Motions of the earth
The earth has two basic movements:
This is the movement of the earth with the sun and the rest of the solar system in an orbit around the centre of the MilkyWay Galaxy. This, however, has little effect upon the changing environment of the earth.
1. Rotation: The spinning of the earth around its axis is called the rotation of the earth. The axis is the imaginary line passing through the centre of the earth. The earth completes one rotation in 23 hours, 56 minutes and 4.09 seconds.
It rotates in an eastward direction opposite to the apparent movement of the sun. The earth’s axis is inclined at an angle of 66½q to the orbital plane as it moves around the sun. We can say, the earth’s axis is tilted at an angle of 23½q (Figure 2.10) from a perpendicular to the elliptic plane. The velocity of earth’s rotation varies depending on the distance of a given place from the equator.The rotational velocity at the poles is nearly zero. The greatest velocity of the rotation is found at the equator. The velocity of rotation at the equator is 1,670 km per hour.
Effects of earth’s rotation: The rotation of the earth causes the following effects:
1. The apparent rising and setting of the sun is actually caused by the earth’s rotation which results in the alternate occurrence of day and night everywhere on the earth’s surface.
2. Rotation of the earth is also responsible for the difference in time between different places on the earth. A 24 hour period divided by 360 degrees gives a difference of 4 minutes for every degree of longitude that passes the sun. The hour (60 minutes) is thus 1/24 of a day.
3. When you observe through a moving train, trees, houses and fields on the other side of the track appear to move in the direction opposite to that of the speeding train. The apparent movement of the sun and the other heavenly bodies in relation to the rotating earth is similar. As the earth rotates from west to east, the sun, moon, planets and stars appear to rise in the east and set in the west.
4. Rotation causes the working of the Coriolis force which results in the deflection of the winds and the ocean currents from their normal path.
5. Tide is caused by the rotation of the earth apart from the gravitational pull of the sun and the moon.
Rotation causes a flattening of Earth at the two poles and bulging at the Equator. Hence, there is a difference in diameter at the poles and equator.
Circle of Illumination: The line around the earth separating the light and dark is known as the circle of illumination (Figure 2.11).
It passes through the poles and allows the entire earth to have an equal amount of time during the daylight and night time hours. This line can be seen from space, and the exact location of the line is dependent on the various seasons.
The movement of the earth in its orbit around the sun in an anti-clockwise direction, that is, from west to east is called revolution of the earth. The earth revolves in an orbit at an average distance of 150 million km.
The distance of the earth from sun varies time to time due to the elliptical shape of the orbit. About January 3rd the earth is closest to the sun and it is said to be at Perihelion (‘peri’ means close to and Helios means sun). At Perihelion, the distance is 147 million km.
Around July 4th the earth is farthest from the sun and it is said to be at Aphelion (Ap means away and Helios means sun). At Aphelion the distance of the earth is 152 million km away from the sun.
The period taken by the earth to complete one revolution around the sun is 365 days and 6 hours (5 hours, 48 minutes and 45 seconds) or 365¼ days. The speed of the revolution is 1,07,000 km per hour. The speed is 30 km per second. The bullet from a gun travels with a speed of 9 km per second.
The period of time the earth takes to make one revolution around the sun determines the length of one year. The earth takes 365 days and 6 hours to complete one revolution. Earth takes 365.25 days to complete one trip around the Sun .That extra quarter of a day presents a challenge to our calendar system, which has one year as 365 days. To keep our yearly calendars consistent with our orbit around the Sun once in, every four years we add one day.
The extra day added to is called a leap day, and the year the extra day is added to is called a leap year. The extra day is added to the month of February which has 29 days in a leap year.
How many birth days a person, whose life span supposed to be 60 years, would have seen in his/ her life time, if born on 29th February?
The revolution of the earth around the sun results in the following
· Cycle of seasons,
· Variation in length of days and nights,
· Variation in distribution of solar energy over the earth and the temperature zones.
Let us know!
How to calculate leap year? Take any year and divide by 4 or 100 or 400. If it is divisible (whole number with no reminder), it is a leap year.
Students’ activity: calculate and identify the leap years from the following years 1992, 1995, 2000, 2005, 2008, 2010, 2012, 2014, 2017, 2020, 2024, 2030, 2035, 2040 and 2044.