- Earth is the only known planet where life exists.
- Its surface area is covered with two-third of water that is why we call it a blue planet.
- Earth is the third planet from the sun, the densest planet in the solar system, the largest of the solar system’s four terrestrial planets.
- In size, it is the fifth-largest planet. It is the largest terrestrial planet. The other terrestrial planets are Venus, Mars, and Mercury.
- It is slightly flattened at the poles, that is why its shape is described as a Geoid. Geoid means an earth-like shape.
- Earth revolves around the Sun, but its average distance from it is 149 million kilometers / 93 million miles. In astronomy, this is 1 AU – or an astronomical unit.
- Scientists have researched and estimated that our Earth is around 4.5 billion years old. Earth formed at around the same time as the rest of our Solar System.
- Earth revolves around the Sun once every 365.25 days – this is known as one Earth year.
- Only 3% of Earth’s water is fresh and 97% of it is salty.
- The surface of Earth is covered by water, around 71%, only 29% of Earth’s surface is covered by land.
- The atmosphere of Earth is divided into 6 layers – the troposphere, stratosphere, mesosphere, thermosphere, exosphere, and ionosphere.
- Earth has only one satellite – the Moon, but it also has a couple of temporal artificial satellites.
Size and Shape of the Earth
- Earth is not perfect circle it is an oblate spheroid, it is like a sphere, but the distance from pole to pole is less than the distance around the equator (middle).
- The shape of the Earth is called “geoid” that is, ‘an Earth-like shape’.
- Earth is bulged out at equator and flattered at poles because of centrifugal force.
- The earth spins at constant rate but rate of movement is different the equator is moving fastest and poles are not moving (ignoring the fact that earth is orbiting the sun). Because of this movement centrifugal force is pulling matter closer to equator which structure outwards giving earth slightly non-spherical shape.
- Geodesy is the science that studies the shape and size of the Earth.
- Earth’s circumference and diameter differ because its shape is classified as an oblate spheroid or ellipsoid, instead of a true sphere. This means that instead of being of equal circumference in all areas, the poles are squished(slightly flattened at the North and South Poles), resulting in a bulge at the equator, and thus a larger circumference and diameter there.
- The equatorial bulge at Earth’s equator is measured at 26.5 miles (42.72 km) and is caused by the planet’s rotation and gravity.
Motions of the Earth
- Motion is the action or process of moving or of changing place or position.
- The Earth is constantly in motion, revolving around the Sun and rotating on its axis. These motions account for many of the phenomena we see as normal occurrences: night and day, changing of the seasons, and different climates in different regions.
- The earth has two movements, rotation and revolution.
- Rotation- A rotation is a circular movement of an object around a center (or point) of rotation.
- A three-dimensional object rotates always around an imaginary line called a rotation axis.
- If the axis is within the body, and passes through its center of mass the body is said to rotate upon it, or spin.
- Rotation causes days and nights. It takes 23 hours, 56 minutes, and 4.09 seconds for a sidereal day and an exact 24 hours for a mean solar day.
- Earth rotates on its axis from west to east(counter-clockwise). The speed of Earth’s rotation is 1,674.4 km/h or 1,040.4 miles per hour at the equator.
- The earth has a 23.45° tilt of axis.
- While the Earth is spinning on its axis, it is revolving around the Sun in a counter-clockwise direction. It takes the Earth one full year to complete one full revolution around the Sun. This path is known as the Earth’s orbit. The mean distance of the Earth from the Sun is about 93 milling miles and the distance varies by 3 million miles, forming a slightly oval path.
- The revolution of the Earth around the Sun traverses a distance of 595 million miles in 365 days, 6 hours, 9 minutes, and 9.5 seconds. This means a speed of 18 miles a second (or 66,000 miles per hour) while at the same time rotating once each twenty-four hours.
- It takes approximately 365.25 days for Earth to orbit the Sun — a solar year.
- In an ordinary year, if you were to count all the days in a calendar from January to December, you’d count 365 days. But approximately every four years, February has 29 days instead of 28. So, there are 366 days in the year. This is called a leap year.
Earth rotates in an elliptical orbit around the Sun
- The orbit of the Earth around the sun is elliptical and not circular. Due to this, the distance between the Earth and the sun keeps changing.
- When this distance is minimum, the Earth is said to be in perihelion (around January 3), When the distance is the maximum, it is said to be in aphelion (around July 4).
- The Earth’s axis points constantly to the same point (the polar star) in the celestial sphere.
- As a consequence, the latitude on the surface of the earth at which the sun’s rays fall vertically keeps changing as the earth moves its orbit around the sun. Due to this, the earth attains four critical positions with reference to the sun.
- Equinoxes: Equinox refers to a day with an equal duration of day and night. We have two equinoxes in a year which are:
- Spring equinox on March 20
- Autumnal equinox on September 22
- Solstice: On the other hand, solstice refers to a day with either the longest day or the shortest. The two solstices in a year are:
- Winter solstice on December 22
- Summer solstice on June 21
- Equinoxes: Equinox refers to a day with an equal duration of day and night. We have two equinoxes in a year which are:
|Time of the year when the sun is nearest to the equatorial plane giving equal lengths of day and night||Time of the year when the sun is farthest from the equatorial plane resulting in long nights and days|
|An equinox occurs at the start of the spring and fall||The solstice occurs during the summer and the winter|
|Happens twice a year||Happens twice a year|
|Occurs on March 20 (vernal equinox) and on September 22 (autumnal equinox)||Occurs on June 21(Summer Solstice) and on Dec 22 (Winter Solstice)|
Latitudes and Longitudes
Latitudes and Longitudes are imaginary lines used to determine the location of a place on earth.
- The Parallel of Latitudes extends from the Equator to 90 degrees North Pole and 90 degrees South Pole.
- If the latitude are drawn at an interval of 1 degree, then in each of the hemispheres there will be 89 latitude lines that will add up to 179 total lines.
- They are mainly the east-west circles that connect all the locations of the Earth.
- The distance between two parallel latitudes is 111km.
- All the latitudes are parallel to the Equator.
Major Parallel of Latitudes
- Equator is the imaginary line that divides the earth into two hemispheres.
- The northern hemisphere and the Southern hemisphere.
- It is the longest line of latitude.
- The Equator covers 40075km out of which 78.8% covers the water area while 21.3% covers the surface area.
- Out of the five parallel latitudes, Arctic Circle is the Northernmost circle which is at 66 and 1/2 degrees north of the Equator.
- The position of the Arctic is not always fixed.
- As per updates, the Arctic is drifting towards the North about 14.5 m (48 ft)/year.
- The Arctic Circle is 16000km long which covers 4% of the Earth’s surface.
- Antarctic Circle is the Southernmost circle which is currently at 66 and a 1/2 degrees south of the Equator.
- Like the Arctic Circle, the Antarctic Circle is also shifting southward about 14.5 m (48 ft)/year.
- The Antarctic Circle is also 16000km long which covers 4% of the Earth’s surface towards the south.
Tropic of Cancer
- It is also known as Northern Tropic and the Sun is directly overhead at this position in June.
- The Tropic of Cancer is located at 23 and a 1/2 degrees in the Northern Hemisphere.
- The position of the tropic of cancer is fluctuating because of the longitudinal alignment. But the distance between the tropic of cancer and the Antarctic Circle remains the same as they move at the same speed.
- The length of the Tropic of Cancer is 36,788 km.
Tropic of Capricorn
- It is the Southern Tropic which is currently at 23 and a 1/2 degree south.
- The length of the Tropic of Capricorn is the same as the Tropic of Cancer.
- The Southern Tropic covers 3% of the total world’s population.
- The sun is overhead once a year in December and therefore experiences the maximum heat.
Geographical Zones on Earth
The temperature zones are also identified based on the latitudes drawn from the Equator to the Poles. The difference between the above five parallels of Latitudes relates to the changes in Climate.
- There are two frigid zones- North Frigid Zone and the South Frigid Zone.
- The North zone lies between North Pole (90-degree) and Arctic Circle whereas the South zone is between South Pole (90-degree) and Antarctic Circle.
- These zones experience the midnight sun and polar night for the part of the year and are the coldest regions on Earth.
- Summers reside here for 2-3 months and the zones experience 24 hour of sunlight during such time.
- Since the sun rays are always slanting here, the region stays cool.
- Again there are two temperate zones- North Zone lying between Arctic Circle and Tropic of Cancer, South Zone lying between Antarctic Circle and Tropic of Capricorn.
- Due to the tepid latitudes Sun rays never fall directly which results in mild weather.
- Therefore, these zones experience all four seasons: summer, spring, autumn and winter.
- The Torrid Zone lies between Tropic of Cancer and Tropic of Capricorn and is called Tropics.
- In this zone the sun passes directly overhead seasonally.
- Hence it experiences the maximum heat once in a year.
- Longitude is the east-west measurement of the prime meridian.
- There are 180 vertical east longitudes of the Prime Meridian and 180 vertical west longitudes of the Prime Meridian.
- The line that passes through the Greenwich, British Royal Observatory is regarded as Prime meridian.
- It is the base Longitude which is 0 degrees from where 180 degrees east and west directions are considered.
- Hence, Prime of Meridian is the base of world time.
- The Prime Meridian divides the Earth into two halves, Eastern and Western hemisphere.
- It is the east of the Prime Meridian that covers the country like Africa, Asia, Europe, Australia, and the islands of Oceania.
- The landmass of the Eastern Hemisphere is larger than the western part. Therefore, 80% of the human population survives in the Eastern Hemisphere.
- Eastern Hemisphere is also termed as Oriental Hemisphere.
- It is the west of the Prime Meridian that covers North and South America and some parts of Africa, Europe, Antarctica, and also Asia.
- The center of the Western Hemisphere is at the Pacific Ocean whose nearest land is Genovesa Island.
Longitude and Time
- The Earth takes 24 hours time to complete one Rotation.
- The Sun takes 12 hours time to cross Eastern and Western hemispheres.
- The Sun crosses 15-degree of Longitude per hour in every four minutes of time.
- To keep uniform the time range in all the territorial limits of the country, the central meridian is regarded as the Standard Meridian whose local time is considered as the standard time for the whole country.
- There are a total of 24 time zones in the world.
- It is by the movement of Earth, moon and the planets, time is measured on Earth.
- The International Date Line passes through the 180-degree line of longitude.
- The difference between 0-degree longitude and IDL is 12 hours.
- The calculation of time stands like this: the earth rotates 1-degree in four minutes. When it is noon at Greenwich, time at 15-degree east of Greenwich will be (15*4=60 minutes) which is 1 hour ahead of Greenwich Time.
Standard Time and Time zones
- If each town were to keep the time of its own meridian, there would be much difference in local time between one town and the other.
- Travelers going from one end of the country to the other would have to keep changing their watches if they wanted to keep their appointments. This is impractical and very inconvenient.
- To avoid all these difficulties, a system of standard time is observed by all countries.
- Most countries adopt their standard time from the central meridian of their countries.
- The local time of places which are on different meridians is bound to differ.
- In India, for instance, there will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam.
- It is, therefore, necessary to adopt the local time of some central meridian of a country as the standard time for the country.
- In India, the longitude of 82½° E (82° 30′ E) is treated as the standard meridian. The local time at this meridian is taken as the standard time for the whole country. It is known as the Indian Standard Time (IST).
- Earth Divided into twenty-four time zones of one hour each and Each zone Covers 15° of longitude.
- The larger country like the USA, Canada, and U.S.S.R. which have a great east-west stretch, it would be difficult to follow a single time zone, therefore, these countries have to adopt several time zones for a practical purpose.
- Both Canada and U.S.A. have five time zones—the Atlantic, Eastern, Central, Mountain and Pacific Time Zones. The difference between the local time of the Atlantic and Pacific coasts is nearly five hours.
- USSR the largest country (with east-west 165° extends) is divided into 11 time zones. Russia now has nine time zones.
Indian Standard Time
- The Indian Government has accepted the meridian of 82.5° east for the standard time which is 5 hours 30 mins, ahead of Greenwich Mean Time.
The International Date Line
- A traveler going eastwards gains time from Greenwich until he reaches the meridian 180°E when he will be 12 hours ahead of G.M.T.
- Similarly, in going westwards, he loses 12 hours when he reaches 180°W. There is thus a total difference of 24 hours or a whole day between the two sides of the 180° meridian.
- This is the International Date Line where the date changes by exactly one day when it is crossed. A traveler crossing the dateline from east to west loses a day (because of the loss in time he has made), and while crossing the dateline from west to east he gains a day (because of the gain in time he encountered).
- The International Date Line in the mid-Pacific curves from the normal 180° meridian at the Bering Strait, Fiji, Tonga and other islands to prevent confusion of day and date in some of the island groups that are cut through by the meridian.
- Some of them keep Asiatic or New Zealand standard time, others follow the American date and time.
Why is the international dateline drawn in a zigzag manner?
- The time difference on either side of this line is 24 hours. So, the date changes as soon as one crosses this line. To avoid any confusion of date, this line is drawn through where the sea lies and not land. Hence, the IDL is drawn in a zig-zag manner.
References: NCERT, G C Leong, Wikipedia