Seasons and Time

Reasons for the occurrence of different Seasons

  • • Revolution of the earth
  • • Tilt of the earth's axis
  • • Parallelism of the earth's axis
  • • The apparent movement of the sun

Parallelism Of The Earth's Axis

The axis of the earth is tilted at an angle of 66½° from the orbital plane and 23½° from the vertical plane. The earth maintains this tilt throughout its revolution. This is known as parallelism of the earth's axis.

Apparent Movement Of The Sun

Since the parallelism of the earth's axis is maintained same throughout the revolution, the position of the sun in relation to the earth varies apparently between Tropic of Cancer (23½° N) and Tropic of Capricorn (23½° S). This is known as the apparent movement of the sun.

Utharayanam

  • • The northward apparent movement of the sun from Tropic of Capricorn to Tropic of Cancer
  • • Uttarayanam starts from 22nd December and lasts for 6 months till June 21
  • • The duration of day in the northern hemisphere gradually increases during this period

Dakshinayanam

  • • The southward apparent movement of the sun from Tropic of Cancer to Tropic of Capricorn
  • • Dakshinayanam starts from 21st June and lasts for 6 months till December 22
  • • The duration of day in the northern hemisphere gradually increases during this period

Special Days

Equinoxes

  • • The apparent position of the sun will be over the equator on March 21 and September 23
  • • So equal amount of sunlight is received in both the hemispheres
  • • The length of day and night will be equal on these days in both the hemispheres
  • • These days are called equinoxes

Summer Solstice

  • • From March 21 onwards, the sun apparently shifts from the equator northwards and reaches vertically over the Tropic of Cancer on June 21
  • • During this day the longer day and shorter night occur in Northern Hemisphere
  • • This day is known as the summer solstice in Northern Hemisphere

Winter Solstice

  • • From September 23 onwards, the sun apparently shifts from the equator southwards and reaches vertically over the Tropic of Capricorn on December 22
  • • Longer night and shorter day occur in Northern Hemisphere on this day
  • • This day is known as winter solstice in Northern Hemisphere

Seasons

Autumn Season

  • • Autumn is the transition period from summer towards winter
  • • Autumn is experienced in the Northern Hemisphere from 23 September to December 22
  • • During this period, the temperature decreases considerably
  • • This is followed by a shortening of day and lengthening of night
  • • The trees generally shed their leaves

Spring Season

  • • Spring is the transition season from winter to summer
  • • The peculiarities of spring are plants sprouting, mango trees blooming and jack fruit tree bearing buds
  • • It is experienced between March 21 and June 21 in the Northern Hemisphere
  • • The duration of day gradually increases during this period

Time

Local Time & Standard Time

In the ancient period, time was calculated based on the apex position of the Sun and the length of the shadow cast by it. When the Sun is vertically overhead, it is noon. Thus the time estimated at each place, based on the position of the Sun, is termed as the local time.

The local time would be different at each longitude. If we start calculating the local time at different places based on the longitude there, there will be more than one time within a country and it creates a lot of confusion. It adversely affects nationwide examinations, train timings and radio broadcasts.

To solve this, the local time at the longitude that passes through the middle of a country is selected as the common time for the whole country. Each country in the world considers the longitude that passes almost through its middle as the standard meridian. The countries with large longitudinal extent estimates more than one local time by considering more than one standard meridian. The local time at the standard meridian is the standard time of that country.

Greenwich Time & Time Zones [GMT]

  • • The zero degree longitude is known as the Greenwich Meridian
  • • It acquires its name from Greenwich, the place where the Royal British Observatory is situated and through which this line passes
  • • Time is calculated worldwide based on the Greenwich Line. Hence this line is also known as the prime meridian
  • • The local time at the prime meridian is known as the Greenwich Mean Time
  • • Based on the Greenwich Merdian, the world is divided into 24 zones, each with a time difference of one hour. These are known as time zones

The International Date Line

  • • 180 degree longitude is known as International Date Line
  • • There is a diffence of 24 hours, at 180° longitude to the east and west of Greenwich
  • • If 180° longitude passes through a country, the places situated East and West of this line will be having two different days
  • • To avoid this, certain necessary adjustments have been effected in this line with the result that it doesn't pass through the corresponding land areas
  • • The line is in such a way that it passes through Bering - strait in Pacific Ocean and avoid some of the inhabited islands
  • • 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 west deduct one day