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Earth & Space

Changes in season

Length of day

Presented by Zoe L. Seda

This presentation may contain copyrighted material.

Sunshine State Standards

• Earth & Space– 1.1 Students know that the tilt of Earth on its own axis

as it rotates & revolves around the sun causes changes in season, length of day

• Topics discussed are– Seasons– Length of Day

changes in season length of day

Review

Important Latitudes to remember

Where are we?

Tampa, FL

Latitude: 27°N Longitude: 82°W

What is a season?

• One of the major divisions of the year, generally based on cyclic changes of climate– One of the four natural divisions of the year, spring,

summer, fall, and winter, in the North and South Temperate zones

• Each season, beginning astronomically at an equinox or solstice, is characterized by specific meteorological or climatic conditions

– The two divisions of the year, rainy and dry, in some tropical regions

Why are there seasons?

The Earth has changes in seasons because Earth's orbit is in the shape of an ellipse, so it gets closer or further away from the sun as it orbits the sun.

This is a common mistake…

Ultimate Cause of Seasons

• Earth’s axis is NOT oriented straight up and down from the North and South Pole

• Instead, Earth is tilted from straight up and down by an angle of ~23.5 degrees of arc– Actually the angle ranges

from 22 to 24.5 degrees

But why?But why?

North Pole

South Pole

Why is it 23.5°?

• No one knows…• Astronomers have different theories:

– About 5 billion years ago, when the Earth was still very young, it was struck by a Mars-sized planet. This impact could have tipped our planet over.

– As the cloud of dust and gas collapsed when the universe was forming, the solar system did not form uniformly, the spinning of the gases and other planets is what made each different, hence the tilting of the planets

– Essentially, the numerical value of this axis tilt is an artifact of the way the Earth formed. It didn't have to have any specific value, and in fact the other planets all have different axis tilts owing to the differing details of their formation.

What does the tilt do?

• It allows the sun’s rays to shine more

directly and for longer periods of time on some locations than other places of Earth

Sunlight Intensity

• Even though the sun’s rays hit the earth in parallel beams, the tilt of the earth towards the sun causes the beams to hit more directly in some places than others

• Because the earth is round, we can see the different angles that sunlight makes as it hits the earth

• The angle of incidence is the angle formed between the sun’s rays and the earth’s surface. The further from the equator North or South one travels, the smaller the angle of incidence becomes, the more surface area is lit by the sun, and the less intense the sunlight is as it is spread over more area

The Two “Types” of Seasons

Temperate SeasonSun Over Equator (March 21)

Sun Over Tropic of Capricorn (December 21)

Sun Over Tropic of Cancer (June 21)

Sun Over Equator (September 21)

Sunlight Reaching Earth at…

• Equinox

Sunlight Reaching Earth at…

• Solstice

Tropical Seasons

• The tropics is the area between the Tropic of Cancer and the Tropic of Capricorn

• In the tropics, the angle of incidence of sunlight remains relatively high throughout the year and seasonal patterns of temperature are not evident

Tropical Seasons

• The year is divided up into wet and dry seasons

• Wet seasons occur during the months of greatest solar heating when the midday Sun is overhead, generating significant vertical uplift or convection of air that is accompanied by the almost daily formation of large thunderstorms

• This zone of convection is called the Inter-Tropical Convergence Zone (ITCZ)

The “itch”

Inter-Tropical Convergence Zone (ITCZ)

ITCZ• Moves with the seasons north and south of the equator

between the Tropics of Cancer and Capricorn • Close to the equator, the ITCZ influences the weather twice a

year during the equinoxes in March and September.• Near the Tropic of Cancer, the ITCZ approaches only during

June and July, and climates at these latitudes generally experience only one wet season and a prolonged dry season throughout the remainder of the year.

• Near the Tropic of Capricorn, the short wet season occurs during December and January. In some parts of the world, for example India, the special pattern of atmospheric pressure and wind which accompanies the wet season, is known as the monsoon.

Again, why are there seasons?

Why do we have days?

• We have day and night because the Earth rotates on its axis.

Day versus Night

Sunlight

Sunlight

Daytime

Nighttime

When where you are is pointed toward the Sun, it is day. Then the Earth rotates you away from the Sun, and it is night.

Tilt of Earth and Days

• The length of a day changes because the earth spins at a tilt

• The length of a day depends on where you are on the earth– sometimes the North Pole points towards the sun,

while the South Pole points away – this gives the North Pole 24 hours of daylight for about

6 months, while the South Pole is plunged into darkness

– on the equator, the sun is always nearly overhead, so the days are more constant with approximately 12 hours of daylight and 12 hours of darkness everyday

What is a solar day?

• Definitions are based on the apparent motion of the Sun across the sky (solar day; solar time)– the reason for this apparent motion is the rotation of the

Earth around its axis, – as well as the revolution of the Earth in an orbit around the Sun

• Also defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination)– The exact moment is dependent on the geographical longitude,

and to a lesser extent on the time of the year – The length of a such a day is nearly constant

apparent

The changing day

• The earth has over time had an increasingly longer day

• The original length of one day, when the earth was new, is actually closer to 21 hours

• This phenomenon is due to the tides raised by the Moon (tidal acceleration) which slows the Earth's rotation – During the Pennsylvanian Period a day was

~22.4 hours long. – In the Devonian Period, a day was ~21.8 hours long. – Earth's rotation appears to be slowing approximately

2 seconds every 100,000 years.

Tidal Acceleration

• As the moon orbits the Earth, the orbital angular momentum of the Moon increases, while it moves away from the Earth.

• As it stays in orbit, the Moon’s velocity decreases: so the tidal acceleration of the Moon is an apparent deceleration of its motion across the celestial sphere. As its kinetic energy decreases, its potential energy increases.

• As a consequence, Earth’s rotation slows down, and the length of the day increases.

• The Moon recedes from Earth at the rate of approximately 38 mm per year.– This in turn, lengthens Earth's day by about 15 microseconds every

year.• This mechanism has been working for 4.5 billion years, since

oceans first formed on the Earth. • There is geological and paleontological evidence that the Earth

rotated faster and that the Moon was closer to the Earth in the remote past.

Civil Day

• In the middle of the 19th century, a common clock time was defined for an entire region based on the mean local solar time at some central meridian

• For the whole world, about 30 such time zones are defined

• The main one is "world time" or UTC (Coordinated Universal Time)

• The present common convention has the civil day start at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone

• A day is commonly divided into 24 hours of 60 minutes of 60 seconds each

Time Zones

What is Daylight Saving Time?

• Notice that there is no “s” at the end of Saving. Adding the s at the end of Saving is a common mistake.

• Benjamin Franklin was the first person to come up with the idea.

• Main purpose of Daylight Saving Time (called "Summer Time" in many places in the world) is to make better use of daylight. We change our clocks during the summer months to move an hour of daylight from the morning to the evening.

• Daylight Saving Time begins for most of the United States at 2:00 a.m. on the first Sunday of April. Time reverts to standard time at 2:00 a.m. on the last Sunday of October. In the U.S., each time zone switches at a different time.

The rule of thumb…

• When changing the clock, we say

In the Spring, on the first Sunday of April the time springs forward an hour!

In the Fall, on the last Sunday of October the time falls back an hour!

Daylight Saving Time is NOT worldwide

• Equatorial and tropical countries (lower latitudes) generally do not observe Daylight Saving Time. Since the daylight hours are similar during every season, there is no advantage to moving clocks forward during the summer.

• Other places that do not observe DST:– China– Arizona, US

Leap Seconds

• In order to keep the civil day aligned with the apparent movement of the sun, leap seconds may be inserted

• A civil clock day is typically 86400 SI seconds long, – but will be 86401 s long in the event of a leap second– or possibly 86399 s in the event of a reverse leap second (this

has never happened yet)• Leap seconds are announced in advance by the

International Earth Rotation and Reference Systems Service – which measures the Earth's rotation and determines whether a

leap second is necessary• Leap seconds occur only at the end of a UTC month, and

have only ever been inserted at the end of June 30 or December 31

What is a sidereal day?

• In astronomy; it is about 3 minutes 56 seconds shorter than the solar day

• It is close to the actual rotation period of the Earth, as opposed to the Sun's apparent motion

• Refers to the rotation of the Earth measured relative to the stars. It is the time it takes the Earth to rotate 360 degrees and is equal to 23 hours, 56 minutes and 4 seconds.

(pronounced sigh-dear'-real)

Sidereal vs. Solar Day

• Because the Earth moves in its orbit around the Sun, the Earth must rotate more than 360 degrees in one solar day

• The Earth must rotate an extra 0.986 degrees between solar crossings of the meridian. Therefore in 24 hours of solar time, the Earth rotates 360.986 degrees.

• Because the stars are so distant from us, the motion of the Earth in its orbit makes an negligible difference in the direction to the stars. Hence, the Earth rotates 360 degrees in one sidereal day .

• A sidereal day lasts from when a distant star is on the meridian at a point on Earth until it is next on the meridian.

Sidereal vs. Solar Day

0°360°

Sidereal Day

It takes the Earth 23 hours and 56 minutes to rotate 360 degrees relative to a distant star

Distant Star Overhead

0.986°0°

360°

Solar Day

It takes the Earth 24 hours to rotate using the sun as our reference.

This means Earth travels more than 360 degrees.

Sun Overhead

References• http://www.polaris.iastate.edu/NorthStar/Unit1/activity1.htm• http://www.polaris.iastate.edu/NorthStar/Unit4/unit4_sub1.htm• http://www.astrosociety.org/education/publications/tnl/29/29.html• http://www.astro.uiuc.edu/classes/archive/astr100/f03_lect1/lecture04_summary.html• http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/lec4.html• http://www.bbc.co.uk/science/space/solarsystem/earth/solsticescience.shtml• http://spot.pcc.edu/~aodman/cycles/cycle.htm• http://hea-www.harvard.edu/ECT/the_book/Chap2/Chapter2.html• http://www.ifa.hawaii.edu/~acowie/class99b/class_4.html• http://www.srh.weather.gov/srh/jetstream/tropics/itcz.htm• http://zebu.uoregon.edu/ph121/l4a.html• http://www.madsci.org/posts/archives/may99/927116739.As.r.html• http://www.madsci.org/posts/archives/oct2001/1002243289.As.r.html• http://analyzer.depaul.edu/paperplate/Seasons.htm• http://www.astro.uiuc.edu/projects/data/Seasons/seasons.html• http://www.livescience.com/technology/050705_leap_second.html• http://okfirst.ocs.ou.edu/train/meteorology/Seasons.html• http://daphne.palomar.edu/jthorngren/tutorial.htm• http://www.bigelow.org/virtual/handson/seasons_rotation.html• http://www.crh.noaa.gov/fsd/astro/season.htm• http://www.oceansonline.com/seasons.htm• http://webexhibits.org/daylightsaving/g.html