### Theory:

Motions of the Earth
• Shape: Spherical
Inclination of Earth’s axis
• Earth rotates on its axis.
• Axis is an imaginary line that runs from the North Pole to the South Pole.
• Axis passes through the centre of the Earth.
• Tilted or inclined from the vertical by an angle of $$23½°$$.
• Makes an angle of $$66½°$$ with the plane of the Earth’s orbit.
Velocity of the Earth’s rotation
• 1670 km per hour at the equator
• 845 km per hour at 60° N and S latitudes and
• Zero at the poles
Rotation
• It is the spinning movement of the Earth on its axis.
• The Earth rotates from west to east (anti-clockwise).
• It takes 23 hours 56 minutes and 4.09 seconds to complete one rotation.
• The line which divides the surface of the Earth into a lighted half and a dark half is called the Terminator Line.
Revolution
• It is the movement of the Earth around the Sun on its elliptical path.
• The Earth takes 365 ¼ days for it to complete one revolution.
• Revolution speed: 30 km per second.
• Inclination of the Earth on its axis and its revolution around the Sun cause different seasons
• The Northern Hemisphere is inclined towards the Sun: 21st March to 23rd September (6 months).
• The Southern Hemisphere is inclined towards the Sun: From Sep 23rd to March 21st (6 months).

Equinoxes
• Equator faces the Sun directly on 21 March and 23 September.
• The day and night are equal throughout the Earth.
Summer Solstice
• On 21st June, the Tropic of Cancer faces the Sun.
• Longest day in Northern Hemisphere and longest night in Southern Hemisphere.
Winter Solstice
• On 22nd December, the Tropic of Capricorn faces the Sun.
• Longest day in Southern Hemisphere and longest night in Northern Hemisphere.
 Summer Solstice Winter Solstice Date 21st June 22nd December Longest day (Hemisphere) North South Longest night (Hemisphere) South North

Aphelion & Perihelion
• Perihelion is the Earth’s closest position to the Sun.
• Aphelion is the farthest position of the Earth from the Sun.