Rotation of the Earth
1. An imaginary line from west to east is taken as axis of the earth. Turning the earth from west to east is called the rotation of earth. Rotation makes the sun to rise every day in the east and set in the west.
2. The earth spins around is aris west to east once in 24 hours.
3. The north and south pole lie at the ends of axis.
4. The rotation axis of the earth makes an angle of 23O30′ with the vertical.
5. The rotation axis of the earth always points towards pole star as the Earth moves round the sun.
6. The tilt of the earth is called its inclination.
Tilted axis effects on day and night:
1. Rotation of tilted axis of the earth is responsible for the different lengths of the days and nights on different parts of the earth.
2. As the earth’s axis is inclined, the sun’s rays are constantly changing as the earth journeys around the sun. Due to this the length of days and nights change.
Earth’s Rotation
1. Shape of the Earth : The earth is slightly flattened at the poles and bulging at the equator i.e. the earth is a oblate spheroid.
2. East-west movement : The sun appears rising in the east and setting in the west because the earth rotates from west to east.
3. Day and night : Day light and night darkness follow a rhythm by which light, heat, humidity and wind movements are affected.
4. Circulation of air and water : The change in the direction of the flow of winds and ocean waters are created by the earth’s rotation.
5. Tidal Forces : The Moon exerts a gravitational pull on the Earth while at the same time the Earth is turning with respect to the Moon. This generates tidal forces.
Revolution of the Earth
The revolution is the motion of the Earth around the Sun.
The path around the sun is called the orbit.
The Earth moves with a greater speed when it is closer to the sun but decreases as it moves away from the sun and the time taken by the Earth for one revolution around the sun is called a year.
1. Perihelion : The Earth is closest to the sun near January 3rd (147.5 million km). At this time the Earth is said to be in perihelion. (This word comes from the Greek word peri meaning ‘near or around’ and helos meaning ‘sun’.)
2. Aphelion : The Earth is farthest from the sun (152.5 million km) near July 4th. At this time the Earth is said to be in aphelion. (This word comes from the Greek word ap meaning ‘away from’ and helos.)
Note : The rotation of the Earth on its inclined axis and the revolution of the Earth around the Sun causes seasons. Differences in temperature on different parts of the Earth are caused due to the angle at which the rays of the Sun falls on the surface of the Earth.
1. If the Earth had been flat instead of spherical, all places on its surface would have acquired the same temperature.
2. If the axis of the Earth was vertical instead of inclined, places on the Earth would have the same temperature throughout the year.
3. If the Earth did not revolve around the Sun, then even with its inclined axis, there would be no seasons.
Four distinct Phases of Seasons
1. Summer : The part of the Earth that tilts towards the Sun has longer days and hence the hot weather.
2. Autumn : The Earth begins to tilt away from the Sun hence makes a cooler weather.
3. Winter : The part of the Earth that is tilted farthest from the Sun receives little sun rays hence days are short and the weather is cold.
4. Spring : The Earth begins to tilt back towards the Sun hence brings the warmer weather.
June 21 : The rays of the Sun fall vertically on the Tropic of Cancer. This is because the North Pole is tilted towards the Sun. On this day the Northern Hemisphere has the longest day and shortest night.
September 23 : On this day the sun rays fall vertically at the Equator and hence at all places days and nights are equal and it is autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
December 22 : The rays of the Sun fall vertically on the Tropic of Capricorn. Hence on this day Northern Hemisphere has the shortest day and longest night.
March 21 : The rays of the Sun fall vertically at the Equator. Hence at all places days and nights are equal and it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
Apparent Migration of the Sun
(i) Northward apparent migration : The Sun appears to be moving north for six months in a year. This northward migration begins after December 22 and is completed on June 21, when the Sun is directly overhead at 23.5O N of latitude (Tropic of Cancer).
(ii) Southward apparent migration : The Sun appears to be moving south for six months in a year. This southward migration begins after June 21 and is completed on December 22, when the Sun is directly overhead at 23.5OS of latitude (Tropic of Capricorn).
Time
Day : The average time required for one rotation of the Earth is called a day. Each day has exactly 24 hours. Each hour has 60 minutes and each minute has 60 seconds.
Month : A month, originally, was the time taken by the Moon to complete one revolution round the Earth. In modern calendars, however, the number of days in a month are not based on the phases of the Moon.
Week : A week is not based on a natrual phenomenon. A month is divided into four weeks of seven days each. The 7-day week was probably the result of the tradition of a day of rest after six days of work. Although the days of the week had no astronomical significance, they were clearly named after the seven moving celestial bodies, namely the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn.
Year : A year is defined as the time taken by the Earth to complete one revolution around the Sun. One year contains 365.25 days.
Julian Calendar : In this calender a year has 365 days and adding an extra day every fourth year. Thus the tradition of the leap year begin.
Year of Confusion
In 48 B.C. when Julius Caesar came to power in Rome, the calendar indicated spring while the weather indicated the middle of winter. To correct this situation, Julius Caesar ordered 80 days to be added to the year 46 B.C. which for obvious reasons came to be known as the ‘year of confusion’.
Gregorian Calendar : The Gregorian calendar is also called the Christian Calendar because it makes use of the birth of Jesus Christ as the starting date.
Years in this calendar are designated A.D. (Latin anno domini meaning ‘in the year of our Lord’) and B.C. (before Christ).
Months of the Year
1. January (31 days) : January is named after the Roman god Janus the god of gates and doors and hence it marks the openings and beginnings.
2. February (28 days) : This word is derived from the Latin word Februa signifying the festival of purification celebrated in ancient Rome during this month. Originally February had 29 days but later one day from this month was added to the month of August giving it 31 days. Today, in a leap year February has 29 days and 28 days in other years.
3. March (31 days) : March is named after Mars, the Roman god of war.
4. April (30 days) : This month has been named as Aprilis by Romans. It is derived from aperire meaning ‘to open’ probably because it is the season when buds open.
5. May (31 days) : This is the fifth month of the year.
6. June (30 days) : This month originally consisted of 29 days. Julius Caesar added one more day to this month, making it 30 days long. Perhaps named after the Roman goddess—Juno.
7. July (31 days) : It was the month in which Julius Caesar was born and named as named Julius in his honour.
8. August (31 days) : It was the sixth month in the Roman calendar and hence called Sextilis (Latin sextus meaning ‘sixth’). It was renamed in honour of Caesar Augustus. A day was taken from February and added to this month to make it equal to 31 days just like July.
9. September (30 days) : In the Roman calendar it was the seventh month and derived its name from septem meaning ‘seven’.
10. October (31 days) : In the Roman calender it was the eighth month and derived its name from octo meaning ‘eight’.
11. November (30 days) : In the Roman calander it was the ninth month. It had 29 days until Julius Caesar gave it 30 days in his calendar.
12. December (31 days) : In the Roman calender it was the tenth month of the year and hence the name from the latin decem meaning ‘ten’.
The Jewish Calendar : It is religious and the official calendar of the state of Israel and is used by the Jewish people throughout the world.
It is a mixed calender of moon and sun, based on lunar months of 29 days alternating with 30 days. An extra month is added every three years based on a cycle of 19 years.
The Islamic Calendar : The Islamic calender is a lunar calendar used in most Islamic countries.
The Islamic year has 12 lunar months. Thirty years make a cycle in which the 2nd, 5th, 7th, 10th, 13th, 16th, 18th, 21st, 24th, 26th and 29th years are leap years of 355 days. The other common years have 354 days.
The Hindu Calendar : The traditional Hindu calendar is luni-solar, based on 12 lunar months. The shortfall of 11 days between the lunar and the solar year is adjusted over a period of three years, when an additional month, adhik mas mal mas (extra month) is added.
Months of the Hindu Calendar
Hindu months Month corresponding on Western calendar
1. Chaitra March-April
2. Vaishakh April-May
3. Jyeshtha May-June
4. Ashadh June-July
5. Shravan July-August
6. Bhadrapad August-September
7. Ashwin September-October
8. Kartik October-November
9. Margashirsha November-December
10. Poush December-January
11. Magh January-February
12. Phalgun February-March
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