UPSC Prelims 2025 Q23: Equator, Tanganyika, Tonlé Sap, Patos Lagoon
Consider the following water bodies:
I. Lake Tanganyika
II. Lake Tonlé Sap
III. Patos Lagoon
Through how many of them does the equator pass?
- Only one
- Only two
- All the three
- None
Answer: (d) None
Statement-wise explanation
- I: incorrect — Lake Tanganyika lies entirely south of the equator, between about 3°S and 9°S. It runs about 670 km north to south in the western arm of the Great Rift Valley and is shared by Burundi, DR Congo, Tanzania and Zambia. Its northern tip, at Bujumbura in Burundi, is still about 3° (over 300 km) south of the equator. The East African lake that the equator does cross is Lake Victoria, through its northern part near Uganda and Kenya.
- II: incorrect — Tonlé Sap is in Cambodia, with its centre near 12.8°N, so it is in the Northern Hemisphere and far from the equator. It is the largest freshwater lake in Southeast Asia. In the monsoon the Mekong floods and reverses the flow of the Tonlé Sap river, which makes the lake grow from about 2,500–3,000 sq km to as much as 10,000–12,000 sq km.
- III: incorrect — Patos Lagoon (Lagoa dos Patos) is in Rio Grande do Sul, Brazil's southernmost state, at about 30°S–32°S. That is roughly 30° south of the equator. It is Brazil's largest lagoon, about 290 km long and over 10,000 sq km in area, and is cut off from the Atlantic by a sandbar. The equator crosses Brazil far to the north, near the mouth of the Amazon.
How to eliminate
Simple map sense settles two of the three. Tonlé Sap is in Cambodia, on the Indochina peninsula north of the Gulf of Thailand. All of Cambodia lies between about 10°N and 15°N, so the equator cannot touch it. Patos Lagoon is in southern Brazil near Uruguay, close to 30°S, so it can be ruled out too. Once these two are gone, 'All the three' and 'Only two' are out, and the choice is between 'Only one' and 'None', depending only on Lake Tanganyika. The trap here is mixing up Africa's Great Lakes. The equator crosses Lake Victoria, not Tanganyika, which lies south of Victoria at 3°S–9°S. Remember: the equator crosses Lake Victoria in Africa, and in South America it crosses the Amazon mouth near Macapá, not any lagoon in southern Brazil.
Know more
What the equator crosses
The equator (0° latitude) is the only parallel that is a great circle. It is about 40,075 km long and splits the Earth into two hemispheres. UPSC often tests map knowledge by naming water bodies and asking which ones a given line crosses. These lines include the equator, the Tropics (23.5°N/S) and the Prime Meridian.
Land and water on the equator (west to east)
| Continent / ocean | Countries / features crossed |
|---|---|
| South America | Ecuador, Colombia, Brazil (Amazon mouth, near Macapá) |
| Africa | São Tomé & Príncipe (waters), Gabon, Rep. of Congo, DR Congo, Uganda, Kenya, Somalia; Lake Victoria |
| Indian Ocean | Maldives (waters between Huvadhu and Fuvahmulah atolls) |
| Asia | Indonesia (Sumatra, Borneo, Sulawesi, Halmahera) |
| Pacific | Kiribati |
Lakes that are often confused
| Water body | Location | Approx. latitude |
|---|---|---|
| Lake Victoria | Uganda, Kenya, Tanzania | ~0°21'N to 3°S (crosses equator) |
| Lake Tanganyika | Rift Valley, 4 countries | ~3°S to 9°S |
| Lake Malawi (Nyasa) | Malawi, Mozambique, Tanzania | ~9°S to 14°S |
| Tonlé Sap | Cambodia | ~12–13°N |
| Patos Lagoon | Southern Brazil | ~30–32°S |
Useful extras
- Lake Victoria is Africa's largest lake by area. It is shallow and sits in a basin between the two arms of the Rift Valley.
- Lake Tanganyika is a rift lake. It is the world's longest freshwater lake and the second deepest (about 1,470 m) after Baikal.
- Tonlé Sap is a UNESCO Biosphere Reserve (1997). Its river reverses direction every monsoon.
- The thermal equator is different from the geographic equator. It moves with the seasons, so don't use it to answer questions about where 0° latitude falls.
NCERT Concepts to Revise
- Lake Tanganyika: Statement I depends on exactly where this Rift Valley lake sits relative to 0° latitude.
- Lake Victoria: The East African lake most often linked with the equator; compare its latitude with Tanganyika's before answering Statement I.
- Parallels of latitude: The question is about latitude, so you need to know how parallels are measured from the equator.
- Mekong: Tonlé Sap is fed and reversed by this river, which helps you place Statement II on the map.
- Thermal equator: Do not confuse the shifting belt of highest temperature with the fixed 0° parallel that the question asks about.