How and why the Earth is tilted are questions still looking for hard fact answers. There are however theories regarding why the Earth and other planets are tilted and the mechanisms of how Earth tilts and what effect the tilt has on the planet and its life forms is mostly understood. Putting it all together may provide a more complete picture for those seeking answers about the unknown.
According to Nasa, Earth, the fifth largest planet in the solar system and third-closest to the Sun and the Moon, was formed at the same time as the rest of the solar system. That would be around 4.5 billion years ago. In the process of formation, Earth and a few other planets ended up tilted instead of perpendicular with their planes of orbit. How this tilt occurred is theorized by some as protoplanets and planetesimals colliding, creating bigger planets and causing a tilt on impact. Another theory is that the Sun, the Moon and the rest of the planets gravitational pull tilts the Earth. These are some theories on how planet Earth got its tilt, but can the degree of the tilt change?
In an interview for Earthsky.org, Richard Gross of NASA’s Jet Propulsion Laboratory says:
“The only way Earth’s tilt or orbit can be affected is if some external force – like an asteroid – hits the Earth.”
In “Conceptual Framework Part2: How have Global Climates Changed?” from AAG Center for Global Geography Education the authors explain a newer theory regarding the forces that seem to affect the axial tilt of the Earth and its orbit that disagrees with the idea that nothing but a collision could alter Earths axial tilt.
Rotational axis and figure axis
Planet earth spins around its axis, which is currently at an angle of about 23.5 Degrees. This is the rotational axis. This axis is an imaginary line that passes through Earth from the North Pole to the South Pole. The celestial Poles are two imaginary points in the sky where the Earth’s rotational axis crosses the celestial sphere, the imaginary rotating sphere of stars. The celestial Poles appear directly overhead to observers at the South and North poles. The apparent circular motion of these stars is due to earth’s rotation, as it spins the celestial poles seem to be fixed in the sky with all other points appearing to rotate around them.
The Earth’s mass is balanced about the figure axis and according to NASA/JPL scientist Margaret Glasscoe earthquakes can shift earth’s figure axis by moving the earth’s mass around. This shift in mass can affect the speed at which the Earth rotates. Margaret explains:
“If you imagine a figure skater who is spinning on the ice, if she were to spread her arms out away from her body, she would spin more slowly. If she brought she arms in closely, she would spin more quickly. In this way, the skater is shifting her mass and changing her angular momentum. This is similar to what is happening to the Earth when a large earthquake shifts its mass around and changes its figure axis. If it changes it enough, it can make the Earth rotate more slowly (lengthening the day) or more quickly (shortening the day).”
It takes Earth 365.25 days to make one full revolution around the sun. This orbit is not quite circular, therefore it is elliptical. Eccentricity is the amount that the orbit is out of roundness (elliptical). Varying from almost 0% to never more than 3.5% over time, the earth’s eccentricity is currently 1.6%. Changing from a nearly circular orbit to an elliptical one would increase and decrease temperatures throughout the year.
The gravitational pull of the moon and to a lesser extent the Sun, alters the rotating axis causing a realignment of this axis called precession. Over approximately 9 000 years, the alteration has been calculated to be about 180 Degrees. This means that Earth, some 10 000 years ago would have been at its closest to the Sun in June and farthest from the Sun in January which is the opposite of today.
The fact that the Earth has seasons is attributed to the earth’s axial tilt. The tilt of 23.5 Degrees seems to agree with earth, providing the seasons in a balanced manner. If there was more of an inclination parts of the world would receive much more Sun while other parts could possibly receive none. Over time, the Earth’s tilt has varied by a range of 2.5 Degrees to 3 Degrees from 21.5 Degrees to 24.5 Degrees. This change also affecting the area of the tropics by the same amount. When the tilt is at its greatest the area of the tropics expands changing the climate in the polar regions that leads to the melting of ice.