# Definition:Cardinal Directions

## Definition

The $4$ cardinal directions:

are the basis of navigation and geographical coordinates.

## Cardinal Directions (Terrestrial)

The $4$ **cardinal directions** are the directions on (or near) Earth's surface defined as follows:

### East

**East** is the direction on (or near) Earth's surface along the small circle in the direction of Earth's rotation in space about its axis.

### West

**West** is the direction on (or near) Earth's surface along the small circle in the opposite direction from Earth's rotation in space about its axis.

### North

**North** is the direction on (or near) Earth's surface along the meridian directly towards the North Pole.

### South

**South** is the direction on (or near) Earth's surface along the meridian directly towards the South Pole.

## Cardinal Points (Celestial)

The **cardinal points** of the celestial sphere are the **north point**, the **sourth point**, the **east point** and the **west point**:

### North Point of Horizon

The point on the celestial horizon through which the principal vertical circle passes is called the **north point of the horizon**.

### South Point of Horizon

The point on the celestial horizon exactly opposite the **north point of the horizon** is called the **south point of the horizon**.

### East Point of Horizon

Let an observer $O$ be facing the **north point of the horizon** $N$, facing directly away from the **south point of the horizon** $S$.

Let the celestial equator meet the celestial horizon at the points $E$ and $W$.

Let $E$ be the point on the right of $O$.

This is called the **east point of the horizon**.

### West Point of Horizon

Let an observer $O$ be facing the **north point of the horizon** $N$, facing directly away from the **south point of the horizon** $S$.

Let the celestial equator meet the celestial horizon at the points $E$ and $W$.

Let $W$ be the point on the left of $O$.

This is called the **west point of the horizon**.

## Also known as

The **(terrestrial) cardinal directions** are often referred to as the **cardinal points**, despite the fact that they are not actually points.

The **(celestial) cardinal points** are also seen referred to as the **cardinal directions** when the context refers to the direction towards those **cardinal points**.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**cardinal points** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**cardinal points**