The Earth-Moon-Sun The Earth-Moon-Sun SystemSystem

Time and SeasonsTime and Seasons

• Measuring time on EarthMeasuring time on Earth– Around 3000 B.C., the Babylonians Around 3000 B.C., the Babylonians

devised a method of timekeepingdevised a method of timekeeping– Because their counting methods were Because their counting methods were

based on 60, they divided a circle into based on 60, they divided a circle into 360 parts called degrees360 parts called degrees

– The symbol for degree (The symbol for degree (oo) was taken ) was taken from their symbol for the Sunfrom their symbol for the Sun

Earth spins and makes one Earth spins and makes one complete turn in about 24 complete turn in about 24 hourshoursThis spinning This spinning

causes the Sun causes the Sun to appear to to appear to move across move across the sky from the sky from east to westeast to west

• If Earth spins approximately 360If Earth spins approximately 360oo in in 24 hours, then it spins through 1524 hours, then it spins through 15o o in in one hour.one hour.

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

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

• How many degrees does Earth spin How many degrees does Earth spin in one hour?in one hour?

• Fifteen degreesFifteen degrees

• A day is added to the time at the A day is added to the time at the International Date LineInternational Date Line– This line is drawn down through the This line is drawn down through the

Pacific Ocean directly opposite the Pacific Ocean directly opposite the Prime Meridian, the starting point for the Prime Meridian, the starting point for the worldwide system of measuring timeworldwide system of measuring time

• Let’s calculate a few time changes…Let’s calculate a few time changes…

• Rotation is the spinning of Earth on Rotation is the spinning of Earth on its axis, an imaginary line drawn its axis, an imaginary line drawn through Earth from its rotation north through Earth from its rotation north pole to its rotational south polepole to its rotational south pole

• The apparent movement of the Sun The apparent movement of the Sun from noon one day until noon the from noon one day until noon the next day is call a solar daynext day is call a solar day

• Rotation measures daysRotation measures days

• Revolution is the motion of Earth in Revolution is the motion of Earth in its orbit around the Sunits orbit around the Sun

• As Earth revolves in its orbit, the Sun As Earth revolves in its orbit, the Sun appears to move through the skies appears to move through the skies compared to the seemingly fixed compared to the seemingly fixed positions of the starspositions of the stars

• Revolution measures yearsRevolution measures years

• The apparent path The apparent path of the Sun during of the Sun during this year is called this year is called the eclipticthe ecliptic

• The ecliptic is The ecliptic is defined as the plane defined as the plane of Earth’s orbit of Earth’s orbit around the Sunaround the Sun

Why do seasons change?Why do seasons change?

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

• Earth’s axis is tilted 23.5Earth’s axis is tilted 23.5oo

• The hemisphere tilted toward the The hemisphere tilted toward the Sun receives sunlight at higher Sun receives sunlight at higher angles than the hemisphere tilted angles than the hemisphere tilted away from the Sun.away from the Sun.

• The greater intensity of sunlight is The greater intensity of sunlight is one reason why summer is warmer one reason why summer is warmer than winterthan winter– But it’s not the only reasonBut it’s not the only reason

• So, as the year progresses, the So, as the year progresses, the number of hours of daylight each day number of hours of daylight each day becomes fewer.becomes fewer.

• Our shortest day of the year is….Our shortest day of the year is….– December 21, when we have the fewest December 21, when we have the fewest

number of daylight hoursnumber of daylight hours

• The hemisphere of Earth that is tilted The hemisphere of Earth that is tilted toward the Sun receives more hours toward the Sun receives more hours of daylight each day than the of daylight each day than the hemisphere tilted away from the hemisphere tilted away from the Sun. Sun.

• This longer period of daylight is the This longer period of daylight is the second reason why summer is second reason why summer is warmer than winter.warmer than winter.

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

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