Universe & Solar System

The Big Bang Theory, proposed by Georges Lemaître (1927) and supported by Hubble's observations of expanding galaxies, is the most widely accepted. Key evidence: Cosmic Microwave Background (CMB) radiation and redshift of galaxies. Steady State (Fred Hoyle, 1948) was disproved when CMB was discovered in 1965. The Nebular Hypothesis relates to solar system formation, not the universe's origin.

Venus (~462°C) is the hottest planet. Classic CDS trap: Mercury is closest to the Sun but has virtually no atmosphere — heat escapes. Venus has a dense CO₂ atmosphere creating a runaway greenhouse effect, trapping all heat. Temperature on Venus is uniform day and night, unlike Mercury's extreme swings (+430°C to −180°C). This is the most repeated CDS solar system question.

IST = GMT + 5h 30min, based on the 82.5°E meridian (passes through Mirzapur, UP). Formula: 82.5 ÷ 15° = 5.5 hours = 5h 30min ahead of Greenwich. India spans ~29° of longitude (68°E–97°E) but maintains one time zone. 1° longitude = 4 minutes of time difference.

Venus rotates east-to-west (retrograde/clockwise viewed from above north pole) — opposite to Earth. So the Sun rises in the West on Venus. All planets except Venus (and partially Uranus) rotate west-to-east. Venus is also unique in having its day (243 Earth days rotation) longer than its year (225 Earth days revolution). This retrograde rotation is a CDS favourite.

Ganymede (orbiting Jupiter) is the largest moon in the solar system — even larger than the planet Mercury (though less massive). Common confusion: Titan is notable for having a thick nitrogen atmosphere (only moon with a significant atmosphere) but is smaller than Ganymede. Triton (Neptune) orbits in a retrograde direction. Ganymede has its own magnetic field.

A comet's tail is always pushed away from the Sun by solar wind and radiation pressure — regardless of which direction the comet is travelling. When approaching the Sun, the tail trails behind. When moving away from the Sun, the tail actually leads ahead of the comet's body. This makes the comet's tail a reliable indicator of the Sun's direction.

Perihelion (closest to Sun ~147 million km) occurs around January 3. Aphelion (farthest ~152 million km) around July 4. Counterintuitive: January = Northern Hemisphere winter yet Earth is closest to Sun. Seasons are caused by axial tilt (23.5°), NOT Earth-Sun distance. This distinction is a direct CDS question.

Mercury is the smallest planet (diameter ~4,880 km). Note: Pluto was reclassified as a dwarf planet by IAU in 2006 — so it is no longer counted as a planet. Mercury has almost no atmosphere, causing extreme temperature swings from +430°C to −180°C. This is WHY Mercury is NOT the hottest planet despite being closest to the Sun.

Seasons are caused by Earth's axial tilt (23.5°) during revolution — NOT by varying distance. In fact, Earth is closest to the Sun in January (NH winter) and farthest in July (NH summer). When the NH is tilted toward the Sun (June), it receives more direct sunlight for longer = summer. The southern hemisphere simultaneously experiences winter.

The International Astronomical Union (IAU) reclassified Pluto as a 'dwarf planet' in 2006, reducing our solar system's planet count from 9 to 8. Pluto fails the third criterion — it has not 'cleared the neighbourhood' around its orbit (overlaps with Kuiper Belt Objects). The 8 planets now are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Auroras (Aurora Borealis/Northern Lights and Aurora Australis/Southern Lights) occur in the Thermosphere (80–600 km), specifically the ionosphere layer within it. Charged solar wind particles interact with atmospheric gases along Earth's magnetic field lines near the poles, releasing energy as colourful light. Troposphere = weather; Stratosphere = ozone; Mesosphere = meteors burn; Thermosphere = auroras + ionosphere.

Geostationary satellites orbit above the Equator (NOT poles) at ~35,786 km altitude, completing one orbit in exactly 24 hours (matching Earth's rotation) — appearing stationary relative to a fixed point on Earth. Polar orbit satellites (like remote sensing satellites) orbit over the poles, covering the entire Earth surface over time. Geostationary satellites are ideal for communication (INSAT), weather monitoring (Meteosat), and TV broadcasting.

The midnight sun (24-hour daylight) is experienced above the Arctic Circle (66.5°N) during Northern Hemisphere summer and above the Antarctic Circle (66.5°S) during Southern Hemisphere summer. Due to Earth's 23.5° axial tilt, areas above 66.5° experience continuous daylight in summer and continuous darkness (polar night) in winter. Countries experiencing midnight sun: Norway, Finland, Iceland, northern Canada, Russia.

Jupiter's Great Red Spot is a massive anticyclonic storm (high-pressure system) that has been observed for over 350 years. It is larger than Earth. Jupiter is also the solar system's largest planet, fastest rotator (~10 hours per day), has the most moons (95 confirmed), and the strongest magnetic field. Saturn has prominent rings; Mars has Olympus Mons; Venus is hottest.

Saturn has the lowest average density (0.69 g/cm³) — less than water (1 g/cm³). It would float if placed in a large enough body of water. Saturn is composed mainly of hydrogen and helium. It is also famous for its extensive ring system (ice and rock particles). Jupiter has the most moons; Jupiter also rotates fastest; Saturn = least dense with most prominent rings.

Light from the Sun takes approximately 8 minutes 20 seconds to reach Earth (distance ~150 million km = 1 Astronomical Unit). Light speed = 3×10⁸ m/s. If the Sun suddenly disappeared, we would not know for over 8 minutes. This distance (1 AU) is the standard unit for measuring distances in the solar system. 1 light year = distance light travels in 1 year (~9.46 trillion km).

India's standard meridian (82.5°E) passes through Mirzapur (now Shankargarh), Uttar Pradesh, determining IST = GMT + 5:30. India spans ~29° of longitude (68°E to 97°E) — this means sunrise/sunset times vary by nearly 2 hours across India (Arunachal Pradesh sees sunrise ~2 hours before Gujarat) but India maintains a single time zone for administrative convenience.

A solar eclipse occurs only during a New Moon — when the Moon is positioned between Earth and the Sun, blocking sunlight. However, not every New Moon causes an eclipse because the Moon's orbit is inclined ~5° to Earth's orbital plane — the Moon usually passes slightly above or below the Sun. Solar eclipses are rarer at any specific location than lunar eclipses because the Moon's shadow covers only a small area on Earth.

Venus has a rotation period of 243 Earth days (a Venusian day) but its revolution period is only 225 Earth days (a Venusian year). So a day on Venus is longer than a year on Venus! Additionally, Venus rotates retrograde (east to west). This combination — retrograde rotation + day longer than year — makes Venus unique among all planets. This is a tricky CDS favourite.

Halley's Comet has a period of ~75–76 years. Last visible in 1986; next expected ~2061. Named after Edmund Halley who used Newton's laws to predict its periodic return. Comets are icy bodies from the Kuiper Belt or Oort Cloud that develop glowing tails (coma) when approaching the Sun — the tail always points away from the Sun due to solar wind. Halley's is the only short-period comet clearly visible to the naked eye from Earth.