Physics № 01
Why Is the Sky Blue and Not Violet? The Full Answer
Skylight carries more violet than blue, yet the sky looks blue. We compute the scattered spectrum and run it through human cone data to show why.
Simulations
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Space Interactive What if we replace the Sun?
Space Interactive Spin a space station: artificial gravity
Space Interactive Could we terraform Mars?
Physics Interactive The muon paradox
Space Interactive How small can you squeeze it?
Physics Interactive Orbits and Kepler’s laws
Space Interactive Scale of the universe
Planets Interactive Planet and star size comparison
Planets Interactive Weight on other planets
Space Interactive Moon phases: why does the Moon change shape?
Space Interactive Why do we have seasons?
Space Interactive Solar and lunar eclipse simulator
Earth Interactive How do tides work?
Space Interactive How long does it take to get to Mars?
Space Interactive Rocket equation: why is orbit so hard?
Physics Interactive Projectile motion simulator: which angle goes farthest?
Physics Interactive Pendulum period formula and simple pendulum simulator
Physics Interactive Half-life and radioactive decay simulator
Quantum Interactive Double slit experiment: waves, particles and watching the slits
Relativity Interactive The twin paradox: how much younger does the traveller return?
Light Interactive Snell’s law: how light bends, reflects and gets trapped
Space Interactive Roche limit simulator: turn a moon into rings
Space Interactive Lagrange points simulator: L1 to L5
Space Interactive Retrograde motion simulator: Mars and Mercury
Space Interactive Gravity assist simulator: a Jupiter flyby
Space Interactive Atmospheric escape simulator: can a planet keep its gas?
Relativity Interactive Gravitational time dilation simulator: black hole clocks
Space Interactive Redshift and blueshift simulator: spectral detective
Space Interactive Exoplanet transit simulator and light curve
Space Interactive Star life cycle simulator and H–R diagram
Space Interactive Orbital resonance simulator: repeating planetary rhythms
Physics Coming soon
How body shape changes lift
Featured
Latest posts
Space 01
Why Is James Webb at L2? The Physics, Computed
Webb orbits L2, 1.5 million km out, so Sun, Earth and Moon stay on one side. L2 isn't where gravity cancels, and sitting at it means an endless eclipse.
Physics 02
Twin Paradox Explained: Why the Traveller Ages Less
The twin paradox explained with a worked 0.8c trip: 10 years on Earth, 6 on the ship. Why acceleration is not the cause, plus Doppler counts.
Space 03
Do Rockets Need to Reach Escape Velocity? No. Here's Why
Escape velocity is 11.2 km/s, but no rocket needs it to leave Earth. It is a coasting speed, not a launch requirement. The real cost is energy and delta-v.
Space 04
How Fast Must a Space Station Spin for 1 g?
A station needs about 30/√r rpm for 1 g. Why small stations feel wrong: head-to-foot gradient, Coriolis, and the radius you really need. Worked numbers.
Space 05
Why We Only See One Side of the Moon (and It's 59%)
The Moon is tidally locked, spinning once per 27.3-day orbit. The torque that locked it, why we see 59% of it, and why it drifts 3.8 cm a year.
Space 06
Why Stars Twinkle but Planets Don't (With the Math)
Stars twinkle because they are points that cross one 7 cm air patch at a time. Planets span hundreds of patches. Here is the arcsecond math, with figures.