Graticule

Graticule is the network of parallels and meridians drawn on a globe. This network helps us in locating dierent places with reference to given latitudes and longitudes. Supposing (Figure 2.5), we say that the latitude of a place A is 45° N and its longitude is 45° W. Then its geometrical position will be at the intersection of 45° north parallel and 45° W meridian. Similarly, point B is 30° S latitude and 30° E longitude.

Some Important Parallels

There are some parallels which are more important than the others. The equator divides the globe into two equal sized-northern and southern hemispheres. This is the longest of all the parallels and is known as the latitude of 0°. This is the basic latitude and all other latitudes are expressed with reference to equator. We know that the earth’s axis is inclined at an angle of 23.5° to its orbital plane. Therefore, 23.5° is the maximum latitude up to which sun’s rays can be perpendicular at any place. Thus 23.5° north and south latitudes have special signi cance. Thparallel of 23.5° north latitude is known as the Tropic of Cancer and that of 23.5° south latitude is called the Tropic of Capricorn.

All places between the Tropic of Cancer and the Tropic of Capricorn experience vertical rays of the sun twice a year. However, it should be remembered that both the tropics experience vertical rays of the sun only once in a year the Tropic of Cancer at the time of summer solstice when the northern hemisphere has maximum inclination towards the sun and the Tropic of Capricorn when the southern hemisphere has maximum inclination towards the sun. The Arctic and the Antarctic circles are at 66.5° north and south latitudes respectively. ese circles enclose the polar zones where there is continuous day or night for more than 24 hours.

Great Circles and Small Circles

A great circle is an imaginary circle on the earth’s surface which divides the earth into two equal parts and its centre lies at the centre of the earth. A great circle is the largest circle that can be drawn on the globe. Equator is a great circle because it divides the globe into two hemispheres known as northern and southern hemispheres. All other parallels are small circles because they are smaller than the equator and divide the globe into two unequal parts. On the other hand, all the meridians are great circles because they divide the globe into two equal parts and their centre lies at the centre of the earth. It is possible to draw an inte number of great circles on the sphere. But it must be remembered that only one great circle can pass through two given points on the globe unless these two points are at the extremities of the same diameter, in which case any number of great circles can be drawn through them. An arc of a great circle represents the shortest distance between two points located on this circle. us great circles are very important for navigation and aviation because a lot of fuel and time is saved by following these circles.

Motions of the Earth and their Effects

The Earth has two motions, namely rotation around its own axis and revolution around the sun.

Rotation

The earth rotates around its axis from west to east, i.e., in anticlockwise direction and completes one rotation in about 24 hours (precisely in 23 hours 56 minutes and 4 seconds). The axis of the earth is an imaginary line passing through the centre of the earth and joining north and south poles. It is tilted at an angle of about 66½° to the plane of the elliptic, i.e., at an angle of about 23½° from the line perpendicular to that plane. The velocity of rotation decreases from equator to poles because the length of parallels also decreases as we move from the equator toward the poles. It is 1700 km per hour at the equator, 850 km per hour at 60° N and S parallels and zero at the poles.

The earth’s rotation has a large number of effects on our everyday life. The main effects are:

  • formation of day and night
  • concept of time
  • dection of winds and ocean currents
  • spheroidal shape of the earth
  • knowledge of directions
  • regular intervals of tides
  • weight of a body is less at the equator than at the poles due to greater centrifugal force at the equator caused by the Earth’s rotation.

Revolution

The movement of the earth around the sun is called its revolution. The earth completes one revolution around the sun in 365 days 5 hours 48 minutes and 45.68 seconds or approximately in 365¼ days (to be exact 365.2422 days). The time taken by the earth to complete one revolution around the sun is known as one year.

Generally a calendar year is of 365 days. The remainder ¼th of the day keeps on accumulating and makes one full day in four years. This is compensated by adding one day aer 4 years, to the month of February making it a month of 29 days in place of 28 days. This is called a leap year. us, a leap year has 366 days in place of 365 and is divisible by the four. Thus 2008 and 2012 were leap years. However, it is worth noting that every century year is not a leap year even if it is divisible by four. It would be leap year only if it is divisible by 400. That is why, the year 2000 was a leap year whereas the years 1800 and 1900 were not leap years. Again 2400 will be a leap year whereas 2100, 2200 and 2300 will not be leap years.

The path along which the earth moves around the sun in the counterclockwise direction is called the earth’s orbit. The shape of the earth’s orbit is elliptical and the sun occupies one focus of the ellipse. This leads to variations in the distance of the earth from the sun. The mean distance of the earth from the sun is about 149.7 million km which increases or decreases by over two million km. The distance between the earth and the sun is least i.e.147.3 million km on January 3. In this position, the earth is said to be in perihelion. After this, the earth moves away from the sun and on July 4, it is at the farthest point, being 152 million km from the sun. At this time, the earth is said to be in aphelion. The average orbital velocity of the earth is 29.8 km per second (or 107,000 km per hour). At this velocity, the earth traverses a distance of about 943 million km along its orbit to complete one revolution around the sun in one year.

The plane in which the earth’s orbit lies is known as the plane of the ecliptic. e plane of the earth’s equator is inclined as an angle of 23½° (more exactly 23°27’) with the plane of the ecliptic. The earth’s axis makes an angle of 66½° with the plane of the ecliptic, and is tilted at 23½° from a line perpendicular to that plane. Thoughout the revolution of the earth around the sun, its axis remains tilted in the same direction and continues to point to the same spot in the heavens called Polaris or polestar. This is known as parallelism of the axis.

Revolution of the earth has the following effects:

  • Change of seasons
  • Variation in the lengths of day and night at dierent latitudes and dierent times of the year
  • Differences in altitudes of sun at noon at dierent times of the year

Change of Seasons

Seasons come and go, one after the other, in a rhythmic fashion, with almost a clockwise precision. Following four factors are responsible for change of seasons:

  • Revolution of the earth around the sun on its orbit.
  • Inclination of the earth’s axis at 66½° to the plane of earth’s orbit.
  • Constant orientation of the earth’s axis towards Polaris (Pole star).
  • Rotation of the earth.

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