Moon Phases
Why the Moon waxes and wanes over 29.5 days. Switch between a realistic 3-D Moon and a top-down Sun-Earth-Moon diagram.
InteractiveEclipses
Why solar and lunar eclipses happen only when a new or full moon lands near a lunar node, in two synced views.
InteractiveThe Metonic Cycle
Why the Moon's phases return to the same dates every 19 years: 235 months almost equal 19 years, the Golden Number, and leap months.
InteractiveThe Lunar Nodes and Standstills
The slowly turning points where the Moon's path crosses the Sun's, the 18.6-year cycle they trace, and a clock for the next major and minor lunar standstills.
InteractiveThe Tides
Why the sea rises twice a day, why high tide slips about 50 minutes later each day, and how the Moon and Sun team up for spring and neap tides.
InteractiveTidal Locking & Libration
Why the Moon keeps one face toward Earth, yet a slow rock called libration lets us see about 59% of its surface. The far side is not the dark side.
InteractiveSun's Declination & Seasons
How Earth's 23.4° tilt swings the Sun's declination over the year, making the equinoxes, solstices and seasons.
InteractiveThe Equation of Time
Why a sundial runs up to a quarter-hour ahead of or behind the clock, and the figure-eight analemma it traces.
InteractivePrecession of the Equinoxes
The 25,920-year wobble of Earth's axis that slowly reshuffles the pole star and the zodiac.
InteractiveApsidal Precession
How the Moon's and the planets' elliptical orbits slowly rotate: the turning of perigee and perihelion.
InteractiveKepler's Three Laws
Ellipses with the Sun at one focus, equal areas in equal times, and T² = a³: the three rules every orbit obeys, live.
InteractiveVenus: the 8-Year Pentagram
The morning and evening star, its ~47° dance with the Sun, and the five-petaled rose it draws over eight years.
InteractiveRetrograde Motion
Why Mars, Jupiter and Venus seem to loop backward: an illusion of Earth overtaking them, traced across the sky.
InteractiveSynodic vs Sidereal Periods
Why the Moon orbits in 27.3 days but cycles its phases in 29.5: two ways to time an orbit, the lapping formula, and the Moon's five different months.
InteractiveSidereal vs Solar Day
Why a solar day runs about 4 minutes longer than a sidereal day, the extra degree Earth turns each day, and why the stars rise 4 minutes earlier each night.
InteractiveMeteor Showers
Watch Earth cross a comet's debris stream, see why every shower meteor traces back to one radiant, and meet the big four.
InteractiveThe Sunspot Cycle
The Sun's 11-year rise and fall in spots, the 22-year magnetic flip, and the butterfly diagram.
InteractiveThe Galactic Year
The Sun's ~230-million-year orbit of the Milky Way: ride one lap, from the first dinosaurs to today.
InteractiveThe Milankovitch Cycles
How the slow flexing of Earth's orbit and tilt, eccentricity, obliquity and precession, paces the ice ages.
InteractiveHow Celestial Navigation Works
How sailors find position from the Sun and stars: the circle of equal altitude, the fix from two sights, and the intercept method.
How this section works
Every topic is built around something you can move and explore, not just read. The visuals are kept faithful to the real geometry, simplified only where a diagram has to be (distances and sizes are rarely to scale, and we say so). Where a topic has a measurable cycle, it links through to the cycles by length list and the calculators so you can put real numbers to what you have just seen.
Every topic here is a hands-on explainer of how the sky actually works. Live now: Moon phases, the tides, tidal locking and libration, eclipses, the Metonic cycle, the lunar nodes, the seasons, the sidereal and solar day, precession, apsidal precession, Kepler's three laws, Venus's 8-year cycle, retrograde motion, synodic vs sidereal periods, meteor showers, the sunspot cycle, the equation of time, the galactic year, the Milankovitch cycles, and how celestial navigation works. New explainers are added regularly.