📖 In-Depth Theory
The Solar System
Our SOLAR SYSTEM consists of:
The SUN — a star at the centre; contains ~99.8% of the total mass of the Solar System.
EIGHT PLANETS — orbit the Sun in approximately circular or elliptical orbits.
DWARF PLANETS — e.g. Pluto, Ceres — smaller bodies.
MOONS — natural satellites orbiting planets.
ASTEROID BELT — rocky bodies between Mars and Jupiter.
COMETS — icy bodies with highly elliptical orbits.
PLANETS (in order from the Sun):
Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune.
Memory: 'My Very Excellent Mother Just Served Us Noodles'
SCALE:
Earth–Sun distance: 1 AU = 1.5 × 10¹¹ m.
Nearest star (Proxima Centauri): ~4.25 light-years.
Our galaxy (Milky Way): ~100,000 light-years across.
1 light-year = 9.46 × 10¹⁵ m.
Orbital Motion and Gravity
Planets, moons and satellites stay in orbit due to GRAVITY.
GRAVITATIONAL FORCE:
All masses attract each other via gravity (a non-contact force).
The Sun's gravity keeps planets in orbit.
Earth's gravity keeps the Moon in orbit.
Gravity acts as the CENTRIPETAL FORCE for circular orbits — always directed towards the centre.
ORBITAL SPEED AND DISTANCE:
Planets closer to the Sun move FASTER — they need more speed to balance the stronger gravitational pull.
Planets further from the Sun move SLOWER — in larger orbits.
NATURAL SATELLITES:
Moon: natural satellite of Earth. Orbital period: ~27 days.
Moons of other planets: hundreds known.
ARTIFICIAL SATELLITES:
High Earth orbit (geostationary): 35,786 km, 24-hour period. Appears stationary. Used for: communications, weather.
Low Earth orbit (LEO): 200–2000 km, ~90 min period. Used for: ISS, Earth observation, GPS.
Higher orbit = slower speed, longer period.
Beyond the Solar System
STARS:
The Sun is a star — a large ball of plasma undergoing nuclear fusion.
Stars are grouped into GALAXIES.
Milky Way: our galaxy, ~200–400 billion stars.
Nearest galaxy to us: Andromeda (~2.5 million light-years).
UNIVERSE:
Contains ~2 trillion galaxies — observable universe ~93 billion light-years in diameter.
Age of universe: ~13.8 billion years.
DISTANCE MEASUREMENT:
Astronomical Unit (AU): average Earth-Sun distance — used within solar system.
Light-year: distance light travels in one year — used for interstellar distances.
Parsec: 3.26 light-years — used by professional astronomers.
SPACE EXPLORATION:
Probes: Voyager 1 and 2, New Horizons, Mars rovers.
Telescopes: Hubble (visible), James Webb (infrared), Chandra (X-ray).
Human spaceflight: ISS — continuous human presence since 2000.
⚠️ Common Mistake
Geostationary orbits are above the EQUATOR and have a period of exactly 24 hours — they appear stationary from Earth. LEO satellites have much shorter periods (90 minutes). The Sun is NOT at the exact centre of planetary orbits — orbits are slightly elliptical, with the Sun at one focus.