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Q & A: The Direction of Earth’s Rotation and Its Tilt

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Most recent answer: 10/22/2007
Q:
When the earth was formed millions of years ago by the "big bang" theory, how did the earth start a steady rotation on a 23.5 degree angle?...and what determined the direction of rotation?
- Kevin
Indianapolis, IN
A:
The Earth rotates because of conservation of angular momentum. An ice skater provides a common example of this effect; as an ice skater brings his or her arms in closer to his or her body, he or she will spin faster.
As the solar system formed from condensing gases, it would by accident have had some angular momentum. In other words, the average velocities of the material on different parts of the cloud weren't quite the same. When nearly all the mass collapsed into the Sun, the only way for the angular momentum to stay the same was for parts of the cloud to stay out of the central star and to rotate quickly around the star. That's why all the planets orbit the same way around the sun. When that material condensed further, forming the Earth and the other planets, these also had to spin about their axes to keep the angular momentum from changing. Most of the planets spin around axes close to the same as the one around which they orbit the Sun. However, accidents that happened to each planet affected the exact rate and direction of the spin.

The Earth has not always rotated at 23.5 degrees. Many factors effect the angle of Earth's tilt. The motion of large air masses can change its tilt by measurable amounts even today. During the major Ice Ages, large sheets of ice on Earth's surface and lowered sea level may have caused the Earth to have an even larger tilt. Small changes in the Earth's surface like continental drift and variations of sea level acting over million of years time may change the tilt significantly.

Last, the Earth's spinning was affected by collisions with other bodies while it was forming. The last major impacts with a body 1000 kilometers across was about 4.5 billion years ago. The impact tore a chunk of material out of the Earth, and formed a ring of rock and debris around our planet which later became the Moon. At least that's our best current understanding of the process.

(published on 10/22/2007)

Follow-Up #1: Planetary orbit and tilt angles

Q:
How common among other planets is the Earth’s tilt in its angle of rotation?
- Mike Tinghitella (age 66)
Fairview, TX, USA
A:
Hi Mike,
The planets’ orbits around the sun are all within a few degrees of a common plane called the ecliptic plane.  Pluto, the odd ball planet recently downgraded in status, is the exception with an orbital tilt of about 17 degrees.   

The spin rotation axes, however, are all over the map.
They seem to be randomly distributed from  Mercury (0.1 degrees), Earth (23.45), to Venus (177 degrees).  These variations are probably are due to random collisions during the early days of the formation of the solar system. 
My source of information comes from http://www.astronomynotes.com/tables/tablesb.htm

LeeH






(published on 10/22/2007)

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