To locate points on earth, we use a set of imaginary lines called latitudes and longitudes. They are drawn with reference to north and south poles. The midpoints between the North Pole and the South Pole are joined to draw a line which divides the earth into two equal hemispheres. This line is called equator or zero degrees latitude. Lines are drawn parallel to the equator and are called Parallels or Latitudes.
The latitudes on the surface of the Earth give the distance north and south of the equator. It is measured in degrees. Every degree is subdivided into 60 minutes and each minute is divided into 60 seconds. It is represented as X0 Y’ Z” meaning X degrees, Y minutes and Z seconds. Everything north or south of the equator is designated either as north latitude or south latitude. The Antarctic Circle (66° 33′ South of Equator), Tropic of Capricorn (23° 26′ South of Equator), Tropic of Cancer (23° 26′ North of Equator), and Arctic Circle (66° 33′ North of Equator), are all circles of latitude.
Longitude or meridian is another set of imaginary lines drawn on the surface of earth connecting North and South Pole and hence it will be perpendicular to latitudes and equator. As all longitudes are equal and there is no distinguishing or natural starting point, we take the meridian passing through the Royal Observatory, Greenwich in London as the prime meridian or zero degrees meridian. The Prime Meridian is 0° (zero degrees), and the farthest away is +180° eastward and -180° westward.
As day and night are caused due to rotation of the earth on its axis when the sun rises at a meridian it will be sunset at the opposite meridian. So the time is measured with the help of sunrise and sunset. We know that a day is divided into 24 hours and a sphere is 3600 each hour is represented by 15 degrees. As the earth rotates from west to east places east of Greenwich will be ahead of Greenwich and those to the west will be behind.
The earth rotates 360 degrees in about 24hrs which mean 15 degrees an hour or 1 second in 4 minutes. Thus, when it is 12 noon at Greenwich, the time at 15 degrees east of Greenwich will be 15×4=60 minutes, i.e. 1 hour ahead of Greenwich Time which means 1 pm. But at 15 degrees west of Greenwich, the time will be behind Greenwich Time by 1 hour i.e. it will be 11 am. Similarly, at 180 degrees it will be midnight when it is 12 noon at Greenwich.
As the time changes depending on longitude, it will be difficult to calculate the time at each point if we are moving east to west or west to east. So we have accepted a convention by which we consider a meridian as standard time meridian for that country and the time of the Standard Time Meridian as the time for that country. In some countries with a large east-west extent, there may be multiple time zones specific to different regions. As per International conventions, each country can choose its time zones which are in multiples of 30 minutes variation from Greenwich Mean Time (multiples of 7 ½ degrees in longitude).
Sun is the source of energy for the earth in form of heat and light and we have experienced that the sun’s heat is more intense during midday when the sun is directly overhead and it is less when the sun is at the horizons. In general, the sun’s light has to pass through a lot more atmosphere (or a greater thickness of air) in the morning and evening to get to a vertical surface than it does when it is at zenith to a horizontal surface.