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2. Changes in season Length of day
Earth & Space
Changes in season
Length of day
Presented by Zoe L. Seda
This presentation may contain copyrighted material.

3. 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

4. Review

5. Important Latitudes to remember

6. Where are we?
Tampa, FL
Latitude: 27°N Longitude: 82°W

7. 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

8. Why are there seasons? WRONG
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.
WRONG
This is a common mistake…

9. Ultimate Cause of Seasons
North Pole
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
South Pole
But why?

10. 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.
Earth is tilted at 23.5 degrees, Uranus is tilted at about 98 degrees
if you were standing on Uranus (if you could) the only difference you would notice was that the sun and the other planets would appear to rise and set in the north and south rather than east/west like on Earth.
Whatever the reason, it's a good thing - if the Earth did not tilt, countries near the poles would be cold and dark all year round. If it tilted too much, the seasons would be very extreme – like on the planet Uranus. Here the winter lasts for 42 years in total darkness

11. 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

12. 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
Sunlight Intensity Not only does the number of daylight hours a location receives change as earth revolves around the sun, but also the intensity of the sunlight received. Think about it: if you were to shine a flashlight straight at a piece of paper, a very intense circle of light would light up a small area. If you were to tilt the flashlight so the light shone on the paper at an angle, the "circle" of light would not be a circle, but an ellipse. The ellipse would be larger in area than the circle. Because the amount of light shining from the flashlight had not changed, but area lit up had increased, the ellipse would be less bright than the circle. The earth’s surface is affected by the sun’s rays in the same way. The sun’s rays shine on earth with an intensity of about 1370 Watts per square meter. 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. 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. Notice in the above diagram that the Earth is receiving sunlight at a 90 degree angle at about 23.5 degrees North in latitude. What time of year is this diagram showing? What would this picture look like if it were showing the vernal or autumnal equinox?

13. The Two “Types” of Seasons
vs.
Temperate
Tropical

14. Temperate Season Sun Over Equator (March 21)
Sun Over Tropic of Cancer (June 21)
Sun Over Tropic of Capricorn (December 21)
vernal equinox: the first day of spring in the Northern Hemisphere (March 21), when the Sun is perpendicular to the equator.
summer solstice: first day of summer in the Northern Hemisphere (June 21), when the Sun is perpendicular to the tropic of Cancer.
autumnal equinox: first day of autumn in the Northern Hemisphere (September 22), when the Sun is perpendicular to the equator.
winter solstice: first day of winter in the Northern Hemisphere (December 21), when the Sun is perpendicular to the Tropic of Capricorn.
Spring equinox - day and night are each 12 hours long and the Sun is at the midpoint of the sky.
Summer solstice - the longest day of the year, when the Sun is at its most northern point in the sky.
Autumn equinox - day and night are each 12 hours long and the Sun is at the midpoint of the sky.
Winter solstice - the shortest day of the year, when the Sun is at its most southern point in the sky.
Summer The day the north pole is nearest the Sun is called the 'summer solstice'. (You can see this from the picture on the right). Looking from Earth, the Sun reaches its highest point in the sky all year. This means it takes the most amount of time to cross the sky. So this is the longest day of the year. Its called the 'summer solstice' and happens around 21 June. Astronomers call this the start of summer and after this date, days start getting shorter.
Autumn As we continue our journey around the Sun, the north pole moves away from the Sun. The Sun rises lower in the sky so the days continue getting shorter. When the Sun is at its mid-point in the sky, we reach the 'autumn equinox', around 22 September. Day and night are both 12 hours long and its the beginning of autumn.
Winter The day when the north pole is furthest from the Sun is called the 'winter solstice'. The Sun crosses the sky at its lowest point all year. Therefore it crosses the sky in the quickest time so this is the shortest day of the year. Winter solstice happens around 22 December and marks the start of winter. From then on, the days start getting longer.
Spring The Earth continues on its path, and our north pole starts moving towards the Sun again. The Sun moves upwards in our skies and the days continue getting longer. Again, we reach a midpoint when day and night are both 12 hours long. This is called the 'vernal (or spring) equinox' and happens around 21 March.
Sun Over Equator (September 21)

15. Sunlight Reaching Earth at…
Equinox
During the equinoxes (March 21 and September 21), a day lasts 12 hours and a night lasts 12 hours at all latitudes.
Sunlight strikes the earth most directly at the equator.

16. Sunlight Reaching Earth at…
Solstice
During the winter solstice (pictured above), the Northern Hemisphere day lasts fewer than 12 hours and the Southern Hemisphere day lasts more than 12 hours.
During the winter solstice, the North Pole has a 24-hour night and the South Pole has a 24-hour day.
During the winter solstice, sunlight strikes the earth most directly at 23.5 degrees South (the Tropic of Capricorn).
During the summer solstice (not pictured), the Northern Hemisphere day lasts more than 12 hours and the Southern Hemisphere day lasts fewer than 12 hours.
During the summer solstice, the North Pole has a 24-hour day and the South Pole has a 24-hour night.
During the summer solstice, sunlight strikes the earth most directly at 23.5 degrees North (the Tropic of Cancer)

17. 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
Within the tropics, the angle of incidence of sunlight remains relatively high throughout the year and seasonal patterns of temperature are not evident. Instead, 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), which moves with the seasons north and south of the equator between the Tropics of Cancer (Northern Hemisphere) and Capricorn (Southern Hemisphere). 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.

18. 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)

19. The “itch” Inter-Tropical Convergence Zone (ITCZ)
The Inter-Tropical Convergence Zone (ITCZ), appears as a band of clouds, usually thunderstorms, that circle the globe near the equator. The solid band of clouds may extend for many hundreds of miles and is sometimes broken into smaller line segments. The ITCZ follows the sun in that the position varies seasonally. It moves north in the northern summer and south in the northern winter. The ITCZ (pronounced "itch") is what is responsible for the wet and dry seasons in the tropics. It exists because of the convergence of the trade winds. In the northern hemisphere the trade winds move in a southwesterly direction, while in the southern hemisphere they move northwesterly. The point at which the trade winds converge forces the air up into the atmosphere, forming the ITCZ. The tendency for thunderstorms in the tropics is to be short in their duration, usually on a small scale but can produce intense rainfall. It is estimated that 40 percent of all tropical rainfall rates exceed one inch per hour. Greatest rainfall typically occurs when the midday Sun is overhead. On the equator this occurs twice a year in March and September, and consequently there are two wet and two dry seasons. Further away from the equator, the two rainy seasons merge into one, and the climate becomes more monsoonal, with one wet season and one dry season. In the Northern Hemisphere, the wet season occurs from May to July, in the Southern Hemisphere from November to February.

20. 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.

21. Again, why are there seasons?

22. Why do we have days?
We have day and night because the Earth rotates on its axis.

23. Day versus Night
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.
Sunlight
Daytime
Nighttime

24. 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

25. 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

26. 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.
Geologic Time
The Palaeozoic Era spans 322 million years, beginning with the Cambrian period 570 million years ago, and finishing with the end of the Permian period 248 million years ago.
The Pennsylvanian period is the division used in North America which corresponds with the Upper Carboniferous period in Europe. The name Pennsylvanian is derived from the coal measures of the state of Pennsylvania which were formed during this period.
The Devonian period begins 412 million years ago and spans 58 million years.

27. 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.
The mass of the Moon is sufficiently large and it is sufficiently close to raise tides in the Earth: the matter of the Earth, in particular the water of the oceans, bulges out to the direction of the Moon (and opposite to it). This follows the Moon in its orbit, which takes about a month. The Earth rotates under this tidal bulge in a day. The actual matter of waters rotate with the Earth, but they rise and fall as the Moon comes overhead. However, the rotation drags the position of the tidal bulge ahead of the position directly under the Moon. As a consequence, there exists a substantial amount of mass that is offset from the line through the centers of the Earth and Moon. Because of this offset, a portion of the gravitational pull it exerts on the Moon is perpendicular to the Earth-Moon line and hence accelerates the latter in its orbit. Conversely, the gravitational pull from the Moon on this mass exerts a torque that decelerates the rotation of the Earth.
So the orbital angular momentum of the Moon increases, while it moves away from the Earth. As it stays in orbit, it follows from Kepler's 3rd law that its 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, the rotational angular momentum of the Earth decreases: its rotation slows down, and the length of the day increases. The corresponding rotational energy dissipates through friction of the tidal waters along shallow coasts, and is lost as heat.
The Moon recedes from Earth at the rate of approximately 38 mm per year. The Earth's day lengthens by about 15 µs 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.

28. 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
For civil purposes, since the middle of the 19th century when railroads with regular schedules
came into use, a common clock time has been 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.

29. Time Zones

30. 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.
Daylight Saving Time also saves energy

31. 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!

32. 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
Jordan decided to implement Summer Time all year round
Israel always has Daylight Saving Time, but until 2005, it was decided every year by the Ministry of Interior

33. 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 SI seconds long,
but will be s long in the event of a leap second
or possibly 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
An extra second will be added to 2005 to make up for the slowing down of the Earth's rotation, officials said this week.
The once-common "leap second" is the first in seven years and reflects the unpredictable nature of the planet's behavior.
The International Earth Rotation and Reference Systems Service in Paris keeps track of time by measuring the Earth's rotation, which varies, and by an atomic clock, which is unwavering. When a difference in the two clocks shows up, the IERS adds or subtracts a second to the year.
For the first time since 1998, the IERS will sneak in an extra second this year to get time back in synch, officials said in a statement Monday.
On Dec. 31, the clock will read like this as it leads into Jan. 1, 2006:
23h 59m 59s h 59m 60s h 00m 00s. Normally, the seconds would roll from 59 directly to 00.
Always on time
"As the Earth is slowing down compared to atomic clock time, noon is going to come a little later. Earth rotation time is falling behind atomic clock time," said Tom O'Brian, Chief of the Time and Frequency Division at the U.S. National Institution of Standards and Technology. "Periodically people have to add time to atomic clock time. When those two times are approaching about a second difference, we add a leap second."
While time has been measured by the planet's rotation for thousands of years, it wasn't until 1949 that scientists developed a clock that kept perfect time.
"An atomic clock keeps time by looking at the fundamental vibrations of atoms," O'Brian said. "It's like middle "C" on a tuning fork – a particular kind of atom has a set of frequencies that can be used to keep time."
The current standard is a cesium atom, which vibrates 9,192,631,770 times per second. As far as scientists know, this doesn't change over time and is the same everywhere on Earth and in space.
Tiny changes
The first leap second was added in 1972, as technology allowed for more accurate timekeeping, and they were all the rage in the beginning. At least one was added every year between 1972 and 1983 before a slight drop-off in the mid-eighties and nineties.
"And then, in 1999 for reasons still unknown, the rotation of the Earth speeded up a bit, so we haven't had to add a second since then," O'Brian told LiveScience in a telephone interview.
Part of the secret behind Earth's changing speeds is tidal force exerted by the Moon, which is responsible for the gradual slowing of our planet's rotation over time. But other slight forces are at work, such as changes in the season, movement of rock in the molten core, and other factors that scientists have yet to uncover.
Seasons, particularly those in the Northern Hemisphere, change the planet's rotational speed predictably during the year. Water evaporates from the sea surface and comes down as rain and snow in the mountains and eventually melts back to the sea.
This creates an effect similar to an ice skater sticking her arms out to slow down a spin, or pulling them close to her body to speed up.
The change is typically miniscule, however.
 "By changing we're talking about a millionth of a second per day," O'Brian said. "But long term slowing is due to the Moon. It's about 1.5/1000th of a second slower per century. The day is longer today than it was in 1905.“
Previous Leap Seconds
12/31/1998 6/30/ /31/1995 6/30/1994 6/30/1993 6/30/ /31/ /31/ /31/1987 6/30/1985 6/30/1983 6/30/1982 6/30/ /31/ /31/ /31/ /31/ /31/ /31/ /31/ /31/1972 6/30/1972

34. What is a sidereal day?
(pronounced sigh-dear'-real)
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.

35. 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 degrees between solar crossings of the meridian. Therefore in 24 hours of solar time, the Earth rotates 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.
In every day life, we use solar time.

36. Sidereal vs. Solar Day

37. Sidereal Day
Distant Star Overhead 0°
360°
It takes the Earth 23 hours and 56 minutes to rotate 360 degrees relative to a distant star

38. 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 0°
0.986°
360°

39. References