SSC
The mean radius of the Earth is approximately:
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Answer: Option B — 6,371 km
Detailed Solution
The mean (average) radius of the Earth is about 6,371 km. This is the radius of a perfect sphere that would have the same volume as the Earth.
Why an average is used:
The Earth is not a perfect sphere. It is slightly flattened at the poles and bulges at the Equator.
Equatorial radius: about 6,378 km.
Polar radius: about 6,357 km.
The mean radius of 6,371 km lies between these values and is used in most calculations.
Other key measurements of the Earth:
- Mean diameter: about 12,742 km (twice the radius).
- Equatorial circumference: about 40,075 km.
- Total surface area: about 510 million square km.
- Volume: about 1.08 trillion cubic km.
- Average density: about 5.5 grams per cubic centimetre.
- Mass: about 5.97 x 10^24 kg.
How the size of the Earth was first measured:
- Around 240 BCE, the Greek scholar Eratosthenes estimated the Earth's circumference.
- He noticed that at noon on the summer solstice, the Sun was directly overhead at Syene (now Aswan, Egypt), while at Alexandria it cast a shadow at an angle of about 7.2 degrees, or one-fiftieth of a circle.
- Knowing the distance between the two cities, he calculated the Earth's circumference fairly accurately.
- In India, the astronomer Aryabhata (5th century CE) also gave an estimate of the Earth's circumference.
Interior of the Earth:
The distance from the surface to the centre is about 6,371 km, made up of:
- Crust: about 5 to 70 km thick.
- Mantle: down to about 2,900 km.
- Outer core: from about 2,900 km to 5,150 km.
- Inner core: from about 5,150 km to 6,371 km at the centre.
Comparison:
Jupiter's radius is about 11 times that of the Earth.
Why not the other options?
- ✗1,737 km is the radius of the Moon.
- ✗12,742 km is the Earth's diameter, not its radius.
- ✗63,710 km is ten times too large.
Quick tip
Remember: Earth's mean radius = 6,371 km (equatorial 6,378 km, polar 6,357 km); diameter = about 12,742 km.
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Earth: Structure, Rocks & Landforms — Preparation
This question tests your understanding of Earth: Structure, Rocks & Landforms, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers the interior of the Earth (crust, mantle and core), types of rocks, plate tectonics and continental drift, earthquakes and volcanoes, and the landforms created by rivers, glaciers, wind and waves. Questions frequently ask about the layers of the Earth and their discontinuities, the three types of rocks (igneous, sedimentary and metamorphic), the Pacific 'Ring of Fire', and which agent forms which landform, such as river meanders and oxbow lakes or glacial cirques and moraines. Alfred Wegener proposed the continental drift theory in 1912. Practise the Earth's structure and landforms questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.