📖 PART 1: Latitudes (Parallels)

• Latitudes range from 0° (Equator) to 90°N (North Pole) and 90°S (South Pole).
• Total number of latitude lines: 181 (including the Equator).
• All parallels of latitude are circles (decrease in circumference from Equator to poles). All parallels of latitude run east-west.
• Distance between any two adjacent parallels (1° apart) = approximately 111 km.

Important Latitudes and Their Names

Latitude	Name	Significance
0°	Equator	Divides Earth into N and S hemispheres; longest parallel; 12 hrs day/night all year
23½°N	Tropic of Cancer	Sun is directly overhead on June 21 (Summer Solstice) — northernmost position of vertical Sun
23½°S	Tropic of Capricorn	Sun is directly overhead on December 22 (Winter Solstice) — southernmost position of vertical Sun
66½°N	Arctic Circle	24-hour daylight on June 21; 24-hour darkness on Dec 22
66½°S	Antarctic Circle	24-hour daylight on Dec 22; 24-hour darkness on June 21
90°N	North Pole	6 months day, 6 months night; all longitudes meet
90°S	South Pole	6 months day, 6 months night; all longitudes meet

Uses of Latitudes

• Used to locate the north-south position of a place on Earth.
• Determine climate zones: Torrid Zone (0°–23.5°), Temperate Zone (23.5°–66.5°), Frigid Zone (66.5°–90°).
• Help in determining duration of day and night.

📖 PART 2: Longitudes (Meridians)

• Total number of meridians: 360 (one every 1°). Each degree can be divided into 60 minutes and each minute into 60 seconds.
• All meridians meet at the North and South Poles.
• The distance between two adjacent meridians decreases from the Equator (111 km apart) to zero at the poles.

Important Longitudes

Longitude	Name	Significance
0°	Prime Meridian (Greenwich Meridian)	Passes through the Royal Observatory, Greenwich, London. Reference meridian for time zones.
180°	International Date Line (IDL)	Opposite of Prime Meridian; passes mainly through Pacific Ocean; when crossed, date changes
82°30'E	Standard Meridian of India (IST)	India's standard time = GMT + 5:30 hours; passes through Mirzapur (UP)

Uses of Longitudes

• Determine the east-west position of a place.
• Determining local time and time zones. Every 15° of longitude = 1 hour difference in time (Earth rotates 360° in 24 hours → 15° per hour).

⏰ PART 3: Time – Local, Standard, and Time Zones

Local Time

• Local Time is time based on the position of the Sun at a specific meridian — it is 12 noon (Local Solar Time) when the Sun is at its highest point (overhead or due south).
• Local time changes by 4 minutes for every 1° of longitude (as Earth rotates 1° every 4 minutes).
• Places to the east have a later (ahead) time than places to the west.

Standard Time

• A country or region adopts a single time for the whole area, based on one standard meridian, to avoid confusion from different local times across the country.
• India's standard time (IST) is based on the 82°30'E meridian (Mirzapur, UP) and is 5 hours 30 minutes ahead of GMT (UTC+5:30).

Greenwich Mean Time (GMT)

• The time at the Prime Meridian (0° longitude) — used as the world reference time. Also called Universal Time Coordinated (UTC).
• All countries calculate their standard time as GMT + or − number of hours corresponding to their standard meridian.

🗓️ PART 4: International Date Line (IDL)

• The International Date Line (IDL) lies broadly along the 180° meridian in the Pacific Ocean.
• When you cross the IDL going eastward (Pacific to Americas): go back one day (subtract 1 day).
• When you cross the IDL going westward (Americas to Pacific/Asia): go forward one day (add 1 day).
• The IDL is not a straight line — it bends around island groups to avoid cutting a single country or island group into two different dates. Deviates around Aleutian Islands (Alaska), Fiji, Samoa.
• Why it is needed: Without the IDL, a person travelling west continuously would find that their calendar is one day behind after circling the globe — IDL corrects for this.

🌐 PART 5: Eastern and Western Hemispheres

• Eastern Hemisphere: All land/area east of Prime Meridian (0°) up to 180° — includes Europe, Asia, Africa, Australia.
• Western Hemisphere: All land/area west of Prime Meridian (0°) up to 180° — includes the Americas.
• Great Circles: Any circle on the Earth's surface whose plane passes through the centre of the Earth. The Equator is the only parallel that is a Great Circle. All meridian pairs (a meridian + its opposite meridian) form Great Circles. Great Circle routes are the shortest distances between two points on Earth — used by ships and aircraft.