Measuring TimeMeasuring Time

and and

The Seasons The Seasons

ObjectivesObjectives

• SummarizeSummarize how Earth’s rotation and revolution provide a how Earth’s rotation and revolution provide a basis for measuring time.basis for measuring time.

• ExplainExplain how the tilt of Earth’s axis and Earth’s movement how the tilt of Earth’s axis and Earth’s movement cause seasons.cause seasons.

Measuring TimeMeasuring Time

• Earth’s motion provides the Earth’s motion provides the basis for measuring time.basis for measuring time.

• A day is determined by Earth’s A day is determined by Earth’s rotation on its axis.rotation on its axis. Each Each complete rotation of Earth on complete rotation of Earth on its axis takes 24 hours.its axis takes 24 hours.

• A year is determined by A year is determined by Earth’s revolution around the Earth’s revolution around the sunsun. Each complete revolution . Each complete revolution of Earth around the sun takes of Earth around the sun takes 365 1/4 days, or one year.365 1/4 days, or one year.

Around and Around We GoAround and Around We Go

• Every second you sit Every second you sit in this classroom, in this classroom,

Earth's orbital motion Earth's orbital motion carries you about 18 carries you about 18 miles through space.miles through space.

Time ZonesTime Zones

• Earth’s surface is divided Earth’s surface is divided into 24 time zones. Each is into 24 time zones. Each is one hour earlier than the one hour earlier than the time in the zone to the east.time in the zone to the east.

• International Date Line: International Date Line: imaginary line which runs imaginary line which runs from the north to the south from the north to the south pole and separates one pole and separates one calendar day from the next.calendar day from the next.– passes through the middle of passes through the middle of

the Pacific Ocean, at 180° the Pacific Ocean, at 180° longitude but deviates to pass longitude but deviates to pass around island groupsaround island groups

U.S. Time ZonesU.S. Time Zones

Daylight Savings TimeDaylight Savings Time

• Because of the tilt of Earth’s Because of the tilt of Earth’s axis, daylight is shorter during axis, daylight is shorter during winter than summer.winter than summer.

• The US uses The US uses daylight daylight savings time savings time to make better to make better use of daylightuse of daylight::

• Daylight savings time: Daylight savings time: clocks clocks are set one hour ahead in are set one hour ahead in April, and are set back one April, and are set back one hour in Octoberhour in October

• ““Spring Ahead”, “Fall Behind”Spring Ahead”, “Fall Behind”

The SeasonsThe Seasons

• Seasons are the result of the tilt of the Earth's axis, which is tilted at 23.5°.

• The number of daylight hours is greater for the hemisphere, or half of Earth, which is tilted toward the Sun.

• Seasons are not caused by Seasons are not caused by how close the Earth is to the how close the Earth is to the sun!sun!

• In fact, Earth is closest to the In fact, Earth is closest to the sun on Jan. 3 and farthest on sun on Jan. 3 and farthest on July 4. We call these?July 4. We call these?

Why We Have SeasonsWhy We Have Seasons

• Summer is warmer than winter because the Sun's rays Summer is warmer than winter because the Sun's rays hit the Earth at a more direct anglehit the Earth at a more direct angle

More About SeasonsMore About Seasons

• Days are much longer than Days are much longer than nights during summer and nights during summer and shorter than nights in the shorter than nights in the winter, due to the tilt of Earth's winter, due to the tilt of Earth's axis.axis.

• When the North Pole tilts away When the North Pole tilts away from the sun, the angle of the from the sun, the angle of the sun’s rays falling on the sun’s rays falling on the Northern Hemisphere is low, Northern Hemisphere is low, and we experience fewer and we experience fewer daylight hours, less energy, daylight hours, less energy, and lower temperatures.and lower temperatures.

EquinoxesEquinoxes• Equinox: Equinox: equal day and equal day and

night…12 hours of eachnight…12 hours of each

• At the equinox, the sun’s At the equinox, the sun’s rays strike Earth at a 90° rays strike Earth at a 90° angle to the equator. angle to the equator.

• The The autumnal equinox autumnal equinox occurs on September 22 occurs on September 22 or 23 and marks the or 23 and marks the beginning of fall in the beginning of fall in the Northern Hemisphere.Northern Hemisphere.

• The The vernal equinox vernal equinox occurs on March 21 or 22 occurs on March 21 or 22 and marks the beginning and marks the beginning of spring in the Northern of spring in the Northern Hemisphere.Hemisphere.

SolsticesSolstices• Solstice:Solstice: point at which the point at which the

sun is as far north or south sun is as far north or south of the equator as possibleof the equator as possible

• Summer SolsticeSummer Solstice: longest : longest day of the year, which day of the year, which occurs on June 21 or 22 and occurs on June 21 or 22 and marks the beginning of marks the beginning of summer in the Northern summer in the Northern Hemisphere.Hemisphere.

• Winter solstice:Winter solstice: shortest shortest day of the year, which day of the year, which occurs on December 21 or occurs on December 21 or 22 and marks the beginning 22 and marks the beginning of winter in the Northern of winter in the Northern Hemisphere.Hemisphere.

ReviewReviewLook closely at where the Sun is hitting the Earth during Look closely at where the Sun is hitting the Earth during

each season:each season:

http://Search.Lycos.com/setup.asp?r=5&src=clear2&query=weather+savvy

1. The Earth spinning on its axis. Rotation

Revolution2. Going around the Sun. Rotation

Revolution

Revolution

4. Causes the Earth’s seasons. Distance from Sun

Tilt

5. A year.

3. Day composed of 24 hours. Rotation

Revolution

RevolutionRotation

6. The moon going around Earth. RevolutionRotation

Identify the motion being shown in each of these pictures. Each team must write their answers for

each figure in the form of a question.

Earth

Figure #1 Rotation

Revolution

Figure #2

Rotation

Revolution

Planet