Sun, Earth and Moon: day and night, Moon phases, tides and eclipses

Most of what we notice in the sky comes from three bodies moving together: Earth spinning, the Moon travelling around Earth, and the Sun lighting them both. The Light Study lets you watch each one happen in 3D.

Why do we have day and night?

The Sun always lights the half of Earth that faces it. Earth spins once about every 24 hours, carrying every place into sunlight (day) and then around to the dark side (night). The line between day and night is called the terminator. Try the Day & Night preset and watch it sweep across the continents.

Why does the Moon change shape?

The Moon makes no light of its own; it shines by reflecting sunlight. Half of the Moon is always lit, but as it travels around Earth we see different amounts of that lit half. Going from one full moon to the next takes about 29.5 days. The phases, in order:

1. New moon (the lit side faces away from us)
2. Waxing crescent
3. First quarter (half of the disc looks lit)
4. Waxing gibbous
5. Full moon
6. Waning gibbous
7. Last quarter
8. Waning crescent

Phases are not caused by Earth’s shadow. That only happens during a lunar eclipse. Try the Lunar Phases preset.

What causes ocean tides?

The Moon’s gravity pulls a little harder on the side of Earth nearest to it and a little less on the far side. That difference stretches the oceans into two bulges, so many coasts get two high tides and two low tides about every 24 hours and 50 minutes. The Sun adds a smaller pull. When the Sun and Moon line up at new and full moon, tides are extra large; these are called spring tides. Try the Ocean Tides preset.

What is the difference between a solar eclipse and a lunar eclipse?

• Solar eclipse: the Moon passes directly between the Sun and Earth, and its shadow falls on Earth. This can only happen at new moon.
• Lunar eclipse: Earth passes directly between the Sun and the Moon, and Earth’s shadow falls on the Moon. This can only happen at full moon. The Moon often turns red, because a little sunlight bends through Earth’s atmosphere and reaches it.

Eclipses don’t happen every month because the Moon’s orbit is tilted about 5° compared with Earth’s, so the Moon usually passes just above or below the shadows. Never look straight at the Sun, even during a partial eclipse; use proper eclipse glasses. Try the Solar Eclipse and Lunar Eclipse presets.

What are the northern and southern lights?

The Sun sends out a stream of electrically charged particles. Earth’s magnetic field steers some of them toward the poles, where they crash into gases high in the atmosphere and make them glow green, red and purple. That glow is the aurora. Try the Polar Lights preset.

Sources NASA Moon phases NASA Moon NASA eclipse orbit data NOAA Tides NASA Auroras Reviewed Sep 27, 2026