Earth and the Universe

Overview

Earth and the Universe forms a foundational topic in the Geography section of OTET Paper II Social Science. This topic tests candidates' understanding of our planet's place in the solar system, the coordinate system used to locate places on Earth, and the movements that cause day-night cycles and seasons. Questions typically focus on factual recall (names of planets, important latitudes) and conceptual understanding (why seasons occur, what causes eclipses).

Mastering this topic requires building a clear mental picture of spatial relationships—Earth's position relative to the Sun, how imaginary lines divide the globe, and how Earth's twin motions create the phenomena we observe daily. These concepts also connect to climate, time zones, and map reading, making them essential building blocks for other geography topics in the syllabus.

Key Concepts

• **Solar System Structure**: The Sun is at the centre, with eight planets orbiting it. In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Pluto was reclassified as a dwarf planet in 2006.

• **Inner vs Outer Planets**: Mercury, Venus, Earth and Mars are rocky terrestrial planets. Jupiter, Saturn, Uranus and Neptune are gas giants. The asteroid belt separates these two groups.

• **Latitude (Parallels)**: Imaginary horizontal lines running east-west, measuring distance north or south of the Equator (0°). Range from 0° to 90°N and 0° to 90°S.

• **Longitude (Meridians)**: Imaginary vertical lines running north-south, measuring distance east or west of the Prime Meridian (0°). Range from 0° to 180°E and 0° to 180°W.

• **Rotation**: Earth spins on its axis once every approximately 24 hours (west to east). This causes day and night and the apparent movement of the Sun across the sky.

• **Revolution**: Earth orbits the Sun once every approximately 365¼ days. This motion, combined with the axial tilt of 23.5°, causes the seasons.

• **Axial Tilt**: Earth's axis is tilted at 23.5° from the perpendicular to its orbital plane. This tilt is the primary cause of seasons—not Earth's distance from the Sun.

• **Eclipses**: Solar eclipse occurs when the Moon comes between Earth and Sun. Lunar eclipse occurs when Earth comes between Sun and Moon, casting shadow on the Moon.

Formulas / Key Facts

| Fact | Detail | |------|--------| | Number of planets | 8 (Mercury to Neptune) | | Largest planet | Jupiter | | Smallest planet | Mercury | | Earth's rotation period | Approximately 24 hours | | Earth's revolution period | Approximately 365¼ days (one year) | | Axial tilt of Earth | 23.5° | | Equator | 0° latitude, divides Earth into Northern and Southern Hemispheres | | Prime Meridian | 0° longitude, passes through Greenwich, UK | | International Date Line | 180° longitude (with deviations), where calendar date changes | | Tropic of Cancer | 23.5°N latitude | | Tropic of Capricorn | 23.5°S latitude | | Arctic Circle | 66.5°N latitude | | Antarctic Circle | 66.5°S latitude | | Total degrees of latitude | 180° (90°N + 90°S) | | Total degrees of longitude | 360° (180°E + 180°W) |