Seasons

Seasons are the four main moments of the climate at Earth. It is badly known that seasons are an astronomical phenomenon indeed

Earth Axis Tilt

Seasons are explained by Earth's orbit and by Earth axis' tilt. The Earth's axis is the line joining Earth north and south poles. It's the axis around which Earth is rotating. This axis is tilted. It is tilted relative to ecliptic. The ecliptic is the plane of the Earth's orbit's around the Sun. Technically, the tilt is computed relative to a perpendicular to the ecliptic. The tilt is about 23.5° in any case

The tilt of the Earth does not vary as Earth is orbiting the Sun. Which makes that at some point of the orbit the axis is tilted away from the Sun as at another point it is tilted towards. Two more positions are when the axis, although tilted still, is neutral relative to the Sun. It is these four main positions -tilt away from the Sun, tilt towards the Sun, tilt neutral, which produce the seasons. This is due to the angles at which the rays of the Sun are reaching Earth being different. When Earth northern hemisphere e.g. is tilted away from the Sun, Sun rays come there at a shallow angle as when the hemisphere is tilted towards the Sun, Sun rays come at a sharp angle. Hence less, or more, heat. The length of days is a cause too. Days are longer during summer and shorter during winter, hence too more or less heat

The Four Seasons

The motion of the Earth around the Sun, relative to the axis tilt, is characterized by four particuliar moments

• Earth' axis northern tip is most away from the Sun, as southern is most toward
• neither axis' northern or southern tip is tilted toward Sun
• axis' northern tip is most tilted towards the Sun, as South is most away
• neither axis' northern or southern tip again is tilted toward Sun

The axis tilt determines four peculiar moments along the orbit. These four moments in turn are determining the seasons. As far as Earth's hemispheres are concerned, seasons are reverted. When it's winter North, is summer South e.g. This is due to that when the northern hemisphere is at a given tilt relative to the Sun, the southern hemisphere is correlatively at the opposite tilt. Strictly speaking equatorial regions do not have any delineated seasons. Temperatures, vegetation, and the length of days are remaining identical all year long

• first moment (Earth axis' North is most away from the Sun, as South is most toward -right on sketch above) determines the northern winter and the southern summer. North, Sun rays are reaching Earth at a shallow angle. Sun is low in sky. It is rising at its farthest southeast and it is setting at its farthest southwest. Days are year's shortest. These data are just the reverse for the southern hemisphere. It's the midnight sun in southernmmost latitudes, above the Antarctic Circle. Astronomically, this moment is called the winter, or the summer, solstice. It takes place about December, 21st
• second moment (neither axis' northern or southern tip is tilted toward Sun -top) determines northern spring and southern fall. North, and South, Sun rays are striking the surface only moderately. Sun is moderately high in the sky. It is rising just East and it is setting just West. Days equal nights. Things are reverted in the southern hemisphere. Astronomically this is called the vernal, or the autumnal, equinox. It takes place about March, 20th-21st
• third moment (axis' northern tip is most tilted towards the Sun, as South is most away -left) determine the northern summer and the southern winter. North Sun rays are reaching Earth at a sharp angle. Sun is high in sky. It is rising at its farthest northeast and it is setting at its farthest northwest. Days are year's longest. At northernmost latitudes, above the Arctic Circle, it is the midnight sun. Data are the reverse in the southern hemisphere. Astronomically this is the summer, or the winter, solstice. It takes place about June, 21st
• last moment (neither axis' northern or southern tip again is tilted toward Sun -bottom) determines northern fall and southern spring. North and South Sun rays are striking moderately again. Sun is moderately high in the sky. It is rising just East and it is setting just West. Days equal nights. Astronomically this is called the autumnal, or the vernal, equinox. It takes place around September, 20th-21st

Another interesting, advanced, point is that Earth's orbit is not exactly a circle but an ellipse. Hence Earth is nearest to the Sun at one point of its orbit and farthest from it at another point. The nearest point is termed the "perihelion", the farthest is the "aphelion". Both words are of Greek origin and mean near ("peri") Sun ("helios"), and far ("a") from Sun. Perihelion differs of aphelion by 3 million miles. Perihelion takes place each year about January, 2nd as aphelion is about July, 4th. As such an inconsistency is of few importance in the northern hemisphere, it is much more important in the southern hemisphere. It's when the southern part of the Earth is tilted towards the Sun that Earth, at the same time, is nearest to the Sun hence southern summers are warmer than northern! On the other hand, the aphelion does not yield colder southern winters however. This might be due to the oceans. Their surface is much more important in the southern hemisphere and they might have a tempering effect

The Cyclical Variations of Earth's Axis and Orbit

Earth's orbit and axis are affected by various cyclical variations. Most of these cycles have been described by Milutin Milankovitch, a Serbian scientist in the 1920s, as part of science he was making about the long-term variations of the Earth climate. One of those variations only, the 'precession", is of importance in astronomy, and has been known since a long time. Others are of few interest to astronomers and are just part of the usual orbital fluctuations in a solar system

• the 26,000-year axis' tilt variation. This variation is most known as "precession". Due to the gravitational tugs of the Sun and the Moon, the Earth axis' tilt describes a circle like a spinning top. Motion is clockwise. It is a little more than half a degree each century. Its most famous effect is that the north celestial pole is apparent wandering along stars. a of the Dragon was the Polaris 6500 years ago as Vega of the Lyre will be 14,000 years from now! The other most noticeable effect of the precession is illustrated by the sketch above. It consists into the -very slow- shift of the seasons as the modification of how the Earth' axis is tilted relatively to the Sun, determines that the tilt which defines winter shift into the tilt defining spring, etc. This shift, on the other hand, does not affect the weather which is brought by each season. In the northern hemisphere, like an example, when the northern tip of the Earth's axis is tilted away from the Sun, it keeps being, meteorologically, winter. The shift of the seasons due to precession, further, occurs on long durations of time -in the order of 6,500 years- which do no allow the Earth dwellers to note the change. The shift of the seasons, at the opposite -albeit as slow and progressive however- is more noticeable on an observational, astronomical point of view as, if the observer, at the moment of the winter solstice, will get snow and cold, he will not, at all, see the same sky at night! Progressively, along a duration of 6,500 years, the constellations he was seeing during the winter nights, will have give way to those he was seeing during spring! This being due to that the seasons, on the orbit, now occur at positions from which the Sun is no more, in perspective, seen against the same background of stars
• the 100,000-year orbit eccentricity variation brings Earth's orbit eccentricity to vary from 0 -about a circle- to 3 times its present value. A circular orbit usually brings a ice age, as we are currently headed to such a circularity in 27,000 years from now
• as the present Earth's axis tilt of 23.5° is in the middle range, the 41,000-year obliquity cycle is adding some degrees, more or less
• the 21,000-year perihelion variation has the aphelion-perihelion axis shifting on the orbit
• another 21,000-year perihelion variation makes the perihelion shifts relative to the calendar year. One year from perihelion to perihelion (an "anomalistic year") is about 25 minutes longer than one year from equinox to equinox (a "tropical year"). The shift is one day each 58 years

The precession, as it combines itself to the fundamentals of the Earth's orbit -and even with the other cyclical variations as described above- certainly, obviously impacts, on the long term, the Earth's climate. The shift, on the orbit, of the moments of the seasons, thus, for example, is shifting the relationship between summers and winters, and the aphelion and the perihelion