Why this matters now
The geographic grid, important parallels, and the longitude-time relationship (time zones, IDL, IST) are classic prelims fundamentals that also support map-based questions.
Latitudes and longitudes
Latitudes (parallels) are angular distances north/south of the Equator (0°), running east-west (0°–90°). Longitudes (meridians) are angular distances east/west of the Prime Meridian (0°, Greenwich), running north-south (0°–180°). Together they form the geographic grid.
Important parallels
Key parallels: the Equator (0°), the Tropic of Cancer (23.5°N) and Tropic of Capricorn (23.5°S), and the Arctic (66.5°N) and Antarctic (66.5°S) Circles. The Tropic of Cancer passes through 8 Indian states — a favourite prelims fact.
Longitude and time
The Earth rotates 360° in 24 hours, so 15° of longitude = 1 hour (1° = 4 minutes); places east are ahead in time. GMT/UTC is reckoned from the Prime Meridian. The International Date Line (~180°) marks the date change. Indian Standard Time (IST) is based on 82.5°E (passing near Mirzapur), making India UTC +5:30.
UPSC angle
Know parallels vs meridians, key lines (Equator, Tropics, Polar Circles; Tropic of Cancer through 8 Indian states), the 15°=1hr time rule, the IDL (180°), and IST (82.5°E, UTC+5:30).
Frequently asked questions
What is the difference between latitude and longitude?
Latitudes are parallels measured north/south of the Equator; longitudes are meridians measured east/west of the Prime Meridian.
How is longitude related to time?
The Earth turns 15° of longitude per hour (1° = 4 minutes), so local time changes with longitude — places to the east are ahead.
What is the International Date Line?
An imaginary line roughly along 180° longitude where the calendar date changes by a day.
On which meridian is Indian Standard Time based?
82.5°E (near Mirzapur), making IST UTC +5:30.