Latitudes and Longitudes are imaginary lines used to determine the location of a place on earth. Since the shape of the earth is ‘Geoid’, the location of a place on the earth can be mentioned in terms of latitudes and longitudes. Example: The location of Agra is 27° 10′ 3.792” N78° 2′ 9.204” E or New Delhi is 28° N, 77° E.
Latitudes are the angular distance of a point on the earth’s surface, measured in degrees from the centre of the earth. As the earth is slightly flattened at the poles, the linear distance of a degree of latitude at the pole is a little longer than that at the equator.
For example at the equator (0°) it is 68.704 miles, at 45° it is 69.054 miles and at the poles, it is 69.407 miles. The average is taken as 69 miles (111km). (1 mile = 1.607 km)
Besides the equator (0°), the north pole (90°N) and the south pole (90° S), there are four important parallels of latitudes–
1) Tropic of Cancer (23½° N) in the northern hemisphere.
2) Tropic of Capricorn (23½° S) in the southern hemisphere.
3) Arctic circle at 66½° north of the equator.
4) Antarctic circle at 66½° south of the equator.
• The mid-day sun is exactly overhead at least once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. This area, therefore, receives the maximum heat and is called the torrid zone.
• The mid-day sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn. The angle of the sun’s rays goes on decreasing towards the poles. As such, the areas bounded by the Tropic of Cancer and the Arctic circle in the northern hemisphere, and the Tropic of Capricorn and the Antarctic circle in the southern hemisphere, have moderate temperatures. These are, therefore, called temperate zones.
• Areas lying between the Arctic circle and the north pole in the northern hemisphere and the Antarctic circle and the south pole in the southern hemisphere, are very cold. It is because here the sun does not raise much above the horizon. Therefore, its rays are always slanting. These are, therefore, called frigid zones.
What is Geodesy?
“Geodesy is the science of accurately measuring and understanding three fundamental properties of the Earth: its geometric shape, its orientation in space, and its gravity field— as well as the changes of these properties with time.”-oceanservice.noaa.gov
• Longitudes are the angular distance, measured in degrees along the equator east or west of the Prime (or First) Meridian.
• On the globe longitude is shown as a series of semi-circles that run from pole to pole passing through the equator. Such lines are also called meridians.
Unlike the equator which is centrally placed between the poles, any meridian could have been taken to begin the numbering of longitude. It was finally decided in 1884, by international agreement, to choose as the zero meridian the one which passes through the Royal Astronomical Observatory at Greenwich, near London.
• This is the Prime Meridian (0°) from which all other meridians radiate eastwards and westwards up to 180°.
• As the parallels of latitude become shorter poleward, so the meridians of longitude, which converge at the poles, enclose a narrower space.
• They have one very important function, they determine local time in relation to G.M.T. or Greenwich Mean Time, which is sometimes referred to as World Time.
Longitudes and Time
• Since the earth makes one complete revolution of 360° in one day or 24 hours, it passes through 15° in one hour or 1° in 4 minutes.
• The earth rotates from west to east, so every 15° we go eastwards, local time is advanced by 1 hour. Conversely, if we go westwards, local time is reduced by 1 hour.
• We may thus conclude that places east of Greenwich see the sun earlier and gain time, whereas places west of Greenwich see the sun later and lose time.
• If we know G.M.T., to find local time, we merely have to add or subtract the difference in the number of hours from the given longitude.
Motions of the earth
Latitudes and longitudes along with the motion of the earth help us understand different phenomena. Primarily earth moves in two ways Rotation and Revolution.
• It takes approximately 24 hrs to complete one rotation.
• Earth rotates along its axis from west to east.
• Days and nights occur due to the rotation of the earth.
• The circle that divides the day from night on the globe is called the circle of illumination.
• Earth rotates on a tilted axis. Earth’s rotational axis makes an angle of 23.5° with the normal i.e. it makes an angle of 66.5° with the orbital plane. The orbital plane is the plane of earth’s orbit around the Sun.
• The Revolution of the earth is responsible for the seasons we observe.
• Earth moves around the sun in 365 days, 6 hours, and 9 minutes. All the usual years are of 365 days and the remaining time is added to a leap year. This is the reason every fourth year, known as the leap year is of 366 days as the additional day is added to the month of February which has 28 days in a usual year and 29 days in a leap year.
• On 21st June, the northern hemisphere is tilted towards the sun. The rays of the sun fall directly on the Tropic of Cancer. As a result, these areas receive more heat.
• The areas near the poles receive less heat as the rays of the sun are slanting.
• The north pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months.
• Since a large portion of the northern hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21st June.
• At this time in the southern hemisphere all these conditions are reversed. It is winter season there. The nights are longer than the days. This position of the earth is called the summer solstice.
• On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the south pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23ó° s), a larger portion of the southern hemisphere gets light. Therefore, it is summer in the southern hemisphere
with longer days and shorter nights. The reverse happens in the northern hemisphere. This position of the earth is called the winter solstice.
• On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox. On 23rd September, it is the autumn season [season after summer and before the beginning of winter] in the northern hemisphere and the spring season [season after winter and before the beginning of summer] in the southern hemisphere. The opposite is the case on 21st March when it is spring in the northern hemisphere and autumn in the southern hemisphere.
• Thus, you find that there are days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.