Universe & Earth

Venus (~462°C) is the hottest planet — NOT Mercury. Mercury is closest to the Sun but has almost no atmosphere, so heat escapes. Venus has a dense CO₂ atmosphere creating a runaway greenhouse effect, trapping all solar heat. Temperatures on Venus are uniform day and night. Mercury's surface swings from +430°C to −180°C. This is one of the most repeated NDA geography questions.

The Tropic of Cancer (23½°N) passes through 8 Indian states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura, and Mizoram. Mnemonic: GR MC JW TM. It does NOT pass through Manipur — a common NDA confusion. The Tropic of Cancer enters India from the Gujarat coast (west) and exits through Mizoram (east).

IST = GMT + 5 hours 30 minutes. India's standard meridian is 82½°E (passing through Mirzapur, UP). 12:00 noon + 5:30 = 5:30 PM IST. Formula: 82.5° ÷ 15° per hour = 5.5 hours = 5h 30min. India is east of Greenwich so it is ahead. This calculation appears in NDA nearly every year.

On 21 June, the Sun is directly overhead the Tropic of Cancer (23½°N) — the Northern Hemisphere's longest day (Summer Solstice). The Southern Hemisphere simultaneously has its Winter Solstice. Key dates: 21 March = Vernal/Spring Equinox; 21 June = NH Summer Solstice; 23 September = Autumnal Equinox; 22 December = NH Winter Solstice. All four dates are directly tested in NDA.

Lunar eclipse: Earth is between the Sun and Moon — Earth's shadow falls on the Moon. Occurs only during Full Moon (Purnima). Solar eclipse: Moon is between Earth and Sun — occurs only during New Moon (Amavasya). A lunar eclipse can be seen from the entire night side of Earth; a solar eclipse is visible only from a narrow path. NDA frequently tests this solar vs lunar eclipse distinction.

Venus has a rotation period of ~243 Earth days (one Venusian day) but its revolution period is only ~225 Earth days (one Venusian year). So a day on Venus is longer than a year on Venus. Venus also has retrograde rotation — it spins east to west, opposite to most planets, meaning the Sun rises in the West on Venus. This combination of retrograde rotation and day longer than year makes Venus unique.

The Milky Way is a barred spiral galaxy — our Solar System is located on the Orion Arm, about 26,000 light-years from the galactic centre. The Milky Way contains 200–400 billion stars. It takes the Solar System approximately 225–250 million years to orbit the galactic centre (one cosmic year). The nearest galaxy to the Milky Way is the Andromeda Galaxy (~2.5 million light-years away).

The International Date Line approximately follows the 180° meridian in the Pacific Ocean, with deviations to keep island groups in the same time zone. Crossing westward across the IDL means advancing one calendar day; crossing eastward means losing one day. The Prime Meridian (0°) passes through Greenwich, England. These two meridians are on opposite sides of Earth. The IDL was established to resolve the calendar date problem for circumnavigators.

Earth's axial tilt is approximately 23.5° (23°27') from the perpendicular to its orbital plane. This tilt is the primary cause of seasons — when the NH is tilted toward the Sun, it receives more direct sunlight for more hours = summer. The Tropics of Cancer and Capricorn (23.5°N and 23.5°S) and the Arctic/Antarctic Circles (66.5°N and 66.5°S) are all directly derived from this tilt angle.

Halley's Comet has an orbital period of approximately 75–76 years. It was last visible in 1986 and will next appear around 2061. Edmond Halley used Newton's laws to predict the comet's return, and it was named after him. Comets develop a luminous tail pointing away from the Sun (due to solar wind) when they enter the inner solar system. Halley's Comet is the only short-period comet clearly visible to the naked eye.

A solar eclipse occurs only during New Moon (Amavasya) — when the Moon is positioned between Earth and the Sun, blocking sunlight. However, not every New Moon causes an eclipse because the Moon's orbital plane is inclined ~5° to Earth's orbital plane. Compare: lunar eclipse occurs only during Full Moon (Purnima) when Earth is between Sun and Moon. NDA tests both conditions regularly.

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, ~21.9 km), deepest canyon (Valles Marineris), and its day is 24 hours 37 minutes — closest to Earth's. Mars has two small moons: Phobos and Deimos. It is the most explored planet after Earth (NASA rovers Curiosity and Perseverance are currently on Mars).

Troposphere (0–12 km, all weather) → Stratosphere (12–50 km, ozone layer) → Mesosphere (50–80 km, meteors burn) → Thermosphere (80–600 km, auroras, ISS) → Exosphere (space). Temperature pattern: Tropo = decreases; Strato = increases; Meso = decreases; Thermo = increases. Mnemonic: 'The Silly Monkey Throws Eggs' (Tropo-Strato-Meso-Thermo-Exo).

Perihelion = Earth's closest point to the Sun (~147 million km), occurring around January 3. Aphelion = farthest point (~152 million km), around July 4. Counterintuitive: Earth is closest to the Sun during Northern Hemisphere winter. Apogee/Perigee apply to the Moon's orbit around Earth (Apogee = farthest from Earth; Perigee = closest to Earth). These terms are frequently confused and directly tested in NDA.

Auroras occur in the Thermosphere (80–600 km altitude) — specifically in 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. Auroras are best seen near the magnetic poles. Troposphere = weather; Stratosphere = ozone; Mesosphere = coldest, meteors burn; Thermosphere = auroras, ISS, radio wave reflection.

Saturn holds the record with 146 confirmed moons (as of 2024), overtaking Jupiter (95 moons). Saturn's largest moon, Titan, is the only moon in the solar system with a thick atmosphere (nitrogen + methane). Jupiter's moon Ganymede is the largest moon overall (larger than Mercury). Note: Jupiter long held this record — NDA may use older data where Jupiter led. Check the question's year context.

The Coriolis effect is caused by Earth's rotation on its axis. Freely moving objects (air, water, missiles) are deflected: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This causes NE trade winds, cyclone rotation (anticlockwise in NH, clockwise in SH), and anticyclone rotation (clockwise in NH). The Coriolis force is zero at the Equator and maximum at the Poles. This is directly tested in NDA.

Tropic of Capricorn = 23.5°S latitude. It marks the southernmost latitude where the Sun can be directly overhead — this occurs on the December Solstice (Winter Solstice in the NH, Summer Solstice in the SH). The Tropic of Cancer = 23.5°N. Arctic Circle = 66.5°N; Antarctic Circle = 66.5°S. Australia, Brazil, and South Africa all have land within the Tropic of Capricorn — this is tested in NDA world geography sections.

Saturn is the least dense planet in the solar system (average density ~0.69 g/cm³ — less than water at 1 g/cm³). It would theoretically float on water if placed in a large enough body. Saturn is composed mainly of hydrogen and helium. It also has the most prominent ring system. Jupiter has the strongest magnetic field and rotates fastest (~10 hours). Saturn's beauty and rings make it a frequent NDA factual question.

The asteroid belt is located between Mars and Jupiter, at 2.2–3.2 AU from the Sun. It contains millions of rocky and metallic bodies — remnants from the early solar system that never coalesced into a planet due to Jupiter's gravitational influence. Ceres is the largest asteroid (also classified as a dwarf planet). The Kuiper Belt (beyond Neptune) and Oort Cloud (far outer solar system) are other regions containing small bodies.