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Rotation, Orbits & The Seasons (Concept 6.2)Explaining the SeasonsNorthern hemisphere: Warm in July; cold in JanuarySouthern hemisphere: cold in July; warm in JanuaryAt one time, people thought the Earth was at the center of the solar system and everything moved around Earth. This was supported by how the sun, plants, & stars, when viewed from earth, seem to move across the skyThey did not know that the earth rotates, or spins, OR that it revolves around the sun once each year.Now, spacecraft can send images of the Earth showing its rotationEventually Nicolaus Copernicus and Galileo Galilei used telescopes to prove Earth was not the center of the universe.How does Earth Move in Space?Earth is in continuous motion in spaceEarth spins on its axis (imaginary pole that runs from N. pole to S. pole through the Earth’s center) This is known as a rotation.Earth rotates once every 23 hours, 56 minutes, and 4.1 seconds (which is called a sidereal day)Our 24-hour day is called a solar day, which is the time it takes for Earth to rotate until the sun appears again in the same place in the skyThe part of earth facing the sun has daytime, the part facing away from the sun is nighttime.Earth’s axis is not perpendicular to its orbital pathEarth tilts at approximately 23.5 Degrees from perpendicular.This tilt impacts the amount of daylight according to latitude.During the winter, the hemisphere experiencing winter is tilted away from the sunThe higher the latitude (farther north and south) the shorter the days get in winter and the longer the days get in the summer.Earth travels around the sun in a slightly elliptical orbit; this is its revolutionAn orbit is the revolution of one astronomical body (Earth) around another astronomical body (the sun).Since Earth’s orbit is not perfectly circular, it is closer to the sun at some times of the year than at othersEarth’s farthest point from the sun occurs in July (94.5 million miles)Earth’s closest point to the sun occurs in January (91.4 million miles)It takes Earth 365.26 days to complete one orbit around the sunThis is just over our 365-day calendar yearEvery 4 years, we have a leap year – one extra day in February -- to account for the extra timeWhy does Earth have Seasons?Seasons are the result of the tilt in Earth’s axis relative to its orbital planeAlthough the axis is always pointing in the same direction, it is not always pointing in the same direction relative to the sun.The North pole is tilted toward the sun between mid-March and mid-SeptemberThe north pole is tilted away from the sun during the other half of the yearThis changes the number of daylight hours in each 24-hour period.Parts of Earth tilted toward the sun receive more sunlight and experience summer, with longer days and generally warm weather When a hemisphere is tilted away from the sun, it experiences winter with shorter days and generally cooler weather Many people think the seasons are a result of being closer/further from the sun, but that is not the case!Solstice & EquinoxThe summer solstice is the first day of astronomical summer and the longest day of the year.This occurs when the most direct sunlight strikes Earth at the farthest point N or S of the equatorThe winter solstice is the first day of astronomical winter and the shortest day of the yearThe vernal equinox, or first day of astronomical spring, and the autumnal equinox, or first day of astronomical autumn, occur when the sun’s rays strike Earth most directly over the equator.Why do Seasons Differ Around the World?Seasons are not the same everyone on Earth at the same time.Seasons in the north and south hemispheres are opposite each other.Warm July here is a cold time in AustraliaCold Christmas here is beach time in AustraliaHowever, not everywhere on earth experiences 4 distinct seasons.Earth’s tilt is responsible for the differences.At any given time, whichever hemisphere is receiving the most direct sunlight experiences longer days and warmer weather.The opposite hemisphere receives less sunlight which means shorter and cooler daysWhere you are in terms of latitude also makes a difference in what kinds of seasons you experienceIn some places, seasonal changes are extreme, while in others, not so much.The farther north or south you are, the more variation you will experience in hours and intensity of sunlightGenerally, the closer a place is to the equator, the less difference there is between seasonsThe equator always receives steady sunlight, no matter which way the Earth is tiltedThis is one reason why in N. America, southern states have milder wintersClimate factors can still create dramatic seasonal differences, such as rainy or dry seasons. ................
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