Geographical Division of Earth & its Relation to Solar System

Shape of Earth

• Oblate spheroid or oblate ellipsoid shape
• Slightly flattering at poles & slightly bulging at equator
• Polar radius approx. 21 km shorter than equatorial radius

Distinct Features

• Length of latitudes decreases from equator to poles i.e., max. at equator & 0 at poles
• As earth’s axis is inclined by 23.5* to its orbital plane, therefore 23.5* is max. latitude upto which sunrays can be perpendicular to any place.
• Means all places between Tropic of cancer & Tropic of Capricorn experience vertical rays of sun twice a year but both the tropics only once.
• Tropic of cancer will get vertical sunrays at summer solstice, when Northern hemisphere of earth is at max. inclination from sunrays.
• Tropic of Capricorn will get vertical sunrays at winter solstice, when southern hemisphere of earth is at max. inclination from sunrays.

Greenwich Meridian

• Also known as Prime Meridian or Time Meridian
• Meridian passing through Royal observatory at Greenwich near London which divides earth in eastern & western hemisphere.

International Date Line

• Exact opposite to Greenwich meridian at longitude of 180*

Graticule

• Network of parallels & meridians drawn on the globe
• Helps to locate a place with given longitudes & latitudes

Great Circles

• Imaginary circles which divide the earth into two equal parts & whose center lies at the center of the earth
• Largest circles that can be drawn on the globe i.e., Equator & all meridians

Rotation of Earth

• Earth’s rotate around its own axis from west to east viz. anticlockwise direction
• Earth’s rotate around the sun (Elliptical path) from west to east viz. anticlockwise direction

• Axis of earth rotation is inclined at 66.5* to its plane of elliptic
• Axis of earth rotation is inclined at 23.5* to perpendicular to the plane of elliptic
• Plane of earths equator to plane of elliptic or earth’s axis to axis of revolution is inclined at 23.5*

• Velocity of earth’s rotation decreases from equator to poles

• Correlated with length of the parallels which also decreases from equator to poles
• velocity at equator & 0 velocity at poles

• Weight of body is less at equator & greater at poles

• Because of greater centrifugal force at the equator (mv^2 / r) due to greater velocity at equator
• Because of higher gravitational force at poles (GMm/r) due to lesser radius of poles than the equator

• The earth moves in space in two distinct ways viz.

• It rotates on its own axis from west to east once in every 24 hours causing day & night
• It revolves around the sun in an orbit every 365(1/4) days causing the seasons & the year

• Throughout the revolution of earth around the sun, its axis remains tilted in the same direction.
• Its axis continues to point to same spot in heaven known as Polaris / polestar/ parallelism of axis.
• Polestar >> Brightest star in the sky in north direction or northern star

Solstices

• When sun is at the greatest distance from the equator
• Its rays falls vertical either at tropic of Cancer or tropic of Capricorn

Summer Solstice

• Earth’s axis leans at max 23.5* in northern hemisphere towards the sun
• Sun’s ray fall vertical at Tropic of cancer around 21/22 June
• This brings summer season in Northern hemisphere
• With this duration of days starts decreasing
• Means June 21/June 22 is longest day in Northern hemisphere.
• Daylights of 14 hrs at Tropic of Cancer; 12 hrs at Equator; 10 hrs at Tropic of Capricorn
• Between Arctic Circle & North Pole Day lasts for 24 hours & between Antarctic Circle & South Pole same duration of night lasts.
• At North Pole Day last for 6 months & at South Pole night last for 6 months approx.

Winter Solstice

• Earth’s axis leans at max 23.5* in southern hemisphere towards the sun
• Sun’s ray falls vertical at Tropic of Capricorn around 21/22 Dec.
• This brings summer season in Southern hemisphere
• With this duration of days starts increasing in Northern hemisphere
• Means Dec 21/June 22 is shortest day in Northern hemisphere
• Daylights of 14 hrs at Tropic of Capricorn; 12 hrs at Equator; 10 hrs at Tropic of Cancer
• Between Antarctic Circle & South Pole Day lasts for 24 hours & between Arctic Circle & North Pole same duration of night lasts.
• At South Pole Day last for 6 months & at North Pole night last for 6 months approx.

Equinoxes

• The sun is vertically overhead at the equator on two days of the year usually on 21 March & 21 September
• Dates changes because a year is not exactly of 365 days
• These two days are termed as equinox means on these two days all parts of the world have equal days & nights.

Seasonal Changes & Their Effect on Temperature

• In summer, sun is overhead & its sunrays fall almost vertically on the earth, concentrating its heat on a small area;
• Temperature therefore rises & summers are always warm
• In winters, the oblique rays of sun come through atmosphere less directly & have their heat absorbed by atmosphere & water vapour;
• Sun rays fall obliquely & spread over greater area, hence temperature remain low.

In addition, days are longer than nights in summer & more heat is received over longer daylight duration; Nights are shorter & less heat is lost. Hence, there is net gain in total heat received & temperature rises in summer. Shorter days & longer nights in winters accounts for reverse effect.

Dawn & Twilight

• The brief period between sunrise & full daylight is called dawn & that between sunset & complete darkness is termed as twilight.
• This is caused by the fact that during the periods of dawn & twilight earth receives diffused or refracted light from the sun while it is still below the horizon.
• Since the sun rises & sets in vertical path at the equator, the period during which refracted light is received is short.

• But in temperate latitudes, the rises & sets in oblique path & hence the period of refracted light is longer than that at equator, which is much longer at poles.

Longitude & Concept of time

• Earth completes 1 rotation in 24 hours means swept 360* in 24 hours >>> 15 * in one hour or 1* in 4 minute
• Since earth rotates west to east places located in the east gain time while those located in the west loose time

• Generally, 12: 00 noon at a place is considered when sun’s altitude is highest &  exactly over the meridian at that place
• Time at all the places located at particular meridian i.e. north & south will be same, however places located at east & west will have different local time

Standard time of a country

• Standard time of a country is local time of a selected longitude crossing through a place in the country of due importance.
• Standard time of India is local time of longitude passing through Allahabad situated at 82.5* East of Prime meridian i.e. Five & half hours ahead of GMT
• Calendar date is changed by one day when someone crosses international date line
• Although line is deviated at some places to mark same date at some countries & islands
• Thumb Rule → Loose 12 hrs west of Prime meridian & gain 12 hrs east of prime meridian

Earth’s Natural Satellite (Moon)

• Moon rotates around the earth in an elliptical orbit as earth rotates around the sun
• Axis of moon is inclined at 58.4 * approx. wrt Plane of ecliptic, as a result distance of moon from earth keeps on changing
• Only 59 % of moon’s surface is visible from earth at the max

Eclipses 

• Partial or total obstruction of light from a celestial body as it passes through shadow of another celestial body
• Apparently, eclipse shall occur every month because of revolution of earth around the sun & moon around the earth, but Plane of moon’s orbit around the earth is inclined at 5.9* to the plane of earth’s orbit around the sun