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It is 12 o’clock. All the watches of that place should be set according to that time.

Local time : 

The local time of a place is the time of its own meridian. The local time is calculated by the position of the Sun at noon at a given place. When the Sun at any place is highest in the sky. It is noon.

Longitude of Madras = 80° E 

Longitude of New York = 74° W 

Difference in Degrees = 80° + 74° = 154° 

Difference in time per degree = 4 minutes 

Difference between the times of two places = 40 x 154 = 616 = 10 hour 16 minutes 

Since Madras lies East of New York, we shall add 10 hours 16 minutes to the local time of New York. The local time at Madras will therefore, be 7.16 A.M. next morning.

Use of Local time is very inconvenient now-a-days in practical life, because if we have to travel through many countries, it will be convenient to us to follow one particular standard time, according to which we can set our watches with respect to a particular country and its standard time with reference to Greenwich or Prime Meridian (0° Longitude) to bring about international uniformity. This is known as Greenwich Mean Time (GMT). The Indian Standard time is 5 hrs 30 minutes ahead of GMT.

The latitudes are the parallel circles with respect to the equator reducing in length northwards and southwards and the poles are the points only. On the other hand longitudes are equal in length drawn from North Pole to South Pole with their intervals reducing towards poles

As our earth is spherical in shape, so it can be best represented by a spherical model like a globe. A globe illustrates the position of continents and oceans, the tilted axis of the earth, its rotation, the sea and air routes etc. So a globe is a perfect model of the earth.

Longitude of Delhi = 77°E 

Longitude of Tokyo = 139°E 

Difference per degree – 139°- 77° = 62° 

Difference of time per degree = 4 minutes , Difference between the time of Delhi and Tokyo = 4 × 62 = 248 minutes = 4 hours 8 minutes 

Since Delhi lies west of Tokyo we shall subtract 4 hours 8 minutes from Tokyo time. Hence local time at Delhi is 10.52 a.m.

When the Sun reaches the highest point in the sky, while crossing the meridian of any place, it is mid-day or 12.00 noon at that place. If watches are adjusted according to this time, then it will be known as local time. But the highest position of the sun can be found only on a sunny day. Therefore it is correct to say that local time is the Sun-time.

IDL is not a straight line, but it deviates in order to classify some scattered Islands in a particular divisions of Hemispheres, so that the day and time can be calculated according to the line, as the natural position of the Islands is haphazard.

Prime Meridian and Equator.

(a) The lines of latitude are parallel to the equator.

(b) The lines of latitude are drawn at an angular distance with respect to the equator.

As the circumference or the equator is nearly about 40,000 km. So 1° angular distance in km. will be 40,000/360 =111 km. approximately.

Every longitude along with its opposite longitude makes a complete circle around the earth and another complete big circle is the equator; while the latitudes make smaller circles along with the poles to be only points. So the equator and the longitudinal lines around the earth are called the Great Circles.

The longitudinal difference at Singapore and Greenwich is 104°, so the time difference is 104 × 4 = 416 minutes i.e. 6 hours 56 minutes. So the time at Singapore will be 12.56 a.m.

Greenwich Mean Time (GMT) :-  It is based on 0° longitude of a place of this name near London. GMT is adopted uniformly by all countries to keep time. It forms the basis of fixing Time Zones.

The time of any place or country is calculated according to the 0° longitude or Greenwich meridian, the time change is of 4 minutes for every 0° longitude. So it is called Greenwich Mean Time.

According to the Indian Standard Time 7.30 p.m. based on 82 1/2° E. Longitude was the time at Mumbai. The longitudinal difference between Mumbai and Ivory Coast at 20° W is 82.5 + 20 = 102.5. So 102.5 × 4 = 410.0 minutes = 6 hours 50 minutes, i.e. the time will be 6 hours 50 minutes less than 7.30 p.m. So the time will be 12.40 p.m

Time difference is 12 – 7.30 = 4.30 or 4 hours 30 minutes = 240 + 30 = 270 minutes So the longitudinal difference is 270/4 = 67.5°. So the longitude is 67.5°W or 67 1/2° W.

Northern hemisphere and Southern hemisphere.

Equator is the parallel of zero degree latitude. It is an imaginary circle round the Earth bisecting it into halves i.e. Northern  Hemisphere and Southern Hemisphere. It serves as reference line for the location of different places on the earth.

Polar Zone receives slanting rays.

Characteristics : 

1. Every meridian has a different local time. 

2. The places on the same meridian have the same local time. 

3. The Eastern places will be ahead of Western places for local time. 

4. Sundial was a simple and old method to determine local time.

When crossing eastward towards America (from Japan to U.S.A.) a day must be subtracted. When crossing the date line ships gain a day from the calendar and have an ‘eight- day week’. It repeats a day. If ship crosses the date line on Monday, the next day will be again Monday and not Tuesday.

(a) Tropic of Cancer — 23 1/2°N 

(b) Tropic of Capricorn — 23 1/2° S. 

(c) Arctic Circle — 66 1/2° N. 

(d) Antarctic Circle — 66 1/2° S 

(e) North Pole — 90°N 

(f) South Pole — 90°S

The longitudinal difference is 92° – 73° = 19°. So the time difference is 19 x 4 = 76 minutes or 1 hours 16 minutes or the time at Mumbai is 9.14 p.m.

Northern Hemisphere and Southern Hemisphere

Rule : On crossing the date line, the ships adjust the dates according to the following rules : 

When crossing westward, towards Asia (from U.S.A. to Japan), a day must be added. When crossing the date line, ship misses a day from the calendar and have ‘six day week’. If a ship crosses the date line on Monday, the next day will be Wednesday and not Tuesday.

180 + 1 = 181

If you examine a globe or map, you will see two sets of lines drawn across it. One set of lines runs north and south of Equator and Parallel to it. These lines are called Parallels of a Latitude. The total number of such lines, if drawn at a distance of one degree, from one another is 180 : 90 North of the equator and 90 South of the equator.

Because North pole to South pole (a hemisphere) has an angle of 180°.

A ship crossing international date line from West to East gains a day. It repeats a day. A day is subtracted from the calendar. So when it is Wednesday mid-night on Asiatic side, it is Tuesday midnight on the American side.

Latitude is the distance of a place north or south of the equator along a meridian. This distance is measured in degree. When we say that the Latitude of Delhi is 28-j North, we mean that Delhi lies North of the equator and its distance from the equator is 28 1/2.

360 – 1 = 359.

The temperature decreases with latitude.

Longitude is the distance of a place from the Prime Meridian while lines of longitude are lines drawn on a map or globe showing the longitude of all the places at the distance from the Prime Meridian. These lines join the north and south poles and cut the Equator at right angles.

1. Latitude of Equator The latitude of the equator is 0°. Here the days and nights are equal throughout the year and the rays of the sun fall exactly vertically at noon on March 21 and September 23. 

2. Latitude of North Pole The latitude of the North pole is 90° North because it is situated north of the Equator at a distance of a quarter of the circumference of the earth. Here days and nights are of six-months duration.

3. Latitude of South Pole Latitude of the south pole is 90° south, because it is situated South of Equator at a distance of a quarter of the circumference of the earth. Here the days and night are of six months duration. 

4. The Latitude of the Tropic of Cancer is 23 1°/2 North. Here the rays of the sun fall vertically at noon on June 21. 

5. The Latitude of the Tropic of Capricorn is 23 1°/2 South. Here the rays of the sun fall vertically at noon on December 22. 

6. The Latitude of the Arctic Circle is 66 1°/2 North. Here once the year (June 21) the day is of 24 hours duration and once in the year (December 22) the night is of 24 hours duration. 

7. The Latitude of the Antarctic Circle is 66 1°/2 South. Here once in the year (December 22nd) the day is of 24 hours duration and once in year (June 21) the night is of 24 hours duration.

Lines of longitude.

The other set of lines join the north and south poles and cuts the equator at right angles. These lines are called Lines of Longitude or Meridians. The total number of such lines, if drawn 1° apart, is 360° – 180° East and 180° West of the Prime Meridian. It should be noted that 180° East and 180° West in the same lines.