The earth spins on its axis at a rate of one rotation per day. This spinning motion is caused by a combination of factors including the earth’s angular momentum, gravity, and conservation of energy. In the opening paragraphs, we’ll answer some key questions about what makes the earth spin.
What force initially set the earth spinning?
When the solar system formed about 4.6 billion years ago, the clouds of gas and dust that condensed into the planets and other objects were likely rotating to some degree. As these clouds collapsed under gravity, they spun faster much like an ice skater spins faster when they pull their arms in. This conservation of angular momentum led the forming earth to spin as well. The earth inherited this initial spin from the angular momentum of the solar nebula.
What keeps the earth spinning today?
The earth continues spinning due to conservation of angular momentum. Angular momentum is a property of rotating objects related to their mass, shape, and speed of rotation. According to the law of conservation of angular momentum, an object will keep spinning at the same rate unless acted on by an external force. Since no large external force is acting to stop the earth’s rotation, it continues spinning day after day.
How fast is the earth spinning?
The earth rotates once every 23 hours, 56 minutes and 4 seconds. This is referred to as one sidereal day. During this time, the planet spins at a speed of about 1,000 miles per hour at the equator. Other key facts about the earth’s rotation:
| Equatorial circumference | 24,901 miles |
| Rate of rotation | 0.000011512 rpm |
| Escape velocity | 25,020 mph |
As can be seen, the earth spins quite slowly given its large size. But the consistent rotation has produced phenomena like night and day and the Coriolis effect.
What direction does the earth spin?
Looking down from space, the earth rotates counter-clockwise or from west to east. This is determined by the original angular momentum of the solar nebula. The planets all revolve around the sun in the same counter-clockwise direction. Oriented with the North Pole up, the earth rotates right to left.
What impact does spin have on the planet?
The earth’s rotation affects many physical systems:
- Produces night and day as areas turn toward and away from the sun
- Shapes winds and ocean currents due to the Coriolis effect
- Causes the flattening of the globe into an oblate spheroid shape
- May influence climate patterns like monsoons
The spin has also had a major impact on biology by creating circadian rhythms matched to the day-night cycle over millions of years.
Is the spin speed changing?
The speed of the earth’s rotation is gradually slowing over time. This deceleration is happening very slowly due to tidal forces between the earth, moon, and sun. The length of a day increases by about 2.3 milliseconds per century. Other impacts from the slower rotation include:
- Slightly weaker Coriolis effects
- Measurably longer days and years
- Necessitates leap seconds to keep civil time accurate
Over billions of years, the earth’s rotation will continue slowing until it becomes tidal locked to the sun like the moon is to the earth. But this will not happen for eons.
What if the earth spun faster or slower?
The speed of the earth’s rotation has a big influence on climate, habitability, and even the appearance of the planet. Some impacts if the spin rate increased or decreased:
| Faster Spin | Slower Spin | |
| Day length | Shorter | Longer |
| Climate effects | Stronger winds and ocean currents from Coriolis effect | Weaker winds and ocean currents |
| Shape of earth | More oblate | Less oblate |
| Gravity | Weaker at equator | More consistent gravity |
A much faster or slower spin would make it hard for life to evolve and thrive. We seem to have the “just right” rotation period.
Conclusion
The earth’s steady and moderate spin rate is due to the conservation of angular momentum from its formation. This spin produces night and day, the Coriolis effect, and shapes geology and biology. While gradually slowing, the rotation speed has remained relatively constant allowing life to flourish and humans to evolve. The exact rate provides the right day length and climate stability for a habitable planet.
