Latitudes, Longitudes, Equator and the International Date Line
The grid of latitudes and longitudes is the coordinate system that helps us locate any place on the Earth and calculate its climate and time. Examinations ask this topic through the values of important parallels, the meaning of the Equator and the Prime Meridian, the difference between latitudes and longitudes, the time relation of 1 degree of longitude, and the working of the International Date Line. The facts are numbers and definitions, so a small table of latitudes and a clear time rule make this chapter very scoring.
Latitude and the important parallels
Latitude is the angular distance of a place north or south of the Equator, measured in degrees from 0 degrees at the Equator to 90 degrees at the poles. Lines of latitude are called parallels because they run parallel to each other and never meet. The important parallels are: the Equator at 0 degrees, the Tropic of Cancer at 23.5 degrees North, the Tropic of Capricorn at 23.5 degrees South, the Arctic Circle at 66.5 degrees North, the Antarctic Circle at 66.5 degrees South, and the poles at 90 degrees. Each degree of latitude covers about 111 kilometres on the ground. Latitude is the main factor that decides the climate and temperature of a place, because places near the Equator receive more direct sunlight than places near the poles.
Longitude and the Prime Meridian
Longitude is the angular distance of a place east or west of the Prime Meridian, measured from 0 degrees to 180 degrees. The lines of longitude are called meridians. The Prime Meridian passes through the Royal Observatory at Greenwich in London and is taken as 0 degrees. Unlike parallels, meridians are not parallel; they are farthest apart at the Equator and meet at the poles. The distance of one degree of longitude is about 111 kilometres at the Equator but becomes zero at the poles. Longitude is the main factor that decides the time of a place, because the Earth rotates 360 degrees in 24 hours, that is 15 degrees in one hour, so 1 degree of longitude equals 4 minutes of time.
Equator and Great Circle
The Equator is the only parallel that is a great circle, meaning it divides the Earth into two equal halves, the Northern Hemisphere and the Southern Hemisphere. A great circle is the largest possible circle that can be drawn on a sphere, and every meridian also forms half of a great circle with its opposite meridian. The Equator is the longest parallel, about 40,075 kilometres long. It passes through several countries including Ecuador, Brazil, Kenya, Uganda and Indonesia. The Equator receives direct sunlight throughout the year and therefore has a hot climate with little seasonal change.
International Date Line
The International Date Line (IDL) roughly follows the 180 degree meridian in the Pacific Ocean, but it zigzags to avoid cutting through land so that islands and countries keep one single date. The date changes when a traveller crosses the IDL: crossing eastward (for example from Asia towards America) the traveller loses one day and repeats the date, while crossing westward (from America towards Asia) the traveller gains one day and the date jumps forward. This is why the two sides of the IDL have dates that differ by one day even though the clock time may be similar.
India and its standard meridian
India lies between 8 degrees 4 minutes North and 37 degrees 6 minutes North latitude and between 68 degrees 7 minutes East and 97 degrees 25 minutes East longitude. The longitudinal extent of about 29 degrees means the sun rises almost two hours earlier in the east than in the west. To have one common time, India uses the Standard Meridian of 82.5 degrees East, which passes near Mirzapur in Uttar Pradesh, and Indian Standard Time is UTC plus 5 hours 30 minutes. The Tropic of Cancer passes through the middle of India.
Quick revision
Remember that latitude runs from 0 degrees at the Equator to 90 degrees at the poles and controls climate, while longitude runs from 0 degrees at Greenwich to 180 degrees and controls time. The key parallels are 0, 23.5 (both tropics), 66.5 (both circles) and the poles at 90. One degree of longitude equals 4 minutes and 15 degrees equals one hour. The Equator is the only parallel that is a great circle. The IDL follows 180 degrees and zigzags; crossing it eastward means losing a day and crossing westward means gaining a day. India's standard meridian is 82.5 degrees East and its time is UTC plus 5 hours 30 minutes.
How to prepare this topic
- Memorise the important parallels: Equator 0, Tropic of Cancer 23.5 N, Tropic of Capricorn 23.5 S, Arctic 66.5 N, Antarctic 66.5 S.
- Learn the golden time rule: 15 degrees of longitude equals 1 hour and 1 degree equals 4 minutes.
- Draw a globe and mark the Equator, the Prime Meridian and the International Date Line to fix their positions.
- Remember that latitude decides climate and longitude decides time, and that only the Equator is a great-circle parallel.
- Keep India's extent and the standard meridian 82.5 degrees East with IST as UTC plus 5 hours 30 minutes ready.
Common exam traps
- Latitude controls climate and longitude controls time; do not interchange their roles.
- One degree of longitude equals 4 minutes of time, so 15 degrees make one hour.
- The Equator is the only parallel that is a great circle; no other parallel is a great circle.
- Meridians are not parallel; they meet at the poles, while parallels never meet.
- Crossing the IDL eastward loses a day and crossing westward gains a day, so the two sides differ by one date.
Frequently Asked Questions
What are the important parallels of latitude and their values?
The important parallels are the Equator at 0 degrees, the Tropic of Cancer at 23.5 degrees North, the Tropic of Capricorn at 23.5 degrees South, the Arctic Circle at 66.5 degrees North, the Antarctic Circle at 66.5 degrees South, and the North and South Poles at 90 degrees. Each degree of latitude covers about 111 kilometres.
What is the difference between latitude and longitude?
Latitude is the angular distance north or south of the Equator, measured from 0 to 90 degrees, and its lines are called parallels; it mainly decides the climate. Longitude is the angular distance east or west of the Prime Meridian, measured from 0 to 180 degrees, and its lines are called meridians; it mainly decides the time. Latitudes are parallel but longitudes meet at the poles.
Why does one degree of longitude equal four minutes of time?
The Earth rotates 360 degrees in 24 hours, which means it turns 15 degrees in one hour and 1 degree in 4 minutes. Because of this, places 1 degree apart in longitude have a time difference of 4 minutes, and 15 degrees apart have a difference of one hour. This is the basic rule for all standard time and time-zone calculations.
What is the International Date Line and how does it work?
The International Date Line roughly follows the 180 degree meridian in the Pacific Ocean and zigzags to avoid cutting through land. Crossing it eastward, for example from Asia towards America, a traveller loses one day and repeats the date. Crossing it westward, from America towards Asia, the traveller gains one day. Thus the two sides of the line differ by one calendar day.
Which meridian does India use as its standard meridian?
India uses 82.5 degrees East, which passes near Mirzapur in Uttar Pradesh, as its standard meridian. Indian Standard Time is therefore UTC plus 5 hours 30 minutes. India lies between 8 degrees 4 minutes North and 37 degrees 6 minutes North latitude and between 68 degrees 7 minutes East and 97 degrees 25 minutes East longitude, with the Tropic of Cancer passing through its middle.