Earth & Moon

Day and Night

Earth rotates on its own axis (an imaginary line from North Pole to South Pole) once every 24 hours. This rotation causes day and night:

• The side of Earth facing the Sun experiences day — sunlight reaches that surface
• The side facing away from the Sun experiences night — in shadow

Earth rotates from west to east, which is why the Sun appears to rise in the east and set in the west.

The boundary between the day and night sides is called the terminator line.

The Seasons

Many students think seasons are caused by Earth getting closer to the Sun in summer — this is a common misconception. In fact, Earth is actually slightly closer to the Sun in January (Northern Hemisphere winter).

Seasons are caused by the tilt of Earth's axis:

Earth's axis is tilted at 23.5 degrees relative to its orbital plane. As Earth orbits the Sun over a year, the tilt means that different hemispheres receive more direct sunlight at different times:

Season (Northern Hemisphere)	Reason
Summer	North Pole tilts towards the Sun; sunlight hits more directly; days are longer
Winter	North Pole tilts away from the Sun; sunlight hits at shallower angle; days are shorter
Equinoxes (spring, autumn)	Neither hemisphere is tilted towards or away; day and night are equal length (12 hours each)

The Southern Hemisphere experiences opposite seasons — when it is summer in the UK, it is winter in Australia.

Why does angle matter? When sunlight strikes at a low angle, the same amount of energy is spread over a larger area, making it less intense. When it strikes directly, the energy is concentrated over a smaller area, making it hotter.

The Moon

The Moon is Earth's only natural satellite — a body that orbits a planet due to gravity.

Key facts:

• Distance from Earth: approximately 384,400 km
• Diameter: about 3,475 km (about 1/4 the diameter of Earth)
• Orbit period: 27.3 days (sidereal — relative to stars); 29.5 days (synodic — new moon to new moon, as seen from Earth)
• The Moon does not produce its own light — it reflects sunlight
• The Moon has no atmosphere, so temperatures are extreme (-173°C to +127°C)
• Synchronous rotation — the Moon rotates on its axis in the same time it takes to orbit Earth, so the same side always faces us (we never see the "dark side" from Earth)

Moon Phases

As the Moon orbits Earth, we see different portions of its illuminated side. This creates the cycle of moon phases, repeating every 29.5 days:

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

• New Moon — Moon is between Earth and Sun; unlit side faces us; not visible
• Full Moon — Earth is between Sun and Moon; fully illuminated side faces us
• Waxing — Moon is getting larger (more illuminated) each night
• Waning — Moon is getting smaller (less illuminated) each night

Eclipses

Solar Eclipse

Occurs when the Moon moves between Earth and the Sun, blocking sunlight from reaching parts of Earth.

• Total solar eclipse — the Moon completely covers the Sun (only in the narrow path of the Moon's umbra shadow)
• Partial solar eclipse — only part of the Sun is covered (in the penumbra shadow)
• Only occurs at New Moon (when the Moon is between Earth and Sun)
• The Moon's apparent size in the sky almost perfectly matches the Sun's — a remarkable coincidence

Lunar Eclipse

Occurs when Earth moves between the Sun and Moon, casting Earth's shadow on the Moon.

• Total lunar eclipse — Moon enters Earth's full shadow (umbra); Moon turns red-orange ("blood moon") because some red light is refracted through Earth's atmosphere onto the Moon
• Partial lunar eclipse — only part of the Moon enters Earth's shadow
• Only occurs at Full Moon (when Earth is between Sun and Moon)
• Visible from anywhere on Earth where the Moon is above the horizon

Tides

The Moon's gravitational pull creates tidal bulges in Earth's oceans:

• The ocean directly below the Moon is pulled towards it, creating a high tide
• On the opposite side of Earth, a second high tide forms due to inertia
• The areas in between experience low tide

As Earth rotates, most coastal areas experience two high tides and two low tides each day.

The Sun also affects tides, but less strongly than the Moon (even though it is more massive, it is much further away):

• Spring tides — Sun, Earth, and Moon are aligned (at New Moon and Full Moon); tides are highest and lowest
• Neap tides — Sun and Moon are at 90° to Earth (at First and Last Quarter); tides are less extreme

The Moon's Effect on Earth

• Tides — crucial for coastal ecosystems and historically important for navigation and fishing
• Stabilises Earth's axial tilt — without the Moon, Earth's tilt would wobble chaotically, causing extreme climate swings
• Slows Earth's rotation — tidal friction is gradually slowing Earth's spin; a day was shorter billions of years ago

Key Terms

Term	Definition
Rotation	Earth spinning on its own axis (causes day and night)
Revolution/orbit	Earth moving around the Sun (takes one year)
Axial tilt	Earth's axis is tilted 23.5 degrees relative to its orbit
Satellite	Object that orbits a planet
Solar eclipse	Moon blocks sunlight from reaching Earth
Lunar eclipse	Earth's shadow falls on the Moon
Spring tide	Extra-high tide when Sun, Earth and Moon align
Neap tide	Smaller tides when Sun and Moon are at 90 degrees to Earth
Synchronous rotation	Moon rotates at same rate as it orbits; same side always faces Earth

Worked Example

Q: It is summer in Australia in December. Explain why, using the concept of axial tilt.

Earth's axis is tilted at 23.5 degrees. In December, the South Pole tilts towards the Sun. This means:

1. Sunlight strikes the Southern Hemisphere more directly (at a greater angle), concentrating energy over a smaller area.
2. Days in Australia are longer in December, so the surface receives more total sunlight.

Therefore, December is summer in Australia. At the same time, the Northern Hemisphere is tilted away from the Sun, receiving less direct sunlight and shorter days — so it is winter in countries like the UK.

Common Mistakes

• Thinking seasons are caused by Earth's distance from the Sun — they are caused by axial tilt, not distance.
• Confusing solar and lunar eclipses — solar = Moon in front; lunar = Earth in front (between Sun and Moon).
• Saying the Moon produces its own light — the Moon reflects sunlight; it produces no light of its own.
• Thinking spring tides only happen in spring — spring tides happen at every New Moon and Full Moon throughout the year.

Tips and Tricks

• For eclipses: think of the order — in a solar eclipse, the order from the Sun is: Sun, Moon, Earth. In a lunar eclipse: Sun, Earth, Moon.
• Seasons: when a hemisphere tilts towards the Sun = summer; tilts away = winter.
• Waxing moon = getting bigger (W for "waxing" = working towards full); waning = getting smaller.
• Spring tides occur at New Moon and Full Moon — both times when Sun, Earth and Moon are roughly aligned.