What is the reason for the variation of amount of sunshine?
Variation in the amount of sunshine reached on the earth is the important
feature of different seasons. The Earth’s revolution and tilt of the axis
are the reasons for this variation of amount of sunshine.
Revolution of earth
The Earth revolves around the Sun in an elliptical orbit. This is known
as revolution. The Earth takes 365 ¼ days to complete one revolution.
Parallelism of the earth’s axis
The axis of the Earth is tilted at an angle of 66½o from
the orbital plane or 23½o from the vertical plane. The
Earth maintains this tilt throughout its revolution. This is known as the
parallelism of the Earth’s axis.
Perihelion and Aphelion
On 3rd January the Sun and the Earth are nearest. This day is
known as Perihelion.
On 4th July the Sun and the Earth are farthest. This day is
known as Aphelion.
Important latitudes
Apparent movement of the sun
Since the parallelism is maintained same throughout the revolution, the
position of Sun in relation to the Earth varies apparently between
Tropic of Cancer (23½oN) and
Tropic of Capricorn (23½o S). This is known as the
apparent movement of the Sun.
Seasons
What is the reason for the occurrence of seasons?
The apparent movement of the Sun due to the inclination of axis is the
reason for the occurrence of seasons.
What are the different seasons?
Spring, Summer, Autumn and Winter are the different seasons.
Seasonal changes are not usually very obvious in the tropical
regions. Why?
Seasonal changes are not usually very obvious in the tropical regions
because of the incidence of large amount of Sun’s rays throughout the
year. Characteristics of different seasons are clearly felt in the mid
latitudinal or temperate zones.
Seasons are not distinctly felt in Kerala. Why?
Seasonal changes are not usually very obvious in the tropical regions
because of the incidence of large amount of Sun’s rays throughout the
year. Kerala lies in tropical region so seasons are not distinctly felt in
Kerala.
Equinoxes
Equal amount of sunlight is received in the northern hemisphere as well
as the southern hemisphere when the Sun is vertically over the equator.
The apparent position of sun during the Earth’s revolution will be over
the equator on March 21 and September 23. During these days the length of
day and night will be equal on both the hemispheres. These days are called
equinoxes
Summer solstice in the northern hemisphere
From 21 March onwards, the Sun apparently shifts from the equator
northwards and reaches vertically over the Tropic of Cancer
(23½oN) on 21st June. This day is known as the
Summer Solstice in the Northern Hemisphere. On this day the Northern
Hemisphere experiences its longest day and shortest night. During this
period it is spring season in Northern Hemisphere.
Peculiarities of spring season
Spring is the season of transition from winter to summer.
During spring season plants sprout and bloom. Mango trees bloom and
jackfruit trees bear buds.
Winter solstice in the northern hemisphere
The sun apparently shifts from the equator from 23rd September and reaches vertically above Tropic of Capricorn (23½o S) on 22nd December. This day is known as winter solstice in the Northern Hemisphere. On this day the Northern Hemisphere experiences its shortest day and longest night. During this period it is Autumn season in the Northern Hemisphere.
Peculiarities of autumn season
Autumn marks the transition from the severity of summer towards winter.
The atmospheric temperature decreases considerably, the day shortens and
the night lengthens. The trees generally shed their leaves to survive from
the forthcoming dry winter.
Which period experiences summer season and winter season in the
northern hemisphere?
The southward apparent movement of the sun begins from 21st
June from Tropic of Cancer (23½o N) and reaches vertically
above the equator on 23rd September. During this period summer
season is experienced in the northern hemisphere.
The northward apparent shift of the Sun begins by 22nd
December from Tropic of Capricorn (23½o S) and reaches
vertically above the equator on 21st March. During this period
northern hemisphere experiences winter season.
Apparent movement of the sun and seasons in both hemispheres
Utharayanam
The Sun sets its northward apparent movement from Tropic of Capricorn
(23½o S) on December 22 and it culminates on Tropic of Cancer
(23½oN) on June 21. This northward apparent movement of the Sun
from Tropic of Capricorn to Tropic of Cancer is termed as Utharayanam. The
duration of day in the northern hemisphere gradually increases during this
period.
Dakshinayanam
The Sun sets its southward apparent movement from Tropic of Cancer
(23½o N) on June 21 and it culminates on Tropic of Capricorn
(23½oS) on December 22. This southward apparent movement of the
Sun from Tropic of Cancer to Tropic of Capricorn is termed as
Dakshinayanam. The duration of day in the southern hemisphere gradually
increases during this period.
Days and nights in polar regions
Polar regions experience continuous days and nights for 6 months. Explain
why?
When the Sun is in Northern Hemisphere (from March 21 to September 23)
the north polar regions experience continuous daylight for six months and
the south polar regions experience continuous night.
When the Sun is in Southern Hemisphere (from September 23 to March 21)
the south polar regions experience continuous daylight for six months and
the north polar regions experience continuous night.
Rotation and calculation of time
The Earth rotates from west to east
It takes 24 hours to complete
one rotation.
24 hours = 24 ×
60 minutes
= 1440 minutes
The Earth takes 1440 minutes to complete 360o rotation.
This means that for the rotation of 1o longitude, the Earth
takes four minutes.
Important meridians(longitudes)
Greenwich Meridian
The longitude which passes through Greenwich, the place where the Royal
British Observatory situated is known as Greenwich
meridian.
This meridian is considered as zero degree longitude.
Greenwich meantime (GMT)
Time is calculated worldwide based on Greenwich line. Hence this line is
known as prime meridian. The local time at the prime meridian is known as
the Greenwich Mean Time (GMT).
Time zones
Based on the Greenwich meridian the world is divided into 24 zones, each
with a time difference of one hour. These are known as time zones.
Standard
time
Most of the countries in the world consider the longitude that passes
almost through the middle as the standard meridian. The local time at the
standard meridian is selected as the common time for the whole country.
This time is known as the standard time of that country. Countries with
large longitudinal extent consider more than one standard meridian and
hence such countries have more than one standard time.
Indian standard time (IST)
The 82½o E longitude that passes almost through the middle of
our country has been fixed as the standard meridian of India. The local
time along this longitude is the Indian Standard Time (IST).
Local time at different longitudes
Relation between IST and GMT
Indian Standard Time is 5 hours 30 minutes ahead of Greenwich Mean
Time.
Example:
when GMT is 10 am
IST is _____ .
when IST is 12
noon GMT is ______ .
when GMT
is12 noon IST is ______ .
when IST is
5 pm GMT is ______ .
Answer:
Indian Standard Time is 5 hours 30 minutes ahead of Greenwich Mean
Time.
Example:
when GMT is 10 am
IST is 3:30 pm .
when IST is 12
noon GMT is 6:30 am .
when GMT is12midnight IST
is 5:30 am.(next day)
when IST is
5 pm GMT is 11:30 am.
International date line
180o E longitude and 180o W longitude are one and the same longitude. If 180o longitude passes through a country, the places situated east and west of this line will be having two different days. This creates some practical difficulties. To avoid this, the line is adjusted so that it doesn’t pass through the corresponding land areas. This line is also adjusted to avoid some inhabited islands in pacific ocean. The travellers who cross this line from the west calculate the time by advancing it by one day and those who cross the line from the east deduct one day. This imaginary line is known as International Date Line.
Time calculation
Local time at 30o E longitude is 8 am. What is GMT and IST?
Time difference between GMT and local
time at 30o E longitude = 2 hours
GMT = 8am −2 hours = 6 am
IST = 6am+ 5hr 30minute = 11:30 am
What will be the time in Japan
(135o E ) when it is 11pm on Monday in India?
Ans:
GMT when IST is 11pm = 11pm – 5hour
30 minutes
= 5:30 pm
Since the longitude of Japan is 135o
E, the time at Japan = 135×4 𝑚𝑖𝑛𝑢𝑡𝑒𝑠 𝑎ℎ𝑒𝑎𝑑 𝑜𝑓 𝐺𝑀𝑇
= 5:30pm +
540minutes
= 5:30pm +
9hours
= 2:30am on
Tuesday
A one day cricket match is scheduled to conduct in Seddon Park in New
Zealand at 2 pm. The standard meridian of New Zealand is 180o
E. If you want to see the live telecast of the match, at what time you
will switch on your television?
Ans:
Since the standard
meridian of New Zealand is 180o E,
the standard time will be 12
hours ahead of GMT.
GMT when
2pm at New Zealand = 2pm – 12 hours
= 2am on the same day
IST = 2am
+ 5hour 30 minutes
= 7:30am
To see the match we should switch on the television at 7:30am on the same day.
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