Chapter 3. Motions of Earth

Overview
• Earth has two type of motions:
(1) Rotation: This is movement of Earth on its axis.
(2) Revolution: Movement of Earth around Sun in its orbit is known as revolution.
• Orbital Plane: plane formed by its orbit is known as Orbital Plane.
• Circle of Illumination: circle which divides day and night on globe is known as circle of illumination.
• If Earth stops rotating then side facing sun will become too warm and other side will remain in darkness and will be freezing cold all time
• Earth takes 365 ¼ days to complete one revolution around sun.
• Six hours saved every year becomes a day in four years, which is added in February. This year of 29 days of February is known as leap year.

Revolution
• Earth is revolving around sun in an elliptical orbit. Throughout its orbit, earth is inclined in same direction.
• A year is generally divided into summer, winter, spring, & autumn seasons due to change in position of earth around sun.

Equinoxes
• On 21st March and 23rd September, sun rays directly fall on equator.
• As both hemisphere receives an equal amount of sunlight therefore there are equal day and equal night on whole earth. Thus it is known as equinox.
• On 23rd September, autumn season is in Northern hemisphere and spring season is in southern hemisphere. reverse happens on 21st March.

Summer Solstice
• On 21st June, northern hemisphere is tilted towards sun. As sun rays directly falls on Tropic of Cancer therefore that area receives more heat.
• Due to slanting of rays in June in northern hemisphere, areas near poles receive less heat than areas around Tropic of Cancer. Six months long days and nights on poles happen due to tilt of Earth on its axis.
• largest day and smallest night at Tropic of Cancer occur on 21st June and reverse happens in southern hemisphere.

Winter Solstice
• On 22nd December, Tropic of Capricorn receives direct sun rays as earth’s southern hemisphere is tilted towards sun.
• Southern hemisphere receives more heat thus longest day and shortest night occur on Tropic of Capricorn and reverse happens in northern hemisphere.

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