Axial tilt, also called obliquity, refers to the angle a planet’s rotation axis makes with the plane of its orbit. The Earth is currently tilted 23.5° from this plane, resulting in many remarkable effects, including the seasons around the planet.
Scientists believe that when an object the size of Mars crashed into the newly formed planet Earth around 4.5 billion years ago, it knocked our planet over and left it tilted at an angle.
Giant Impact Hypothesis
- The impact around 4.5 billion years ago is described in the Giant Impact Hypothesis, which is the current prevailing theory on how the Moon was formed and how Earth got its tilt.
- Ever since this impact, Earth has been orbiting the Sun at a slant. This slant is the axial tilt, also called obliquity.
- Earth’s obliquity angle is measured from the imaginary line that runs perpendicular to another imaginary line; Earth’s ecliptic plane or orbital plane.
- At the moment, Earth’s obliquity is about 23.4 degrees and decreasing. We say ‘at the moment’ because the obliquity changes over time, although very, very slowly.
- Earth’s axial tilt actually oscillates between 1 and 24.5 degrees.
- The reason for this changing obliquity angle is that Earth’s axis also wobbles around itself. This wobble motion is called axial precession, also known as precession of the equinoxes.
- It is caused by the gravitational force from the Sun, the Moon, and other planets.
- Axial precession can be described as a slow gyration of Earth’s axis about another line intersecting it.
- A complete wobble of Earth’s axis takes around 26,000 years.
- It outlines the shape of a pair of cones or two spinning tops connected at the tips, which would be at the center of Earth.
- Greek astronomer Hipparchus of Nicea is historically credited as the man who first proposed that Earth’s axis gradually shifts, though very slowly.
- Hipparchus made his discovery around 130 BCE, based on comparisons of astronomical observations more than a century apart.
- A common misconception is that seasons are caused by the Earth moving farther or closer away from the Sun.
- The variation of which hemisphere faces the Sun at a given time, is in fact what causes seasons on Earth.
- When the Northern Hemisphere is facing the Sun, seen in the Fig below it is Summer there.
- The first day of summer, or the summer solstice, occurs when the Northern Hemisphere is maximally facing the Sun.
- This also marks the first day of Winter in the Southern Hemisphere.
- Since the Earth’s rotation axis remains fixed, when the Earth moves to the opposite side of the Sun, it is Winter in the Northern Hemisphere, and Summer in the Southern Hemisphere, as it is now the Southern Hemisphere that receives the majority of the Sun’s incoming energy.
- The tilt also produces effects such as the Midnight Sun, where the Sun never sets during some summer nights in very high-latitude
- The Sun, as seen in the Arctic or Antarctic, where the tilt of the Earth’s axis, relative to the plane of its orbit, produces at least one 24-hour period of daylight, and one of night, in every year.
- At the poles, both day and night are theoretically six months long, though the actual periods of light and dark are modified by the twilight periods.
- The effect of the tilt of the axis is seen in lower latitudes as a lengthening of daylight hours in the summer and their shortening in the winter.
- The tilt of the Earth results in the poles not receiving as much energy as the equator – at a 23.5° tilt, the poles only get around 40% of the energy the equator gets.
- This 23.5° tilt is also not set indefinitely as it changes over long periods of time (around 40,000 years), ranging between 22.1° – 24.5° (a factor in natural climate change).
- Both of these factors allow for ice to build up year-after-year in high latitudes, eventually creating massive ice sheets.
- This ice has drastic effects on the Earth’s climate system due to its high albedo, among other things.
- The wobbling has an interesting effect on climate and on navigation. Eleven thousand years ago, the North Star was Vega; currently, the North Star is Polaris.
- The North Star has changed over the past 11,000 years, and in another 11,000 years it will switch back to Vega again.
- The North Star changes because of precession and is slightly influenced by the change in the angle of the Earth’s tilt.
- The axis tilt of the Earth defines the “tropics”, the “tropic” being the low latitude where the Sun is directly overhead for any day of the year.
- At present this is at 23° 26′ 22″ and is decreasing. T
- his will result in a less well-defined difference between summer and winter over a span of a few thousand years.
- During the past 100 years the tropics have become about 2 km narrower.
- Observers in Taiwan have placed monuments at the “line of return” or the latitude of the tropic.
- The oldest surviving dates to 1908 and is more than 1 km from the present latitude of the tropic.
- Axial tilt contributes to the Milankovitch cycles that have changed the Earth’s climate in the past.
- A century ago, Serbian scientist Milutin Milankovitch hypothesized the long-term, collective effects of changes in Earth’s position relative to the Sun are a strong driver of Earth’s long-termclimate, and are responsible for triggering the beginning and end of glaciation periods (Ice Ages).
- Specifically, he examined how variations in three types of Earth orbital movements affect how much solar radiation (known as insolation) reaches the top of Earth’s atmosphere as well as where the insolation reaches.
- These cyclical orbital movements, which became known as the Milankovitch cycles, cause variations of up to 25 percent in the amount of incoming insolation at Earth’s mid-latitudes (the areas of our planet located between about 30 and 60 degrees north and south of the equator).
- The Milankovitch cycles include:
- The shape of Earth’s orbit, known as eccentricity;
- The angle Earth’s axis is tilted with respect to Earth’s orbital plane, known as obliquity; and
- The direction Earth’s axis of rotation is pointed, known as precession.
- Earth’s annual revolution around the Sun isn’t perfectly circular, but it’s pretty close.
- Over time, the pull of gravity from our solar system’s two largest gas giant planets, Jupiter and Saturn, causes the shape of Earth’s orbit to vary from nearly circular to slightly elliptical.
- Eccentricity measures how much the shape of Earth’s orbit departs from a perfect circle.
- These variations affect the distance between Earth and the Sun.
- Eccentricity is the reason why our seasons are slightly different lengths, with summers in the Northern Hemisphere currently about 4.5 days longer than winters, and springs about three days longer than autumns.
- As eccentricity decreases, the length of our seasons gradually evens out.
- The difference in the distance between Earth’s closest approach to the Sun (known as perihelion), which occurs on or about January 3 each year, and its farthest departure from the Sun (known as aphelion) on or about July 4, is currently about 5.1 million kilometers (about 3.2 million miles), a variation of 3.4 percent. That means each January, about 6.8 percent more incoming solar radiation reaches Earth than it does each July.
- When Earth’s orbit is at its most elliptic, about 23 percent more incoming solar radiation reaches Earth at our planet’s closest approach to the Sun each year than does at its farthest departure from the Sun.
- Currently, Earth’s eccentricity is near its least elliptic (most circular) and is very slowly decreasing, in a cycle that spans about 100,000 years.
- The total change in global annual insolation due to the eccentricity cycle is very small.
- Because variations in Earth’s eccentricity are fairly small, they’re a relatively minor factor in annual seasonal climate variations.
- The angle Earth’s axis of rotation is tilted as it travels around the Sun is known as obliquity.
- Obliquity is why Earth has seasons.
- Over the last million years, it has varied between 22.1 and 24.5 degrees perpendicular to Earth’s orbital plane.
- The greater Earth’s axial tilt angle, the more extreme our seasons are, as each hemisphere receives more solar radiation during its summer, when the hemisphere is tilted toward the Sun, and less during winter, when it is tilted away.
- Larger tilt angles favor periods of deglaciation (the melting and retreat of glaciers and ice sheets).
- These effects aren’t uniform globally — higher latitudes receive a larger change in total solar radiation than areas closer to the equator.
- Earth’s axis is currently tilted 23.4 degrees, or about half way between its extremes, and this angle is very slowly decreasing in a cycle that spans about 41,000 years.
- It was last at its maximum tilt about 10,700 years ago and will reach its minimum tilt about 9,800 years from now.
- As obliquity decreases, it gradually helps make our seasons milder, resulting in increasingly warmer winters, and cooler summers that gradually, over time, allow snow and ice at high latitudes to build up into large ice sheets.
- As ice cover increases, it reflects more of the Sun’s energy back into space, promoting even further cooling.
- As Earth rotates, it wobbles slightly upon its axis, like a slightly off-center spinning toy top.
- This wobble is due to tidal forces caused by the gravitational influences of the Sun and Moon that cause Earth to bulge at the equator, affecting its rotation.
- The trend in the direction of this wobble relative to the fixed positions of stars is known as axial precession. The cycle of axial precession spans about 25,771.5 years.
- Axial precession makes seasonal contrasts more extreme in one hemisphere and less extreme in the other.
- Currently perihelion occurs during winter in the Northern Hemisphere and in summer in the Southern Hemisphere.
- This makes Southern Hemisphere summers hotter and moderates Northern Hemisphere seasonal variations.
- But in about 13,000 years, axial precession will cause these conditions to flip, with the Northern Hemisphere seeing more extremes in solar radiation and the Southern Hemisphere experiencing more moderate seasonal variations.
- Axial precession also gradually changes the timing of the seasons, causing them to begin earlier over time, and gradually changes which star Earth’s axis points to at the North Pole (the North Star).
- Today Earth’s North Stars are Polaris and Polaris Australis, but a couple of thousand years ago, they were Kochab and Pherkad.
- There’s also apsidal precession.
- Not only does Earth’s axis wobble, but Earth’s entire orbital ellipse also wobbles irregularly, primarily due to its interactions with Jupiter and Saturn.
- The cycle of apsidal precession spans about 112,000 years.
- Apsidal precession changes the orientation of Earth’s orbit relative to the elliptical plane.
- The combined effects of axial and apsidal precession result in an overall precession cycle spanning about 23,000 years on average.
Thus, the small changes set in motion by Milankovitch cycles operate separately and together to influence Earth’s climate over very long timespans, leading to larger changes in our climate over tens of thousands to hundreds of thousands of years.