Every point on the surface of the Earth needs to be identified precisely. A location may be described informally using landmarks, but for scientific, navigational, and geographical purposes, we use a systematic coordinate system based on angular measurements.

The Earth is a sphere (actually an oblate spheroid — slightly flattened at the poles and bulging at the equator). To locate any place on it, we use two sets of imaginary lines: Latitudes (Parallels) and Longitudes (Meridians). Together, they form a network called the Graticule or Grid.

Latitudes are imaginary horizontal circles drawn on the surface of the Earth, parallel to each other and to the Equator. They are therefore also called parallels.

• The Equator is the largest circle and is drawn at 0°. It divides the Earth into the Northern and Southern Hemispheres.
• Moving north from the Equator, latitudes are labelled North (N). Moving south, they are labelled South (S).
• The North Pole is at 90°N and the South Pole is at 90°S. At the poles, the latitude is a single point, not a circle.
• The total angular distance from the Equator to either pole is 90°. There are thus 181 parallels in total (0° + 90 in North + 90 in South).
• As we move away from the Equator towards the poles, the circles of latitude become progressively smaller.
• All parallels of latitude run in an East–West direction.
• The distance between two consecutive degrees of latitude is approximately 111 km (about 69 miles).

Apart from the Equator (0°), five parallels are of special astronomical and geographical importance:

Longitudes are imaginary semicircles running from the North Pole to the South Pole. Unlike parallels, they are not parallel to each other — they converge at both poles. They are also called meridians.

• All meridians are of equal length — each is a semicircle of 180°.
• Meridians run in a North–South direction.
• They measure angular distance East or West of the Prime Meridian.
• Longitudes range from 0° to 180° East and 0° to 180° West, giving 360 meridians in total.
• The distance between two consecutive meridians at the Equator is approximately 111 km, but this distance decreases as we move towards the poles (where all meridians meet at a point).
• All meridians meet at both the North and South Poles at right angles to the parallels of latitude.

Since any meridian could be chosen as 0°, there was a need for international agreement. The meridian passing through the Royal Observatory at Greenwich, near London, UK, was chosen as the reference meridian in 1884 at the International Meridian Conference in Washington, D.C. It is called the Prime Meridian or the Greenwich Meridian.

• The Prime Meridian is at 0° longitude.
• It passes through Greenwich, England; France; Spain; Algeria; Mali; Burkina Faso; Ghana; and the Southern Ocean.
• Time all over the world is calculated with reference to this meridian — known as Greenwich Mean Time (GMT) or Universal Time Coordinated (UTC).

Fig. 1.5 — The Ujjayinī prime meridian used in ancient Indian astronomy. Cities marked with a circle (Kurukṣhetra, Rohitaka/Rohtak, Mālavangara/Nagar, Ujjayinī/Ujjain, Māhiṣhmati/Maheshwar, Kumārī/Kanyakumari) are mentioned in astronomical texts as being on this meridian (at 75.8°E). Modern cities are marked with squares. (Source: NCERT)

When latitudes and longitudes are drawn together on a globe, they form a grid-like pattern. This network is called the geographical grid or graticule. Any location on Earth can be specified precisely using these two coordinates.

Fig. 1.3 — This globe shows both parallels of latitude (running East–West, measuring N/S distance) and meridians of longitude (running N–S, measuring E/W distance). The Equator at 0° and the Prime Meridian at 0° are highlighted in red. (Source: NCERT)

Notice from the figure that:

• Parallels of latitude are circles running East–West; they are all horizontal.
• Meridians of longitude are semicircles running North–South; they converge at the poles.
• The Equator is the longest latitude; the Poles are points.
• All meridians are equal in length; no parallel equals the length of the Equator except itself.

The Earth is divided into four hemispheres by the Equator and the Prime Meridian/Antimeridian (180° line):

Fig. 1.4 — This sketch shows how the Prime Meridian (Greenwich Meridian) divides the Earth into the Western and Eastern Hemispheres, while the Equator divides it into the Northern and Southern Hemispheres. (Source: NCERT)

Because the Earth rotates from West to East, places to the East receive sunlight (and thus noon) earlier than places to the West. This creates differences in local time.

• Local Time (Solar Time): The time at a particular place based on the position of the Sun. It is noon (12:00 PM) when the Sun is at its highest point (due south in the Northern Hemisphere).
• Since the Earth covers 360° in 24 hours, it rotates at 15° per hour or 1° every 4 minutes.
• Places to the East have a later sunrise, so their local time is ahead of places to the West.
• For every 1° of longitude moved eastward, local time increases by 4 minutes.

Fig. 1.7 — This graph shows longitudes at the bottom and local time at the top, with reference to the Prime Meridian at 0° (12 PM). Each colour is a zone of 15° centred on a meridian, representing a one-hour time difference. Notice how time decreases westward and increases eastward. (Source: NCERT)

If every town followed its local time, it would create chaos for railways, communication, and commerce. Hence, countries adopt a Standard Time based on one or a few selected meridians.

The world is divided into 24 standard time zones, each roughly 15° wide in longitude. Each time zone is one hour ahead of (or behind) the adjacent one.

• Zones to the East of Greenwich are ahead (+) of GMT.
• Zones to the West of Greenwich are behind (–) of GMT.
• In practice, time zone boundaries follow political boundaries (state/country borders) rather than strict 15° meridians, to avoid splitting countries.
• Large countries like Russia, USA, Canada, Australia, and China span multiple time zones.
• Some countries use half-hour or quarter-hour offsets (e.g., India = +5:30, Nepal = +5:45, Iran = +3:30).

Fig. 1.8 — A world map of time zones, showing the standard times (with respect to GMT) for a few countries. Note that Russia has the most time zones (11), while China uses a single time zone despite its vast east-west extent. The International Date Line is on the far left/right. (Note: international borders are approximate, not exact.) (Source: NCERT)

Fig. 1.9 — Time zones (with respect to GMT) in Africa and Eurasia. India is at +5.5 (IST = GMT+5:30). Nepal is at +5.75 (GMT+5:45). China, despite spanning 5 geographic time zones, uses a single standard time of +8. (Source: NCERT)

India follows a single standard time for the entire country, known as Indian Standard Time (IST).

• IST is based on the meridian at 82°30'E longitude.
• This meridian passes through Mirzapur (near Allahabad/Prayagraj) in Uttar Pradesh.
• IST is 5 hours 30 minutes ahead of GMT (UTC +5:30).
• India extends from 68°7'E to 97°25'E, a difference of about 29°, which translates to nearly 2 hours difference in local time between the easternmost and westernmost points.
• Despite this, India follows a single time zone to avoid administrative and logistical complications.
• The choice of 82°30'E is convenient as it gives an exact 5:30 offset from Greenwich.

The International Date Line (IDL) is an imaginary line that runs roughly along the 180° meridian in the middle of the Pacific Ocean.

• When you cross the IDL going eastward (towards Americas), you subtract one day.
• When you cross the IDL going westward (towards Asia/Australia), you add one day.
• The IDL is not a straight line — it zigzags to avoid passing through islands, countries, and territories, preventing them from having two different dates simultaneously.
• It deviates around the Aleutian Islands, Fiji, Tonga, and other Pacific island groups.
• The IDL is on the opposite side of the globe from the Prime Meridian (0°).
• If you fly westward around the world, you gain time — and crossing the IDL means you must move the calendar back one day to compensate. This is the scenario in Jules Verne's novel Around the World in Eighty Days.