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
|Axis of Earth||Earth rotating around an imaginary line running through North pole & South pole via its center|
|Tropic of Cancer||Parallels at 23.5* north of equator|
|Tropic of Capricorn||Parallels at 23.5* south of equator|
|Arctic Circle||Parallels at 66.5* north of Equator|
|Antarctic circle||Parallels at 66.5* south of Equator|
- 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
- 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*
- Network of parallels & meridians drawn on the globe
- Helps to locate a place with given longitudes & latitudes
- Imaginary circles which divides 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
- When sun is at the greatest distance from the equator
- Its rays falls vertical either at tropic of Cancer or tropic of Capricorn
- 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 month approx.
- 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 month approx.
- 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
|Perigee||Nearest point to the moon’s orbit from Earth|
|Apogee||Farthest point to the moon’s orbit from Earth|
|Sidereal Month||Moon completes 1 rotation in 27 days 7 hrs & 43 min approx. wrt earth|
|Synoptic Month||Moon completes 1 rotation in 29 days 12 hrs & 44 min approx. wrt sun|
- 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