Solar system & our earth

Overview & Fundamental Concept

Complete preparation guide for Solar system & our earth under AISSEE Class VI General Knowledge (Syllabus Item 9). Master high-yield facts, exam matrices, solved models, and 10 interactive drill MCQs with detailed explanations.

• The Sun is a medium-sized yellow dwarf star (spectral class G2V) situated at the gravitational hub of our solar system, holding 99.86% of the solar system's entire aggregate mass.
• Nuclear fusion in the solar core transforms hydrogen into helium at an core temperature of ~15,000,000°C (15 million °C); the outer visible layer (Photosphere) registers ~5,500°C to 6,000°C.
• Sunlight speed is ~300,000 km/s (3 × 10⁸ m/s), taking exactly 8 minutes and 20 seconds (500 seconds) to bridge the 149.6 million km (1 Astronomical Unit / AU) distance to Earth.
• Planetary status criterion (IAU 2006 Prague Resolution): A celestial body must orbit the Sun, possess sufficient mass for spherical hydrostatic equilibrium, and have cleared the neighbourhood around its orbit.
• Dwarf Planet status: Pluto, discovered in 1930 by Clyde Tombaugh, was demoted to 'Dwarf Planet' in August 2006 because it crosses Neptune's orbit and has not cleared its neighbourhood. Other dwarf planets include Ceres (Asteroid Belt), Eris, Haumea, and Makemake.

• Mercury (Budh): Closest planet to the Sun; fastest revolution (88 Earth days); has zero atmosphere to regulate thermal energy, generating extreme swings from +430°C on dayside to -180°C on nightside; smallest planet; 0 moons.
• Venus (Shukra): Known as 'Earth's Twin' due to near-identical diameter (12,104 km vs 12,742 km) and mass; hottest planet in the solar system (~465°C) caused by a runaway greenhouse effect from 96.5% dense carbon dioxide and sulfuric acid clouds; rotates retrograde (clockwise: East to West); day (243 Earth days) is longer than its year (225 Earth days); brightest natural object in the night sky after the Moon ('Morning Star' & 'Evening Star'); 0 moons.
• Earth (Prithvi): Known as the 'Blue Planet' because liquid water covers 71% of its surface; only confirmed harbor of life; atmosphere contains 78% Nitrogen, 21% Oxygen, and 0.04% Carbon Dioxide; axis tilted at 23.5°; orbital revolution of 365.25 days produces a Leap Year every 4 years; 1 natural satellite (Moon).
• Mars (Mangal): Known as the 'Red Planet' due to surface iron oxide (rust); thin atmosphere composed primarily of CO₂; home to Olympus Mons (shield volcano 22 km high, 3 times the height of Mt. Everest, largest volcano in solar system) and Valles Marineris (4,000 km long canyon system); 2 small potato-shaped captured asteroid moons: Phobos ('Fear') and Deimos ('Panic' - smallest moon in solar system).
• Jupiter (Brihaspati): Largest planet in the solar system (1,300 Earths could fit inside; 2.5 times the mass of all other planets combined); fastest axial rotation (~9 hours 55 minutes), creating an oblate equatorial bulge; famous Great Red Spot is a giant anticyclonic storm raging for over 350 years; 95 confirmed moons; Ganymede is the largest moon in the solar system (larger than Mercury and Pluto).
• Saturn (Shani): Second-largest planet; encircled by a spectacular ring system spanning 282,000 km but only ~10 to 30 meters thick, composed of 99% pure water ice chunks; lowest density of any planet (0.687 g/cm³), meaning it is lighter than water and would physically float in a sufficiently vast ocean; 146 confirmed moons (most in solar system); Titan is its largest moon (2nd largest in solar system) featuring a dense nitrogen atmosphere and liquid hydrocarbon (methane/ethane) lakes.
• Uranus (Arun): Seventh planet; discovered by Sir William Herschel in 1781 using a telescope; pale cyan-green due to methane; tilted at an extreme 97.8° on its rotational axis, causing it to orbit the Sun rolling on its side; exhibits retrograde rotation (East to West) along with Venus; 28 confirmed moons (Titania, Oberon, Miranda).
• Neptune (Varun): Outermost major planet; discovered in 1846 through mathematical calculations by Urbain Le Verrier and Johann Galle; deep azure blue color; coldest planetary atmosphere (-218°C) and fastest supersonic winds exceeding 2,100 km/h; orbital period is 164.8 Earth years; largest moon Triton orbits retrograde (opposite to Neptune's rotation) and features active nitrogen geysers.

• Earth's Axial Motions: Axial Rotation (23 hours 56 minutes ~ 24 hours, West to East) produces the cycle of Day and Night; Orbital Revolution (365 days 5 hours 48 minutes ~ 365.25 days around the Sun in an elliptical orbit) generates the Annual Cycle and Year.
• The Cause of Seasons: Earth's rotational axis is tilted at 23.5° relative to its orbital plane (or 66.5° to the plane of the ecliptic). The tilt combined with revolution causes different hemispheres to tilt toward or away from the Sun, creating Summer, Winter, Spring, and Autumn.
• Summer Solstice (June 21): Northern Hemisphere tilts directly toward the Sun ($23.5^\circ\text{N}$ Tropic of Cancer receives vertical rays); longest day and shortest night of the year in India and Northern Hemisphere; 24 hours continuous daylight at the North Pole.
• Winter Solstice (December 22): Southern Hemisphere tilts toward the Sun ($23.5^\circ\text{S}$ Tropic of Capricorn receives vertical rays); shortest day and longest night of the year in India; 24 hours continuous darkness at the Arctic Circle.
• Equinoxes (Equal Day and Night = 12 hrs each across the globe): Occurs twice annually when the Sun is directly overhead at the Equator: Vernal / Spring Equinox (March 21) and Autumnal Equinox (September 23).
• The Moon (Earth's Sole Natural Satellite): Average distance is 384,400 km; gravity is exactly 1/6th of Earth's surface gravity ($1.62\text{ m/s}^2$ — an 60 kg person on Earth weighs only 10 kg on the Moon); Moon has no atmosphere, no clouds, no liquid water, and sound cannot travel across it (vacuum).
• Synchronous Rotation: The Moon takes 27.3 days (sidereal month) to rotate on its axis and exactly 27.3 days to revolve around Earth; as a direct result, the exact same face (Near Side) of the Moon always points toward Earth.
• Moon Exploration Milestones: Apollo 11 (NASA) landed first humans on July 20, 1969 — Neil Armstrong was the 1st person on the Moon, followed by Edwin 'Buzz' Aldrin, while Michael Collins piloted the command module.
• India's Chandrayaan Missions: Chandrayaan-1 (Oct 22, 2008) discovered water molecules ($H_2O/OH$) on lunar soil using Moon Mineralogy Mapper; Chandrayaan-3 (Launched July 14, 2023) achieved historic soft-landing on the Lunar South Pole on August 23, 2023 (Lander: Vikram, Rover: Pragyan). August 23 is now celebrated nationally as 'National Space Day'!

• Solar Eclipse (Surya Grahan): Occurs when the Moon moves directly between the Sun and Earth, casting its shadow (Umbra & Penumbra) onto Earth's surface; can only occur during the New Moon (Amavasya) phase.
• Lunar Eclipse (Chandra Grahan): Occurs when Earth passes directly between the Sun and Moon, casting its shadow over the lunar face; can only occur during the Full Moon (Purnima) phase; the Moon appears copper-red ('Blood Moon') due to Rayleigh scattering through Earth's atmosphere.
• Ocean Tides: Gravitational pull exerted by the Moon (predominant due to proximity) and the Sun creates rhythmic rising and falling of ocean waters twice a day (every 12 hours 25 minutes); Spring Tides (highest high tides) occur during New Moon and Full Moon when Sun, Moon, and Earth align linearly (Syzygy); Neap Tides (lowest tidal range) occur at First and Third Quarter Moon when Sun and Moon pull at 90° right angles.
• Comets: Cosmic snowballs composed of frozen gases, rock, and dust orbiting the Sun in highly eccentric elliptical paths; as they approach the solar heat, volatile gases vaporize into a glowing coma and solar radiation pressure pushes dust/ions into tails pointing directly AWAY from the Sun; Halley's Comet orbits every 76 years (last observed 1986, next appearance in 2061).
• Meteoroid vs Meteor vs Meteorite Hierarchy: A Meteoroid is a small space rock traveling in interplanetary space; a Meteor ('Shooting Star') is the glowing luminous streak formed when the rock vaporizes due to friction in Earth's Mesosphere; a Meteorite is any surviving rock fragment that survives atmospheric entry and strikes the Earth's crust (e.g., Lonar Crater Lake in Maharashtra was created by a meteorite impact).
• Major Constellations: Ursa Major (Great Bear / Saptarishi - 7 prominent stars whose outer two 'pointer stars' Dubhe and Merak point directly to Polaris / Dhruva Tara in the north); Orion (The Hunter - contains bright stars Betelgeuse and Rigel, and 3 belt stars); Cassiopeia (Queen - distinct 'W' or 'M' shaped northern constellation).