Interactive physics simulator
The Moon — Earth's Natural Satellite
Watch the Moon orbit Earth in real time, visualise all 8 lunar phases with accurate terminator rendering, explore tidal forces, and inspect physical data — all computed live from real orbital mechanics.
Moon Phase & Orbital Simulator
Click any phase name to jump to it. Drag speed to fast-forward time. Toggle tidal vectors and orbit trail in the panel.
Live Orbital Data
- Phase
- New Moon
- Illumination
- 0%
- Phase Angle
- 0°
- Distance
- 384,400 km
- Orbital Speed
- 1.02 km/s
- Day (Synodic)
- 0.0
- Tidal Force
- —
- Period
- 29.53 d
What is the Moon?
The Moon is Earth's only natural satellite — a rocky, airless body orbiting Earth at an average distance of 384,400 km. With a diameter of 3,474 km (about ¼ of Earth's), it is the fifth-largest moon in the Solar System and the largest relative to its host planet.
The Moon formed approximately 4.5 billion years ago, most likely from debris ejected when a Mars-sized body (Theia) collided with the early Earth — the Giant Impact Hypothesis.
- Sidereal period: 27.321 days (one orbit relative to the stars)
- Synodic period: 29.53 days (new moon to new moon — one phase cycle)
- Orbital eccentricity: 0.0549 (slightly elliptical)
- Axial tilt: 1.54° relative to its orbital plane
Key Formulas
v = 2πr / T v = orbital speed (m/s), r = orbital radius (m), T = period (s)
g = GM / R² G = 6.674×10⁻¹¹ N·m²/kg², M = mass, R = radius
f = (1 − cos θ) / 2 θ = phase angle (0° = New Moon, 180° = Full Moon), f = fraction illuminated (0 to 1)
ΔF = 2GMmr / d³ M = Moon mass, m = Earth test mass, r = Earth radius, d = Earth–Moon distance
v_esc = √(2GM/R) = 2.38 km/s — too low to retain an atmosphere long-term
Lunar Phases Explained
Phases arise because we see different fractions of the Moon's sunlit hemisphere as it orbits Earth. The Moon itself does not emit light — it reflects sunlight.
| Phase | Angle | Illumination | Rise | Set |
|---|---|---|---|---|
| 🌑 New Moon | 0° | 0% | ~06:00 | ~18:00 |
| 🌒 Waxing Crescent | 45° | ~15% | ~09:00 | ~21:00 |
| 🌓 First Quarter | 90° | 50% | ~12:00 | ~00:00 |
| 🌔 Waxing Gibbous | 135° | ~85% | ~15:00 | ~03:00 |
| 🌕 Full Moon | 180° | 100% | ~18:00 | ~06:00 |
| 🌖 Waning Gibbous | 225° | ~85% | ~21:00 | ~09:00 |
| 🌗 Last Quarter | 270° | 50% | ~00:00 | ~12:00 |
| 🌘 Waning Crescent | 315° | ~15% | ~03:00 | ~15:00 |
Solved Examples
Calculate the orbital speed of the Moon around Earth.
- Given: orbital radius r = 3.844 × 10⁸ m, period T = 27.32 days = 2.36 × 10⁶ s
- Orbital circumference = 2πr = 2π × 3.844 × 10⁸ = 2.415 × 10⁹ m
- v = circumference / T = 2.415 × 10⁹ / 2.36 × 10⁶
Answer: v ≈ 1023 m/s ≈ 1.02 km/s
What is the surface gravity on the Moon?
- Given: M = 7.342 × 10²² kg, R = 1.737 × 10⁶ m, G = 6.674 × 10⁻¹¹ N·m²/kg²
- g = GM / R² = (6.674 × 10⁻¹¹ × 7.342 × 10²²) / (1.737 × 10⁶)²
- g = 4.899 × 10¹² / 3.017 × 10¹²
Answer: g_moon ≈ 1.62 m/s² (about 1/6 of Earth's gravity)
How long does it take for the Moon to complete one synodic period (new moon to new moon)?
- The sidereal period (one orbit relative to stars) = 27.32 days
- During this time Earth also moves ~27° around the Sun
- Extra time for Moon to catch up to same Sun-Earth-Moon angle: ~2.21 days
- Synodic period = sidereal period + catch-up time
Answer: Synodic period ≈ 29.53 days
Common Mistakes
- Thinking the Moon emits light — the Moon only reflects sunlight.
- Confusing the sidereal period (27.32 d, orbit) with the synodic period (29.53 d, phases).
- Believing a lunar eclipse happens every Full Moon — the 5.1° orbital tilt means most Full Moons miss Earth's shadow.
- Thinking tides are caused only by the Moon — both the Moon and the Sun contribute; when aligned they produce spring tides.
- Claiming the Moon has no gravity — it has g = 1.62 m/s² (1/6 of Earth's).
Practice Questions
1. What are the 8 phases of the Moon in order?
New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent.
2. Why does the Moon always show the same face to Earth?
The Moon is tidally locked — its rotation period equals its orbital period (both ~27.32 days). This is caused by Earth's gravitational tidal forces slowing the Moon's rotation over billions of years.
3. A student weighs 600 N on Earth. What is their weight on the Moon?
Weight on Moon = mass × g_Moon. Since g_Moon ≈ 1.62 m/s² and g_Earth = 9.8 m/s², weight on Moon = (600/9.8) × 1.62 ≈ 99.2 N. (About 1/6th).
4. What is the difference between a Solar eclipse and a Lunar eclipse?
Solar eclipse: Moon passes between Earth and Sun — Moon's shadow falls on Earth (Moon blocks Sun). Lunar eclipse: Earth passes between Sun and Moon — Earth's shadow falls on Moon.
5. What causes tides on Earth?
Tides are caused by the Moon's differential gravitational pull on different parts of Earth. The side facing the Moon is pulled more strongly, creating a bulge. The opposite side experiences less pull (relative to Earth's centre), creating a second bulge. These bulges produce high tides.
Frequently Asked Questions
How far is the Moon from Earth?
The Moon's average distance is 384,400 km (about 60 Earth radii). Distance varies from 356,500 km (perigee) to 406,700 km (apogee) because its orbit is elliptical.
Why does the Moon appear to change shape?
The Moon does not change shape — we see varying fractions of its sunlit half as it orbits Earth. These changing lit portions are called lunar phases.
What is a supermoon?
A supermoon occurs when a Full Moon coincides with perigee (closest orbital point). The Moon appears up to 14% larger and 30% brighter than at apogee.
Does the Moon rotate on its own axis?
Yes — once every 27.32 days, which equals its orbital period. This tidal locking means we always see the same nearside hemisphere.
What is the far side of the Moon?
The hemisphere we never see from Earth. It was first photographed by the Soviet Luna 3 probe in 1959. It has more craters and fewer dark volcanic maria than the nearside.
What are maria on the Moon?
Maria (singular: mare) are large, dark, ancient lava plains formed by volcanic eruptions 3–4 billion years ago. They cover about 16% of the lunar surface.
What causes a Blue Moon?
A Blue Moon is either the second Full Moon in a calendar month, or the third Full Moon in a season with four Full Moons. It occurs roughly every 2.5 years.
How does the Moon affect ocean tides?
The Moon's gravity raises two ocean bulges — one toward the Moon and one away. As Earth rotates under these bulges, coastlines experience two high tides and two low tides roughly every 24 hours 50 minutes.
What is a lunar eclipse?
A lunar eclipse occurs when Earth passes directly between the Sun and Moon, casting its shadow on the Moon. Total lunar eclipses can turn the Moon reddish ("blood moon") because Earth's atmosphere refracts red sunlight into the shadow.
What is the Moon's gravity compared to Earth?
The Moon's surface gravity is 1.62 m/s² — about 1/6 of Earth's 9.8 m/s². An object weighing 60 N on Earth weighs only ~10 N on the Moon.
What is tidal locking?
Tidal locking is when one body's rotation period matches its orbital period around another body. Earth's tidal forces gradually slowed the Moon's spin over ~1 billion years until it synchronized with its orbit.
Could humans live on the Moon?
The Moon has no atmosphere, extreme temperature swings (−173°C to 127°C), and is exposed to solar radiation. Humans could live there only inside pressurised habitats. Ice water detected in permanently shadowed polar craters could support future colonies.