Universe & Solar System

Venus (avg. 462°C) is the hottest planet — NOT Mercury. Mercury is closest to the Sun but has virtually no atmosphere, so heat escapes. Venus has a dense CO₂ atmosphere that traps all heat (runaway greenhouse effect). Temperature on Venus is uniform day and night. This is AFCAT's most repeated solar system question — if you know nothing else, remember Venus = hottest.

IST = GMT + 5 hours 30 minutes. India's standard meridian is 82½°E (passes through Mirzapur, UP). Since India is east of Greenwich, it is ahead: 12:00 noon + 5:30 = 5:30 PM IST. Formula: longitude ÷ 15 = hours ahead/behind GMT. 82.5 ÷ 15 = 5.5 hours = 5h 30min. AFCAT tests this calculation directly — memorise it as a formula.

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 failed the third criterion: it has not 'cleared the neighbourhood' around its orbit — it overlaps with other Kuiper Belt objects. Today our solar system has 8 planets + 5 recognised dwarf planets: Pluto, Eris, Ceres, Makemake, Haumea.

Olympus Mons on Mars is approximately 21.9 km high and ~600 km wide at the base — nearly 3× the height of Mount Everest. It is a shield volcano. Mars has no tectonic plates, so lava kept piling up in the same spot for billions of years. Earth's Mauna Kea is tallest from ocean floor to summit, but Olympus Mons wins on absolute height above surrounding plains.

Jupiter's Great Red Spot is an anticyclonic storm larger than Earth that has been observed for over 350 years. Jupiter is the solar system's largest planet, composed mainly of hydrogen and helium. It also has the most moons (95 confirmed). Saturn has rings; Mars has Olympus Mons; Neptune has the Great Dark Spot (periodic, not permanent). Jupiter's Great Red Spot = AFCAT favourite.

Halley's Comet has a period of approximately 75–76 years. It was last visible in 1986 and is expected next around 2061. It was named after Edmund Halley who predicted its return using Newton's laws. Comets are icy bodies that develop tails of gas and dust when they approach the Sun. Halley's Comet is the only short-period comet clearly visible to the naked eye from Earth.

The International Date Line approximately follows the 180° meridian in the Pacific Ocean, bending around island groups (Aleutian Islands, Fiji, Kiribati) to avoid splitting countries/territories. When you cross westward from east, you gain one calendar day. When you cross eastward, you lose one day. The Prime Meridian (0°) passes through Greenwich, England — opposite side of Earth.

Venus has retrograde rotation — it spins east to west (clockwise when viewed from above the North Pole), opposite to Earth and most other planets. This means on Venus, the Sun rises in the west and sets in the east. Uranus also has retrograde rotation but spins on its side (axial tilt ~98°). Venus also has the slowest rotation — one Venusian day is longer than its year.

Earth's axial tilt is approximately 23.5° (23°27') from the perpendicular to its orbital plane. This tilt is responsible for the seasons — when the Northern Hemisphere is tilted toward the Sun (June), it receives more direct sunlight = summer. When tilted away (December) = winter. Without this tilt, there would be no seasons. The tropics (Cancer 23.5°N and Capricorn 23.5°S) correspond directly to this angle.

Solar eclipse = Moon between Earth and Sun (New Moon). Lunar eclipse = Earth between Sun and Moon (Full Moon) — Earth's shadow falls on Moon. Solar eclipses are less commonly observed than lunar eclipses because the Moon's shadow covers only a small area on Earth, while a lunar eclipse is visible from the entire night side of Earth simultaneously.

Mercury is the smallest planet in our solar system (since Pluto's reclassification in 2006 as a dwarf planet). Mercury's diameter is about 4,880 km — roughly 38% of Earth's size. It has no atmosphere (or a very thin one), resulting in extreme temperature swings: +430°C on the Sun-facing side and -180°C on the dark side. This is why Mercury is NOT the hottest despite being closest to the Sun.

Auroras (Aurora Borealis in the North, Aurora Australis in the South) occur in the Thermosphere (80–600 km altitude), 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. The thermosphere also contains the ionosphere that reflects radio waves. Troposphere = weather; Stratosphere = ozone; Mesosphere = meteors burn.

Seasons are caused by Earth's axial tilt (23.5°) during revolution around the Sun — NOT by Earth's varying distance. In fact, Earth is actually slightly closer to the Sun in January (Northern Hemisphere winter) than in July. When the Northern Hemisphere is tilted toward the Sun, it receives more direct sunlight and longer days = summer. The Southern Hemisphere experiences opposite seasons simultaneously.

Saturn is the least dense planet in the solar system — its average density (0.69 g/cm³) is less than that of water (1 g/cm³), meaning it would float if placed in a large enough body of water. This is because Saturn is made primarily of hydrogen and helium gas. Jupiter has the most moons (95 confirmed). Jupiter also has the strongest magnetic field. Jupiter rotates fastest (~10 hours).

The Spring/Vernal Equinox occurs around 21 March — when the Sun is directly above the Equator, giving equal day and night everywhere on Earth. 21 June = Summer Solstice in NH (longest day, Sun directly above Tropic of Cancer). 22 December = Winter Solstice in NH (shortest day, Sun above Tropic of Capricorn). 23 September = Autumnal Equinox. These four dates are directly tested in AFCAT.

Mars is called the Red Planet because of iron oxide (rust) on its surface, giving it a reddish appearance. Mars has the solar system's largest volcano (Olympus Mons), deepest canyon (Valles Marineris), and its day is 24h 37min — closest to Earth's. Mars has two small moons: Phobos and Deimos. NASA's Mars rovers (Curiosity, Perseverance) have been studying its surface. Mars is the most explored planet after Earth.

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

Saturn has the most prominent and extensive ring system — visible through even modest telescopes. Saturn's rings are made of ice particles, rocky debris, and dust, stretching up to 282,000 km from the planet. While Jupiter, Uranus, and Neptune also have ring systems, they are faint and not easily visible. Saturn's rings are thought to be remnants of comets, asteroids, or moons that broke apart.

Light from the Sun takes approximately 8 minutes and 20 seconds to reach Earth — a distance of about 150 million km (1 Astronomical Unit). Light travels at 3×10⁸ m/s. This means that when we look at the Sun, we see it as it was 8 minutes ago. If the Sun disappeared right now, we would not know for 8 minutes. This distance (150 million km) is defined as 1 AU (Astronomical Unit), the standard unit for measuring distances in the solar system.

India's standard meridian (82½°E longitude) passes through Mirzapur (now Shankargarh), Uttar Pradesh. It determines IST = GMT + 5:30. India adopted a single time zone despite spanning ~30° of longitude (68°E to 97°E) — this means sunrise/sunset times vary significantly across the country (Arunachal Pradesh sees sunrise ~2 hours before Gujarat). This is a directly tested AFCAT factual question.