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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
season, length of day
changes in season
• Topics discussed are
– Seasons
– Length of Day
Review
Important Latitudes to remember
Where are we?
Tompkinsville, KY
Latitude: 37°N
Longitude: 86°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
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
But why?
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 Season
Sun Over Equator
(March 21)
Sun Over
Tropic of
Cancer
(June 21)
Sun Over Tropic
of Capricorn
(December 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.
What is a solar day?
• Definitions are based on the apparent
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
Again, why are there seasons?
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.
Why do we have days?
• We have day and night because the Earth rotates
on its axis.
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
Sunlight
Daytime
Nighttime
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
Tidal Acceleration
• As the moon orbits the Earth, the orbital
angular momentum of the Moon increases,
while it moves away from the Earth.
• As a consequence, Earth’s rotation slows
down, and the length of the day increases.
• 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.
• For the whole world, about 30 such time zones are
defined
• 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?
• 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?
(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.
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
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
Solar Day
It takes the Earth
24 hours to rotate
using the sun as
our reference.
Sun
Overhead
0°
0.986°
360°
This means Earth
travels more than
360 degrees.
References
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