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Transcript
EARTH’S DAYS, YEARS, AND SEASONS W H AT D ET E R M I N E S T H E LENGTH OF A D AY ? • All planets spins on its axis • Axis – imaginary straight line running from pole to pole • Rotation – spinning of a body on its axis • Day – the time it takes a planet to complete one full rotation on its axis – 1 day on Earth: 24 hours • Only half the Earth faces the sun at a time – Daytime • The other half of the Earth faces away from the sun – Nighttime – 1 day on other planets does not equal 24 hours W H AT D ET E R M I N E S T H E LENGTH OF A YEAR? • The Earth is traveling around the sun at nearly 30 km/s • Revolution – motion of a body that travels around another body in space • Earth completes a full revolution around the sun in 365 ¼ days, or about one year – Every four years, we add the ¼ days and add one more day to February • Leap year: February 29th • Earth’s orbit is elliptical, or oval-shaped and not circular – In January, Earth is 2.5 million km closer to the sun than it is in July • This causes minimal changes in Earth’s temperature W H AT C O N D I T I O N S ARE AFFECTED BY T H E T I LT O F EARTH’S AXIS? Earth’s axis is tilted at 23.5° Axis always points towards the North Star TEMPERATURE • The angle at which the sun’s rays strike each part of Earth’s surface changes as Earth moves in its orbit – When the North Pole is tilted toward the sun, the sun’s rays strike the Northern Hemisphere more directly – This causes the Northern Hemisphere to receive a higher concentration of sunlight, which makes it warmer • Summer – When the North Pole it tilted away from the sun, the sun’s rays strike the Northern Hemisphere less directly – This causes the Northern Hemisphere to receive a lesser concentration of sunlight, which makes it colder • Winter TEMPERATURE CON’T • The spherical shape of Earth also affects how the sun warms up an area – Temperatures are higher at areas where the sun directly hits the Earth – Temperatures are lower at areas where the sun hits the Earth at an angle DAYLIGHT HOURS • All locations on Earth have an average of 12 hours of light each day • The actual number of daylight varies based on the time of year and location – Areas tilted towards the sun receive more hours of daylight – Areas tilted away from the sun receive less hours of daylight MIDNIGHT SUN • During summer in the Northern Hemisphere, the time in each day that it is light increases as you move towards the North Pole • Areas north of the Arctic Circle have 24 hours of daylight called “midnight sun” – At the same time, the Antarctic Circle has 24 hours of darkness called “polar night” – These conditions reverse with the seasons WHAT CAUSES SEASONS? • Most locations on Earth experience seasons • Each season is characterized by a pattern of temperature and other weather trends • Near the equator, the temperatures are almost the same year-round • Near the poles, there are very large changes in temperatures from winter to summer • We experience a change in seasons due to: – the changes in the intensity of sunlight (angle of the Earth) – the number of daylight hours • As Earth travels around the sun the area of sunlight in each hemisphere changes • Equinox – sunlight shines equally on both hemispheres – September and March • Solstice – area of sunlight is at a maximum in one hemisphere and at a minimum in one hemisphere – December and June