The Earth's **rotational motion**—its spinning on its axis—originated from the processes that formed the Solar System about **4.6 billion years ago**. Here’s how it happened:
### **1. Formation of the Solar Nebula**
- The Solar System began as a **giant molecular cloud** (mostly hydrogen and helium with dust) that collapsed under gravity.
- As it collapsed, conservation of **angular momentum** caused it to spin faster (like an ice skater pulling in their arms).
### **2. Protoplanetary Disk Formation**
- The spinning cloud flattened into a **protoplanetary disk** (the **solar nebula**), with most material orbiting the center (which became the Sun).
- Small particles collided and stuck together, forming **planetesimals** (early planetary building blocks).
### **3. Earth’s Formation and Inherited Rotation**
- As Earth formed from collisions of planetesimals and larger bodies (like **Theia**, which may have led to the Moon’s formation), the combined angular momentum of these impacts set Earth spinning.
- The **conservation of angular momentum** ensured that the rotation persisted.
### **4. Why Does Earth Keep Spinning?**
- In the vacuum of space, there’s little friction to stop Earth’s rotation.
- The initial spin has been gradually slowed by **tidal forces** (mostly the Moon’s gravitational pull), but Earth still rotates once every **~24 hours**.
### **Key Factors:**
- **Angular Momentum Conservation**: The original spin of the solar nebula was passed down to planets.
- **Collisional Accretion**: Impacts during Earth’s formation contributed to its rotation.
- **Tidal Effects**: The Moon and Sun have slightly modified Earth’s rotation over time.
### **Did Something "Start" Earth’s Spin?**
No single event "started" it—Earth’s rotation is a natural result of the way the Solar System formed. Without an external force to stop it, the spin continues due to inertia.