time. 4. Under this system, clocks are set one hour ahead of standard time in March which provides an additional hour of daylight during the evening. 5. Also saves energy because the use of electricity decreases. 6. In, November, clocks are set back one hour to return to standard time. 7. Countries, that are in the equatorial region do not observe daylight savings time because there are not significant changes in the amount of daylight time. 8. Daylight is about 12 hours every day of the year.

Winter Solstice

1. By December, the north pole is tilted the farthest point away from the sun. 2. on December 21 or 22, the sun's rays strike Earth at a 90 degree angle along the Tropic of Capricorn, which s located at 235 degrees south latitude. 3. called the winter solstice. 4. marks the beginning of winter in the northern hemisphere 5. during this, the northern hemisphere has the fewest daylight hours. 6. sun follows its lowest path across the sky. 7. places north of the arctic circle then have 24 hours of darkness. 8. places south of the antarctic circle have 24 hours of daylight at that time.

Seasonal Weather

1. Changes in the angle a which the sun's rays strike Earth's surface cause the seasons. 2. When tilted away from the sun, the temperatures are cooler. 3. winter 4. When tilted towards the sun, the temperatures are warmer. 5. summer 6. While the Northern hemisphere is point towards the sun and experiencing summer, the Southern hemisphere will point away from the sun experiencing winter.

The Seasons

1. Earth's axis is tilted at 234 degrees. 2. As it revolves around the sun, the axis always points toward the North Star. 3. Thus, during each revolution, the North Pole sometimes tilts towards the sun and sometimes tilts away from the sun. 4. When the North Pole points towards the sun, the Northern Hemisphere has longer periods pf daylight than the Southern Hemisphere. 5. When the North Pole points away from the sun the Southern Hemisphere has longer periods of daylight. 6. The angle at which the sun's rays strike each part of Earth's surface changed as Earth moves through its orbit. 7. When the North Pole tilts towards the sun, the rays strike the Northern Hemisphere ore directly (warmer). 8. When the North Pole tilts away from the sun, the rays strike the Northern hemisphere less directly (cooler).

Measuring Time

1. Earth's motion provides the basis for measuring time. 2. the day and the year are based on periods of Earth's motion. 3. day is determined by Earth's rotation on its axis. 4. each complete rotation of earth on its axis is one day which is divided into 24 hours. 5. year is determined by Earth's revolution around the sun. 6. each complete revolution of Earth around the sun takes around 365 days (1 year). 7. a month is based on the moon's motion around Earth. 8. a month is determined as roughly one-twelfth of a year.

Evidence of Earth's Rotation

1. If you gaze at a constellation in the evening sky, you might notice that it appears to be moving in the space of an arc. 2. this change in position occurs for the same reason that the sun appears to move across the daylight sky. 3. earth's rotation makes the constellations appear to move in this way.

Law of Ellipses

1. Kepler's first law, the law of ellipses, states that each planet orbits the sun in a path called an ellipse, not a circle. 2. An ellipse is a closed curve whose shaped is determined by two point (foci) within the ellipse. 3. For the planets, one focus is located within the sun. 4. elliptical orbits can vary in spines. 5. some are elongated spines. 6. other orbit shaped are almost perfect circles. 7. the shape of an orbit can be described by a numerical quantity called eccentricity. 8. Eccentricity the degree of elongation of an elliptical orbit. 9. determined by the diving the distance between the fact of the ellipse by the length of the major axis. 10. A circle = 0 11. An extremely elongated orbit = 1

Law of Equal Areas

1. Kepler's second law, the law of equal areas, described the speed at which objects travel at different points in their orbits. 2. Discovered that Mars moves fastest in its elliptical orbit when it is closest to the sun. 3. He calculated that a line from the center of the sun to the center of an object sweeps through equal areas in equal periods of time. 4. Imagine this: a line connects the center of the sun to the center of an object in orbit around the sun. 5. When the object is near the sun, the line is short. 6. Moves rapidly and the line goes through a short, wide, pie-shaped area. 7. When the objects is far from the un, the line is long. 8. Moves slowly and the line goes through a long, thin, pie-shaped area. 9. Kepler's second law states that equal areas are covered in equal amounts of time as an object orbits the sun.

Law of Periods

1. Kepler's third law, the law of periods, describes the relationship between the average distance of a planet from the sun and the orbital period of the planet. 2. Orbital Period: the time required for a body to complete a single orbit. 3. According the Kepler's law, the cube of the average distance (a) of a planet from the sun is always proportional to the square of the period (p). 4. A3 = p2 5. Scientists can find out how far away the planets are from the sun by using this law because they can measure the orbital periods by observing the planets. 6. If they know the orbital period, they can find out this distance. 7. Example: If Jupiter's orbit is 11.9 8. 1192 = 141.61 9. ?3 = 142 10. A = 5.2

Evidence of Earth's Revolution

1. The position of a constellation in the evening sky changed not only because of Earth's rotation but also because of Earth's revolution around the sun. 2. the constellations appear lower in the sky in arch than they do in February. 3. this change is a result of Earth's revolution. 4. as it revolves around the sun, the night side of Earth faces in a different direction of the universe. 5. as earth moves, different constellations are visible in the night sky from month to month an from season to season.

Equinoxes

1. The seasons fall and spring begin on days called equinoxes. 2. Equinox: the moment when the sun appears to cross the celestial equator. 3. Celestial equator: an imaginary line in the sky directly overhead from the equator on Earth. 4.During an equinox, the Sun's rays strike Earth at an a 90 degree angle along the equator. 5. The hours of daylight and darkness are approximately equal everywhere on Earth on that day. 6. Autumnal equinox: occurs on September 22 or 23 each year and marks the beginning of fall in the Northern hemisphere. 7. Vernal equinox: occurs on March 21 or 22 of each year and marks the beginnings of spring in the Northern Hemisphere.

Summer Solstices

1. The seasons of summer and winter begin on days called solstices. 2. each year on June 21 or 22, the North Pole's tilt towards the sun s greatest. 3. On this day, the sun's rays strike Earth at a 90 degree latitude. 4. Called the summer solstice and it marks the beginning of summer in the northern hemisphere. 4. solstice means 'sun stop' and refers to the fact that the northern hemisphere the sun follows its highest path across the sky on this day and then moves lower every day afterwards. 5. the Northern hemisphere has its most hours of daylight during the summer solstice. 6. the further north of the equator you are, the longer the period of daylight you have. 7. north of the Arctic Circle, at 66.5 degrees north latitude, there are 24 hours of daylight during the summer solstice. 8. south of the Antarctic Circle, there are 24 hours of darkness at the time.

Time Zones

1. Using the sun as the basis for measuring time, we define moon is the time when the sun is highest in the sky. 2. because of its rotation, the sun is highest above different locations on Earth at different times of days. 3. earth's surface has been divided 24 standard time zones to avoid problems created by different local times. 4. in each zone, noon is set as the time when the sun is highest over the center of that zone. 5. earth's circumference equals 360 degrees. 6. if you divide this by the 4 hours needed for one oration you find that Earth rotates about 15 degrees per hour. 7. Each time zone covers about 15 degrees. 8. the time in each zones is one hour earlier than the time in the zone to the east.

Early Models

1. more than 2000 years ago, the greek philosophers Aristotle suggest an earth-centered, or geocentric model of the solar system. 2. in this model, the sun, stars and planets revolved around the Earth. 3. but, this model did not explain why the planets sometimes appeared to move backward in the sky relative to the stars. 4. by 150 c.e., the greek astronomer Claudius Ptolemy, proposed changes to this model. 5. he thought the planets moved in small circles as they revolved in larger circles around Earth. 6. could explain the backwards movements. 7. in 1543, a polish astronomer, Nicolaus Copernicus, proposed a sun-centered, or heliocentric, model of the solar system. 8. in this model, the planets revolved around the sun in the same direction but at different speed and distances from the sun. 9. the planets they move slower than Earth look like they are moving backwards.

International Date Line

1. there are 24 standard time zones and 24 hours in a day, but there must be some point on Earth's surface where the date changes. 2. the International Date Line was established to prevent confusion. 3. it is an imaginary line that runs through the Pacific Ocean. 4. When it is Friday west of the line, it is Thursday east of the line. 5. The line is drawn so that it does not cut through island or continents. 6. Thus, everyone living within one country has the same date.

Kepler's law

1. twenty years before Galileo used a telescope, the Danish astronomer Tycho Brahe made detailed observations of the positions of the planets. 2. after his death, one of his assistants, Johannes Kepler, worked with his observations. 3. his studies led Kepler to develop three laws that explained planetary motion.