# Why the Moon Has Phases

Why the Moon has phases: the boiling Sun, the sunlit half of a real\-textured Moon, its orbit, and the phases seen from Earth over a lake and a city

By Numa

Topic: Space

[Open the interactive Jig](https://ask.jig.com/j/LVriCDTHMzwG)

## Guide

### 1. A Moon that changes shape

Tonight the full Moon hangs over the lake, round and bright\. In a week it will be half a Moon, and in two weeks it will vanish\. Why does it keep changing shape?

One full cycle of phases takes about 29\.5 days\.

### 2. It starts with the Sun

The Sun is a boiling ball of hot plasma\. Its bright cells rise, cool and sink in minutes, like a pot of thick soup\. It is the only lamp in our corner of space\.

Each bright cell is about the size of France\.

### 3. Half the Moon is always lit

The Moon makes no light of its own\. Sunlight falls on the half facing the Sun, and the other half is in night\. Drag the scene to look around to the dark side\.

Moon rock is as dark as worn asphalt\. It only looks bright against the black sky\.

### 4. The Moon circles Earth

Every month the Moon travels once around Earth\. The Sun keeps lighting the side that faces it, so the sunlit half always points toward the Sun, wherever the Moon is\.

Not to scale: the real Moon is 30 Earth widths away, and the Sun 400 times farther\.

### 5. What we see from Earth

From Earth we see a different share of the sunlit half as the Moon goes round\. Slide the Moon around its orbit: between us and the Sun it is new, opposite the Sun it is full\.

Phases are not Earth's shadow\. That only happens in a lunar eclipse\.

### 6. A crescent over the city

A few days after new Moon, a thin crescent follows the Sun down in the west\. Its lit edge faces the sunset, and the dark part glows faintly with light reflected from Earth\.

That faint glow is called earthshine\.

### 7. Full Moon: Earth in the middle

At full Moon, Earth sits between the Sun and the Moon, so we see the whole sunlit face\. It rises as the Sun sets and lays a shining path across the water\.

Stars twinkle because our air bends their light\. In space they shine steadily\.