Obliquity of the ecliptic
The obliquity of the ecliptic (sometimes also called simply obliquity) is the angle of inclination of the Earth's axis of rotation with respect to a perpendicular to the plane of the ecliptic. He is responsible for the seasons of the year. It is not constant but changes over time due to two cycles: 1. A short cycle of 18.6 years with an oscillation of 9.2 seconds of arc on the value of its inclination, called nutation movement. 2. Due to another much longer cycle of about 40,000 years that makes it oscillate between 21.8° and 24.4°.
The plane of the terrestrial equator and that of the ecliptic intersect in a line that has the Aries point at one end, and the Libra point at the diametrically opposite end. When the Sun crosses the Aries point, the spring equinox occurs (around March 20-21, beginning spring in the northern hemisphere and the beginning of autumn in the southern hemisphere), and from which the Sun is in the northern celestial hemisphere; until it reaches the Libra point, on the autumnal equinox (around September 22-23, beginning autumn in the northern hemisphere and spring in the southern hemisphere).
In 1917 it was exactly 23° 27′ 00". The following table shows the values (with precision to the second of arc) for the last and following years. It is currently declining at the rate of 0.47" by year.
| 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 |
|---|---|---|---|---|---|---|---|
| 23° 26′ 14′′′ | 23° 26′ 13′′′ | 23° 26′ 13′′′ | 23° 26′ 13′′′ | 23° 26′ 12′′′ | 23° 26′ 12′′′ | 23° 26′ 11′′′′ | 23° 26′ 11′′′′ |
Other planets
For the other planets, the angle formed by their axis of rotation with the plane of their orbit is called axial inclination and is responsible for the seasons on these planets.
In the case of Mars, the inclination is 25º, very similar to Earth. Since the year on Mars is twice as long, it causes seasons that are also twice as long as those on Earth. The cut of both planes marks the vernal point of the planet or the beginning of spring in the Martian northern hemisphere.
Saturn's satellite Titan also appears to have seasons.