Three balls in space. Their dance makes our days, our months, our years, and some amazing things in the sky.
Earth spins like a top. The Moon circles Earth. Earth circles the Sun. Pick a speed and watch.
The Sun lights up one half of Earth at a time. As Earth spins, you ride from the sunny side into the shadow and back. One spin is one day.
The Moon doesn't glow. The Sun lights half of it, always. As it circles us, we see more or less of that sunny half. Drag the Moon around, or tap a phase.
Sometimes the three line up perfectly. Then someone's shadow lands on someone else.
So why not every month? The Moon's path is tipped a little. Most months its shadow sails just above or below Earth. Only about twice a year do they line up.
Earth is tipped over a bit, and it stays tipped the same way all year. When your half leans toward the Sun you get summer: higher Sun, longer days. Leaning away is winter.
The Moon's gravity stretches Earth's oceans into two bulges. As Earth spins, your beach passes through both bulges, so you get two high tides a day.
The pictures above squeeze everything together. Here is the real scale.