CDS Geography · Earth in Space, Maps and Coordinates
Latitude, Longitude, Time and Maps
The Earth's shape and spin, what latitude tells you about a place, how to turn a difference in longitude into a difference in time, and the isolines and diagrams of map work.
Why this matters
Nine CDS questions from 2017 to 2025: 1 EASY, 6 MODERATE, 2 HARD. Four came in 2023 alone. Only one is a calculation (the local time at 90° E), but the 4-minutes-a-degree rule behind it also settles the questions on Indian Standard Time.
Concept 1 of 4: The Earth's shape and rotation
The Earth is almost a ball, a little flattened at the poles and fatter at the equator. It spins once a day from west to east, as one solid body. So every place turns through the same angle in an hour, but a place on the equator has a much bigger circle to go round than a place near a pole, so it moves much faster.
Definition
The facts to hold:
- Shape: an oblate spheroid (also called a geoid), flattened at the poles. The equatorial diameter (about 12,756 km) is a little longer than the polar one (about 12,714 km).
- Eratosthenes (Greek, about 240 BC) measured the Earth's circumference from the Sun's angle at Syene (Aswan) and Alexandria. Today's figure is about 40,000 km.
- Rotation: from west to east, once in about 24 hours, so the Sun rises in the east.
- Angular velocity (degrees turned per hour, 15°) is the same everywhere.
- Linear velocity (distance travelled per hour) is greatest at the equator, about 1,670 km an hour, and zero at the poles.
- Revolution round the Sun takes about 365¼ days; the axis is tilted 23½° from the upright.
| Fact | Value |
|---|---|
| Shape | Oblate spheroid, flattened at the poles |
| Circumference at the equator | About 40,000 km |
| Eratosthenes' two cities | Syene (Aswan) and AlexandriaQ CDS 2024 (II): Alexandria and Syene, not Troy or Thebes. |
| Direction of rotation | West to east |
| Angular velocity | 15° an hour, the same at every placeQ CDS 2024 (I): angular velocity is the same everywhere; linear velocity is greatest at the equator, least at the poles. |
| Linear velocity | About 1,670 km an hour at the equator; zero at the poles |
| Tilt of the axis | 23½° from the upright |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (I) 2024 — General Knowledge · Q90Moderate
Example 1 · Earth in Space, Maps and Coordinates · Latitude, Longitude, Time and Maps
Which of the following statements is/are correct?
1. Angular velocity for all locations on the Earth's surface is the same while linear velocity varies.
2. Linear velocity is maximum at the equator and minimum at the poles.
Select the correct answer using the code given below.
Angular speed is the same, linear speed is not
Every place turns through 15° an hour. The distance covered in that hour is greatest at the equator and falls to zero at the poles. A statement that mixes the two up is false.
Concept 2 of 4: Latitude and what it tells you
Latitude says how far north or south of the equator a place is. That one number decides how high the Sun climbs, so it shapes a place's temperature, rainfall and plants, and how long its days are in summer and winter. To find which of two places is nearer the equator, just compare their latitudes.
Definition
The facts to hold:
- Latitude runs from 0° at the equator to 90° at each pole. The Tropic of Cancer is at 23½° N, the Arctic Circle at 66½° N.
- Drawn every 1°, there are 89 parallels in each hemisphere; with the equator that makes 179. The poles are points, not lines.
- One degree of latitude is about 111 km on the ground. Because the Earth is flattened at the poles, it grows slightly from about 110.6 km at the equator to about 111.7 km near the poles.
- At the equator day and night are about 12 hours each all year. The gap between the longest and shortest day grows with latitude, until at the poles there are six months of day and six of night.
- India: Indira Point (Great Nicobar), the southernmost point, is at about 6°45′ N. The mainland runs from about 8°4′ N to 37°6′ N, and the Tropic of Cancer passes through its middle.
| Line or place | Latitude or fact |
|---|---|
| Equator | 0° |
| Indira Point, Great Nicobar | About 6°45′ N, India's southernmost pointQ CDS 2017 (II): all three statements about latitude were true, including Indira Point at about 6°45′ N. |
| Panaji | About 15½° NQ CDS 2023 (II): Panaji is the closest of these four capitals to the equator. |
| Hyderabad | About 17½° N |
| Mumbai | About 19° N |
| Bhubaneswar | About 20° N |
| Tropic of Cancer | 23½° N |
| Arctic Circle | 66½° N |
| Parallels drawn every 1° | 89 in each hemisphere plus the equator: 179Q CDS 2025 (II): both statements true; 179 parallels, and a degree of latitude changes slightly in length. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2017 · CDS (II) 2017 — General Knowledge · Q98Moderate
Example 2 · Earth in Space, Maps and Coordinates · Latitude, Longitude, Time and Maps
Which of the following statements related to latitude are true?
1. Rainfall, temperature and vegetation vary with latitude.
2. The difference between the longest day and the shortest day increases with latitude.
3. Indira Point is located approximately at 6°45' N latitude.
Select the correct answer using the code given below.
Closest to the equator means lowest latitude
For places north of the equator, the one with the smallest latitude is nearest the equator. Compare the numbers; do not judge by how far south a city feels on the map.
The poles are not parallels
Counting parallels at 1° gives 179, not 181: the two poles are points.
Concept 3 of 4: Local time and longitude
The Earth turns 360° in 24 hours, which is 15° every hour, or 1° every 4 minutes. The Sun reaches places in the east first, so a place east of another is ahead of it in time, and a place to the west is behind. Every time question is this one rule.
Definition
The rule and the standard times:
- 360° in 24 hours, so 15° = 1 hour and 1° = 4 minutes.
- Time difference = difference in longitude × 4 minutes. East is ahead, west is behind.
- If the two places are on opposite sides of Greenwich, add their longitudes to get the difference.
- Indian Standard Time uses the meridian of 82°30′ E, which passes near Mirzapur (Uttar Pradesh). So IST is 5 hours 30 minutes ahead of Greenwich Mean Time.
- India keeps one standard time for the whole country, the Andaman and Nicobar and Lakshadweep islands included.
- The International Date Line runs roughly along 180°. Crossing it eastward (Asia to America) the date goes back one day; westward it goes forward one day.
Time difference from longitude
- 4 minutes of time
- 1 hour of time
Worked example
When it is noon at a place on 30° E, what is the local time at a place on 45° W?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q87Easy
Example 3 · Earth in Space, Maps and Coordinates · Latitude, Longitude, Time and Maps
If it is 10:00 a.m. at Greenwich Mean Time (GMT), then what will be the time at 90° E longitude?
IST is ahead, not behind
India is east of Greenwich, so Indian Standard Time is 5 hours 30 minutes ahead of GMT. A statement saying it is behind is false.
Opposite sides of Greenwich: add the longitudes
Between 30° E and 45° W the difference is 75°, not 15°. Subtract only when both places are on the same side of Greenwich.
One time zone for all of India
The Andaman and Nicobar Islands and Lakshadweep keep Indian Standard Time like the mainland. India has no second official time zone.
Concept 4 of 4: Isolines and diagrams
An isoline joins places that share the same value of something, so the name tells you what: iso means equal, and the rest of the word names the thing measured. Diagrams follow a similar logic: pick the one whose shape matches what you want to show.
Definition
Isolines, by what they join:
- Isotherm: temperature. Isobar: pressure. Isohyet: rainfall.
- Isohel: sunshine. Isonif: snowfall. Isoneph: cloudiness.
- Isohaline: salinity. Isobath: depth of water. Isoseismal: earthquake intensity.
- Isohypse (contour): height above sea level.
Diagrams, by what they show:
- Pie chart: the parts of one whole. Bar chart: compares amounts. Line graph: change over time.
- Scatter graph: whether two sets of data are related.
- Triangular graph: three parts that add up to 100%, such as sand, silt and clay in a soil.
| Line or diagram | What it shows |
|---|---|
| Isotherm | Equal temperature |
| Isobar | Equal pressure |
| Isohyet | Equal rainfall |
| Isohel | Equal sunshine |
| Isonif | Equal snowfall |
| Isoneph | Equal cloudiness |
| Isohypse (contour) | Equal height above sea levelQ CDS 2023 (II): an isohypse joins equal height, not cloudiness. |
| Isobath | Equal depth of water |
| Scatter graph | Whether two sets of data are relatedQ CDS 2023 (I): population size against number of services needs a scatter graph. |
| Triangular graph | Three parts adding up to 100% |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (II) 2023 — General Knowledge · Q97Hard
Example 4 · Earth in Space, Maps and Coordinates · Latitude, Longitude, Time and Maps
Which of the following pairs of Isolines and its properties are correctly matched ?
1. Isonif : Snowfall
2. Isohel : Sunshine
3. Isohaline : Salinity
4. Isohypse : Cloudiness
Select the correct answer using the code given below :
Isoneph, not isohypse, is cloudiness
Isoneph joins equal cloudiness. Isohypse is another name for a contour, joining equal height.
Two variables need a scatter graph
A pie chart or bar chart shows one set of data. To see whether two sets rise and fall together, plot them as points on a scatter graph.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Local time and longitude
Time difference from longitude
Reference tables (3)
The Earth's shape and rotation7 rows
| Fact | Value |
|---|---|
| Shape | Oblate spheroid, flattened at the poles |
| Circumference at the equator | About 40,000 km |
| Eratosthenes' two cities | Syene (Aswan) and AlexandriaQ CDS 2024 (II): Alexandria and Syene, not Troy or Thebes. |
| Direction of rotation | West to east |
| Angular velocity | 15° an hour, the same at every placeQ CDS 2024 (I): angular velocity is the same everywhere; linear velocity is greatest at the equator, least at the poles. |
| Linear velocity | About 1,670 km an hour at the equator; zero at the poles |
| Tilt of the axis | 23½° from the upright |
Latitude and what it tells you9 rows
| Line or place | Latitude or fact |
|---|---|
| Equator | 0° |
| Indira Point, Great Nicobar | About 6°45′ N, India's southernmost pointQ CDS 2017 (II): all three statements about latitude were true, including Indira Point at about 6°45′ N. |
| Panaji | About 15½° NQ CDS 2023 (II): Panaji is the closest of these four capitals to the equator. |
| Hyderabad | About 17½° N |
| Mumbai | About 19° N |
| Bhubaneswar | About 20° N |
| Tropic of Cancer | 23½° N |
| Arctic Circle | 66½° N |
| Parallels drawn every 1° | 89 in each hemisphere plus the equator: 179Q CDS 2025 (II): both statements true; 179 parallels, and a degree of latitude changes slightly in length. |
Isolines and diagrams10 rows
| Line or diagram | What it shows |
|---|---|
| Isotherm | Equal temperature |
| Isobar | Equal pressure |
| Isohyet | Equal rainfall |
| Isohel | Equal sunshine |
| Isonif | Equal snowfall |
| Isoneph | Equal cloudiness |
| Isohypse (contour) | Equal height above sea levelQ CDS 2023 (II): an isohypse joins equal height, not cloudiness. |
| Isobath | Equal depth of water |
| Scatter graph | Whether two sets of data are relatedQ CDS 2023 (I): population size against number of services needs a scatter graph. |
| Triangular graph | Three parts adding up to 100% |
Watch out for (8)
- Angular speed is the same, linear speed is not→ The Earth's shape and rotation
- Closest to the equator means lowest latitude→ Latitude and what it tells you
- The poles are not parallels→ Latitude and what it tells you
- IST is ahead, not behind→ Local time and longitude
- Opposite sides of Greenwich: add the longitudes→ Local time and longitude
- One time zone for all of India→ Local time and longitude
- Isoneph, not isohypse, is cloudiness→ Isolines and diagrams
- Two variables need a scatter graph→ Isolines and diagrams
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