It can be defined as the angular distance measured at the centre of the Earth with espect to any point on the meridian. These are depicted in degrees, minutes and seconds. Equator is called the 0° latitude. These are imaginary circles, drawn on the surface of the earth, at an interrval of 1° on both the hemispheres making the total number of latitudes to 181. 23 l/2 ° latitude on the Northern hemisphere is called the Tropic of Cancer (23 l/2°N), whereas in the southern hemisphere the same latitude is called the Tropic of Capricorn (23 l/2 ° S). 66 l/2° latitude in the northern hemisphere is called the Sub-Arctic Circle (66 l/2° N) whereas 66 l/2° latitude in the southern hemisphere is called Sub-Antarctic Circle (66 l/2 ° S). The latitudinal distance of 1° is about 11 km.
Angular distance of any place from the Prime Meridian, is called longitude. It is also measured in degrees, minutes and seconds. The Prime Meridian is at 0° and is known as the Greenwich Line as it passes through Greenwich, in London. Part of the Earth on the eastern side of the Prime Meridian is called Eastern hemisphere and that on the western side of it is called Western hemisphere.
Since, the Earth is Geoid shape so it rotates by 360° in 24 hours. Therefore, the earth takes 4 minutes to move by 1° of longitude. Since the Sun rises in the east and the Earth moves from west to east, time on the eastern hemisphere is ahead of the Greenwich and that on the western hemisphere is behind it. This is the reason behind differences in local times at different places on the Earth. For every 15° of longitudes there is a difference of one hour in time. In this way The time on the 180° E longitude is 12 hours ahead of the Greenwich and that on the 0°-180°W longitude is 12 hours behind the time at Greenwich. Hence, there is a difference of 24 hours in the time in east and west of the 180° longitude.
It is an imaginary line drawn at the 180° longitude, avoiding the continuous land parts. It is bent at 75° N latitude towards east to avoid division of Siberia and to separate Siberia and Alaska. There is a difference of 24 hours or one day in east and west of it. Therefore, one day is either gained or lost while crossing this line. While crossing it from east to west one day is gained and while going from west to east one day is lost.
Local Time: The time at a particular place on the Earth, as calculated by the position of the Sun, is termed as the Local Time of that place. In India there is a difference of 2 hours in the local time of the eastern most (Arunachal Prades) and the western most (Dwarka in Gujarat) parts.
Standard Time: This is the imaginary line passing through mid of the standard Meridian in the middle of the country. Standard Time of the country is followed all over the country to avoid the inconvenience caused by the difference in local times of different place in the country. For example, 82M>° E longitude, passing through Naini, near Allahabad (Uttar Pradesh), is the Standard Meridian of India. The time at this longitude is the Standard time of India, called Indian Standard Time (1ST).
Some helpful facts to calculate the Standard Time of a Place: