Space

Our Solar System

The Solar System consists of the Sun and everything bound to it by gravity: 8 planets, their moons, dwarf planets, asteroids, comets, and vast amounts of dust and gas. It formed approximately 4.6 billion years ago from a collapsing cloud of gas and dust (a solar nebula).

Our Solar System is located in the Milky Way galaxy, about 26,000 light-years from the galactic centre. The Milky Way contains an estimated 200 to 400 billion stars.

The Sun

The Sun is the star at the centre of our Solar System and the source of nearly all energy on Earth.

• A massive ball of hot plasma about 1.4 million km in diameter (109 times Earth's)
• Made mainly of hydrogen (~75%) and helium (~25%)
• Generates energy through nuclear fusion — hydrogen nuclei fuse to form helium in the core, releasing enormous amounts of energy as light and heat
• Surface temperature: ~5,500°C; core: ~15 million°C
• Distance from Earth: ~150 million km; light takes about 8 minutes to reach us
• The Sun will continue fusing hydrogen for another ~5 billion years before expanding into a red giant

The Eight Planets

Mnemonic: My Very Excited Mother Just Served Us Nachos

Planet	Type	Notable facts
Mercury	Rocky/Terrestrial	Smallest planet; closest to Sun; no atmosphere; temperature range -180°C to 430°C
Venus	Rocky/Terrestrial	Hottest planet (465°C average, greenhouse effect); thick CO2 atmosphere; spins backwards
Earth	Rocky/Terrestrial	Only known planet with life; 71% ocean surface; one large moon
Mars	Rocky/Terrestrial	Red planet (iron oxide); thin CO2 atmosphere; Olympus Mons (22 km high); 2 moons
Jupiter	Gas giant	Largest planet; Great Red Spot storm; 95 known moons including Europa
Saturn	Gas giant	Spectacular ring system; least dense planet; 146 known moons
Uranus	Ice giant	Rotates on its side (axial tilt ~98°); blue-green colour from methane
Neptune	Ice giant	Furthest from Sun; strongest winds (~2100 km/h); deep blue colour

Day, Night, and Seasons

Day and night: Earth rotates on its axis once every 24 hours. The side facing the Sun experiences day; the opposite side experiences night. As Earth rotates, different parts move in and out of sunlight.

Seasons: Earth orbits the Sun once every 365.25 days with its axis tilted at 23.5 degrees. This tilt causes seasons:

• When the Northern Hemisphere tilts towards the Sun: longer days, more direct sunlight → summer in the Northern Hemisphere, winter in the Southern
• When it tilts away: shorter days, less direct sunlight → winter in the Northern Hemisphere

Seasons are NOT caused by Earth changing its distance from the Sun — Earth is actually slightly closer to the Sun in January (Northern Hemisphere winter).

The Moon

Earth's only natural satellite, the Moon orbits Earth approximately every 27.3 days.

• The Moon does NOT produce its own light — it reflects sunlight
• The same side of the Moon always faces Earth (synchronous rotation)
• The Moon's gravity causes ocean tides — the Moon pulls Earth's water toward it, creating a bulge

Moon phases: The Moon's phases are caused by the changing angle at which we see the lit half of the Moon as it orbits Earth:

New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent → New Moon

The cycle repeats approximately every 29.5 days (synodic month).

Lunar and solar eclipses:

• Lunar eclipse: Earth is directly between the Sun and Moon; Earth's shadow falls on the Moon
• Solar eclipse: Moon is directly between the Sun and Earth; Moon's shadow falls on Earth

Gravity in Space

Gravity is the fundamental force of attraction between all objects with mass.

Key principles:

• Gravity acts over any distance — it never completely disappears, only weakens
• The greater the mass of an object, the stronger its gravitational pull
• Gravity decreases with distance (specifically, with the square of the distance — inverse square law)
• Gravity keeps planets in orbit around the Sun; moons in orbit around planets

Without the Sun's gravity, planets would travel in straight lines into space (Newton's First Law). Gravity provides the centripetal force that curves their path into an orbit.

Weight in space: Astronauts on the International Space Station (ISS) are not truly weightless — they are in freefall, continuously falling towards Earth while also moving sideways fast enough to miss it. This produces the sensation of weightlessness.

Beyond the Solar System

Light-year: the distance light travels in one year — approximately 9.46 trillion km or 63,241 AU. Used to measure vast astronomical distances.

• Nearest star to the Sun: Proxima Centauri, about 4.24 light-years away
• The Milky Way is about 100,000 light-years in diameter
• The nearest large galaxy: Andromeda, about 2.5 million light-years away
• The observable universe: about 93 billion light-years in diameter

Galaxies: vast collections of billions of stars, gas, dust, and dark matter bound together by gravity. There are an estimated 2 trillion galaxies in the observable universe.

The Milky Way is a barred spiral galaxy. Our Solar System orbits the galactic centre once every ~225 million years — sometimes called a "galactic year."

Key Terms

• Solar System — the Sun and everything gravitationally bound to it
• Nuclear fusion — process powering stars; hydrogen nuclei fuse to form helium, releasing energy
• Orbit — the curved path of an object around another due to gravity
• Light-year — the distance light travels in one year (~9.46 trillion km)
• Galaxy — a system of billions of stars bound together by gravity
• Moon phase — the changing appearance of the Moon as seen from Earth during its orbit
• Axial tilt — the angle of Earth's rotation axis relative to its orbital plane (23.5 degrees)

Common Mistakes

• Thinking seasons are caused by Earth's changing distance from the Sun — they are caused by axial tilt
• Saying the Moon produces its own light — it reflects sunlight
• Confusing a lunar eclipse (Earth's shadow on the Moon) with a solar eclipse (Moon's shadow on Earth)
• Saying astronauts in the ISS have no gravity — they are in freefall (continuous orbit), not in a gravity-free zone

Tips and Tricks

• Day/night = Earth rotates (24 hours); Year = Earth orbits (365 days) — rotation vs revolution
• Seasons: Northern Hemisphere tilts towards Sun in June → summer; tilts away in December → winter
• Light-year is a unit of distance, not time — a common misconception
• Planet order mnemonic: My Very Excited Mother Just Served Us Nachos (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)