Rotation and Revolution

The axis on which earth rotates makes an angle of 66 1⁄2° with the plane of ecliptic and is tilted 23 1⁄2° from a line perpendicular to that plane. This tilt of earth is called inclination of the Earth's Axis.

Occurrence of day and night is an effect of the Earth's rotation.

Two effects of West to East rotation of the Earth are:

1. It determines the direction in which the Sun, Moon and stars appear to rise and set.
2. Ocean currents and winds are deflected towards right in the Northern Hemisphere and towards the left in the Southern Hemisphere. This is called Coriolis Effect.

The earth rotates about its axis and experiences a centrifugal force which cause a bulge at equator. To balance this bulge there is slight flattening of the Earth at two Poles.

The motion of the earth along its elliptical orbit around the Sun in approximately 365 1/4 days (365.25 days) is termed as Revolution.

The effects of revolution are as follows:

1. Occurrence of different seasons.
2. It causes Spring and Autumn Equinoxes.
3. It causes Summer and Winter Solstice.
4. Changing altitude of the mid-day sun at different times of the year.

The term 'Solstice' means an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. It occurs on June 21 (Summer Solstice) and December 22 (Winter Solstice).

The time when the Sun crosses the plane of the Earth's Equator making length of the day equal to length of night all over the earth, is known as 'Equinox'. It occurs on March 21 and September 23.

The formation of the day and night is due to rotation of earth. While rotating, the part of earth facing the Sun has day and the other part has night. The Circle of Illumination is the imaginary line which shows clear demarcation between the phenomena of day and night on the Earth.

The Summer Solstice occurs on June 21. It occurs when the Sun appears directly overhead Tropic of Cancer in the Northern Hemisphere.

The Winter Solstice occurs on December 22. It occurs when the Sun appears directly overhead Tropic of Capricorn in the Southern Hemisphere. The Solstice occurs due to combined effect of revolution and inclination of Earth.

The Winter Solstice (December 22) is the longest day in the Southern Hemisphere.

The Summer Solstice (June 21) is the longest day in the Northern Hemisphere.

The path along which the Earth travels when moving round the Sun is called the orbit of earth. One complete revolution of the Earth takes approximately 365 1/4 days.

The Arctic Circle experience 'Midnight Sun' on June 21.

One complete revolution of earth takes 365 1/4 days. Every four years, the extra 1/4 day makes a complete day and the next year has 366 days. The year with 366 days instead of 365 days is called leap year and it comes after every four years.

The dates when days and nights are of equal duration on the Equator are March 21 and September 23.

The effect of the inclined axis on the duration of day and night is that their duration differs as shown in the diagram below:

1. At the Equator, place C would be at position C' after 12 hours.
   Distance CO = Distance OC'
   ∴ Length of day = Length of night.
2. On the Tropic of Cancer place B will be at B' after 12 hours.
   But distance BX is greater than B'X.
   ∴ Days are longer than nights.
3. On the Arctic Circle place A will be at position A' after 12 hours. AA' remains in continuous light in summer for six months.
   ∴ Day of 24 hours near the North Pole in summer.
4. Place D on the Tropic of Capricorn in the Southern Hemisphere will be at position D' after 12 hours.
   Distance DY (in light) is less than the distance D'Y (in darkness)
   ∴ Days are shorter than nights.
5. Place E on the Antarctic Circle remains in continuous darkness in winter.
   ∴ Night for 24 hours near the South Pole in winter for six months.

The period of Rotation of the earth is 23 hr, 56 minutes, 4.09 sec. This period is termed as the Sidereal Day.

The Earth is said to be in perihelion when the distance between the Sun and the Earth is least or about 147 million km. The date of this phenomenon is 3rd January.

The speed of Earth’s revolution varies according to the change in its distance from the Sun. The velocity is greatest at Perihelion and least at Aphelion. This variation in speed is due to effect of gravitational pull of the Sun.

Following are the effects of speed of Earth's rotation:

1. All moving bodies on the Earth's surface, i.e. ocean currents and winds are deflected towards the right in the Northern Hemisphere and left in Southern Hemisphere.
2. Cyclones and anti-cyclones are deflected causing atmospheric changes.
3. It affects the pressure belts and movement of air on the surface of earth.
4. The bulging of the air at the Equator and flattening at the poles is also due to rotation.

Two special features of Summer Solstice are:

1. Daylight lasts for the entire 24 hours beyond the Arctic Circle towards the North Pole.
2. The length of day increases with increasing latitude North of the equator.

The two motions of the Earth are rotation and revolution.

A definite path or a curved trajectory in which a planet revolves around the Sun is called its orbit.

The axis on which the Earth rotates is tilted and makes an angle of 23 1⁄2° with the orbital plane. This is known as 'inclination of the Earth'.

Two effects of West to East rotation of the Earth are:

1. The rotation of earth from West to East determines the direction in which the Sun, Moon and stars appear to rise and set.
2. Ocean currents and winds are deflected towards right in the Northern Hemisphere and towards the left in the Southern Hemisphere. This is called Coriolis Effect.

The time when the Sun crosses the plane of the Earth's Equator making length of the day equal to length of night all over the earth, is known as 'Equinox'. It occurs on March 21 and September 23.

The axis on which the Earth rotates is called Earth's axis.

The orbital plane of Earth around the Sun is called Plane of Ecliptic.

The Earth is said to be in perihelion when the distance between the Sun and the Earth is least or about 147 million km. It occurs on 3rd January.

The Earth is said to be in aphelion when the distance between the Sun and the Earth is maximum or about 152 million km. It occurs on July 4.

All the moving bodies on the Earth's surface like ocean currents and winds are deflected towards the right in the Northern Hemisphere and left in the Southern Hemisphere due to Coriolis Force created because of the rotation of Earth. This is called Ferrell’s Law.

The movement of earth on its own axis is called Rotation.

Altitude of the Sun varies at a place according to seasons because when the Sun is overhead the Tropic of Cancer, the northern hemisphere experiences summer season and the southern hemisphere experiences winter season. When the Sun is overhead the Tropic of Capricorn, the northern hemisphere experiences winter season while the southern hemisphere experiences summer season.

Seasons are reversed in Northern and Southern Hemisphere because of inclination the Earth's axis. Because of its tilted position each hemisphere is inclined alternatively towards the Sun. The hemisphere that is near the Sun has summer and the hemisphere that is away from the Sun has winter.

Days and nights are equal at all places on the Earth on March 21 because the Sun is overhead at noon along the Equator. At this position, neither of the poles is tilted towards the sun.

Winds get deflected from their normal path because of the rotational movement of the Earth (Coriolis Effect). In Southern hemisphere the deflection is to the left while in Northern hemisphere the deflection is towards right.

At the Arctic Circle (66 1⁄2°) during summer on June 21 the Sun never sets in mid-summer and there is daylight for 24 hours as North Pole is inclined towards the Sun. Therefore this region is known as the 'Land of Midnight Sun'.

Singapore is situated near the Equator and Tokyo and Russia are situated on higher latitudes. The speed of rotation at Singapore is much faster than that at Tokyo or Russia because as the latitude increases the speed of rotation decreases.

Mid-day Sun can be seen overhead in Chennai twice a year, but not even once in Delhi because Delhi lies to the north of Tropic of Cancer which is the northern most limit of overhead Sun. Sun rays are never vertical beyond the tropics.

On the other hand, Chennai has overhead Sun when the Sun moves northwards from the equator and when the Sun moves southwards towards the equator, as it lies within the tropical region.

The Poles experience six-month day and six-month night due to the inclination of the earth's axis. Because of its tilted position, each pole is inclined alternatively towards the Sun for six months resulting in continuous day and night, which last for 6 months.

Noon is hotter than morning because slanting rays of the sun are received in the morning. At noon, direct rays of the sun are received. Since slanting rays of the sun need to cover more distance than direct rays, they lose more heat.

The variation in the lengths of day and night goes on increasing towards Poles because of the tilted axis of the Earth. As the latitude increases the variation in the lengths of night and days increases and finally at poles, there is continuous day or night for 6 months.

The Tropic of Cancer in the Northern Hemisphere and Tropic of Capricorn in the Southern Hemisphere mark the limit of overhead sun. Beyond the Tropics, the Earth only receives slanting rays of the Sun because of the inclination of the Earth. Therefore the Sun is never overhead beyond the Tropics.

The Poles experience six-month day and six-month night due to the inclination of the earth's axis. Because of its tilted position each pole is inclined alternatively towards the Sun for six months resulting in continuous day and night.

Below diagram shows Summer Solstice in the Northern Hemisphere:

Below diagram shows the position of the Earth on December 22 with Equator, Arctic Circle, and Antarctic Circle labelled:

Below diagram shows the phenomenon of day and night:

Below diagram shows the tilt of the Earth's axis and the revolution causing seasons and variation in the length of day and night:

Below diagram shows Perihelion and Aphelion:

West to east

Elliptical

23 hr 56 minutes 4.09 sec

Coriolis force is generated due to earth’s rotation.

21st June

22nd December

23rd September

22nd December

23rd September

Norway

Aphelion

Winter Solstice

As Australia lies in the Southern Hemisphere, the Sun is overhead on the Tropic of Capricorn during this time. So it is hot in Australia. But India is in Northern hemisphere, which is tilted away from the Sun. Due to the inclination of earth's axis, opposite seasons occur in the two countries Australia and India, which lie in different hemispheres.

Sirin was born on 29th February, a date that only appears in a leap year, which occurs every four years. That's why she gets to celebrate her actual birthday only in leap years like 2016, 2020, 2024, and 2028.

Both A and R are true and R is not the correct explanation of A.

Explanation — All the moving bodies like oceans and winds are deflected towards the right in the Northern Hemisphere and towards left in the Southern Hemisphere due to Coriolis Force created because of the rotation of the Earth. This is called Ferrell's Law.

Both A and R are true and R is the correct explanation of A.

Explanation — 21st June is the longest day in Northern Hemisphere as the sun provides vertical rays on the Tropic of Cancer on this date. This is termed as the Summer Solstice.