See the current lunar phase, illumination percentage, and where today sits in the 29.5-day cycle. Upcoming key phases at a glance.
Tidal friction between the Earth and moon is gradually transferring angular momentum from Earth's rotation to the moon's orbit. The result: Earth's day lengthens by about 1.4 milliseconds per century, and the moon drifts 3.8 centimetres further away each year. When the dinosaurs were alive, a day was roughly 23 hours. In the distant past, the moon would have appeared noticeably larger in the sky.
What we call the lunar phases are a consequence of geometry. As the moon orbits Earth, the angle between the sun, Earth, and moon shifts continuously, changing how much of the moon's illuminated face is visible from the surface. One full cycle takes 29.53 days, known as the synodic month. It starts at the new moon, when the lit side faces entirely away from us, and peaks at the full moon, when Earth sits directly between the sun and moon.
The four key phases occur when that sun-Earth-moon angle hits exactly 0°, 90°, 180°, and 270°. Everything in between is a gradual transition. The illumination percentage shown above is derived from that angle directly: 0% at new moon, 50% at each quarter, 100% at full.