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The Earth-Moon-Sun System. Time and Seasons. Measuring time on Earth Around 3000 B.C., the Babylonians devised a method of timekeeping Because their counting methods were based on 60, they divided a circle into 360 parts called degrees

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Presentation Transcript

Time and seasons
Time and Seasons

  • Measuring time on Earth

    • Around 3000 B.C., the Babylonians devised a method of timekeeping

    • Because their counting methods were based on 60, they divided a circle into 360 parts called degrees

    • The symbol for degree (o) was taken from their symbol for the Sun


Earth spins and makes one complete turn in about 24 hours
Earth spins and makes one complete turn in about 24 hours

This spinning causes the Sun to appear to move across the sky from east to west


  • If Earth spins approximately 360o in 24 hours, then it spins through 15o in one hour.

  • A time zone is an area 15o wide in which time is the same

  • For convenience, time zones are modified to fit around city, state, and country borders, and other key sites.



  • A day is added to the time at the International Date Line

    • This line is drawn down through the Pacific Ocean directly opposite the Prime Meridian, the starting point for the worldwide system of measuring time

  • Let’s calculate a few time changes…





Why do seasons change
Why do seasons change? ecliptic

  • Seasons change on Earth because the number of hours of daylight each day varies and also because the angle at which sunlight strikes.

  • Earth’s axis is tilted 23.5o





  • The Sun reaches an equinox when it is directly above the Earth’s equator, and the number of daylight hours equals the number of nighttime hours all over the world

  • The solstice is the point at which the Sun reaches its greatest distance north or south of the equator, the Tropic of Cancer and Tropic of Capricorn, respectively


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