Motion of the Earth and Effect of Inclination and Critical Positions of Earth

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Motion of the Earth & Effect of Inclination

  • Earth’s axis is inclined at 23.5 degree from the line perpendicular to the plane of ecliptic (the plane of orbit of the earth around the sun)

  • Change of season occurs due to a combined effect of the revolution of the earth around the sun and the inclination of its axis. If the axis were not inclined there would have been no change in the amount of energy received by a place throughout the year and hence no change of season. Also, day and night would have been equal in length.

Critical Positions of Earth

  • Summer solstice: (June 21) Tropic of Cancer receives vertical rays of the sun. North Pole experiences a continuous day while South Pole experiences a continuous night. Longest day in northern hemisphere.

  • Winter solstice: (December 22)Tropic of Capricon receives vertical rays of the sun. South Pole experiences a continuous day while North Pole experiences a continuous night. Longest day in the southern hemisphere.

  • Equinoxes: (23rd September and 21st March) Equator receives vertical rays of the sun. Day and night are of equal length throughout the world on these days.

  • The season in the two hemispheres are reverse of each other

  • At the equator, the days and nights are exactly equal all the year round. Longer than 24 hours day and night are experienced only in the arctic and Antarctic circles.


  • Time is reckoned with respect to the position of the sun.

  • Time of all places on a given meridian is the same.

  • Time required for one-degree rotation is four minutes.

International Date Line

  • The line at 180-degree distance from the prime meridian is called the International Date line. Moving from the west to east one loses a day if he crosses the line. The date in the Eastern Hemisphere is ahead that in the western hemisphere. The date line is bent and altered to keep some countries in the same time zone.

  • The central meridian for India is the longitude of 82.3-degree E which passes near Allahabad. Indian Standard Time is the time of this meridian. It is 5 hrs 30 minutes ahead of the GMT.

  • Solar Day: Average time period required for the successive passages of the sun over a given meridian. It is 24 hours

  • Sidereal day: Time required for a given star in the sky to return to the same position with respect to the earth. It is four minutes less than the solar day. This difference between solar and sidereal day exists because the position of earth with respect to sun keeps changing due to revolution. However, the position with respect a star at infinity is constant.

  • Solar year: measured with respect to the sun

  • Sidereal year: measured with respect to a star

  • Leap year: Since earth takes slightly more than 365 days for revolution around the sun, one day is added every four years to the calendar. This correction is however too large because the actual solar year is 365.2419 days and not 365.25 days. Hence, the leap year is omitted in the century year unless the century year is divisible by 400 (a leap century). Thus, 1900 was not a leap year while 2000 was.

  • Gregorian Calendar: Julius Ceaser. Pope Gregory XII.


  • Axis of moon makes an angle of 58 degree 43 mins to the ecliptic plane.

  • Diameter: 3480 km

  • Mass: 1/81 of earth

  • Perigee: nearest point to earth (356000 km)

  • Apogee: Farthest point to earth (407000 km)

  • Period of revolution around earth: 29.53 days. This is called synodic month.

  • The time taken by moon to complete one rotation on its axis is exactly same as its sidereal month (27.5 days). Hence, we see the same face of the moon from the earth.

  • Only 59 percent of the total surface of moon is visible from earth.


The synodic month begins when the sun and the moon are in conjunction. At this time, moon is dark and called the new moon. This is followed by various phases and the month ends with again the new moon phase.


  • Regular rise and fall of water level in the seas and oceans. Twice a day. Successive high tides are about 12.5 hours apart. Tides are caused by a combined effect of the gravitational pull of moon and sun on earth. When the forces of sun and moon complement each other, higher tides known as spring tides occur. This occurs on full moon and new moon days when the earth, sun and moon are in the same straight line.

  • When the forces of sun and moon act contrary to each other the tides of lower magnitude known as neap tides occur. Neap tides are observed in the first and the last quarters of the moon.

  • Tides are stronger at the time of perigee and weaker at the time of apogee.

  • Tidal range: difference between the water levels at the time of high tide and low tide. It changes from time and place. Narrow bays have higher tidal range.

  • Bay of Fundy has the highest tidal range in the world.

  • Tides may lead to advance of water upstream into the rivers. This is known as tidal bore.

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