Universe - Orbit of the Earth around the Sun
978-3-14-100890-6 | Page 10 | Ill. 3
Overview
The rotation of our home planet, which on earth appears like the Sun is moving across the sky, is the reason for the 24-hour cycle of day and night. The second movement of the Earth on its almost circular orbital path around the Sun - with an average distance of 149.6 million kilometres - is the cause of the cycle of the year.
The Cycle of the year
For the observer on earth, this complex movement manifests itself as the apparent movement of the Sun through a series of constellations, for example from one vernal equinox to the next. This period of time, with a length of 365.2422 days, is called the tropical year. Since calendar years cannot contain fractions of days, the annual deviation adds up to a whole day about every four years, which is then inserted as a leap day on 29 February. On its elliptical orbit, the Earth reaches the point closest to the Sun (perihelion) in January and the point furthest from the Sun (aphelion) in July. Since the Earth moves faster near the Sun, the northern winter is shorter than the summer.
Reasons for the seasons
Because the Earth's axis of rotation forms an angle of 23.5 degrees to the perpendicular of the orbital plane - and this axis remains fixed in space except for long-term effects (precession of the Earth's axis) and always points to the north celestial pole near the pole star - the seasons are created. For example, on 21 June, the northern hemisphere of the Earth is most turned towards the Sun, which means that in the region north of the Arctic Circle, the Sun no longer disappears below the horizon at this time of the summer solstice.
The seasons of the year occur because the Earth's rotational axis is inclined at an angle of 23.5° to the perpendicular of the ecliptic plane - and, with the exception of long-term changes (axial precession), this axis will remain pointing to the north celestial pole close to the northern Pole Star. The Northern Hemisphere faces the Sun for the longest time on the 21st of June (summer solstice) – during this time the Sun does not sink below the horizon north of the polar circle. However, on December 21st (winter solstice) the Sun is invisible north of the polar circle all the time.
At the time of equinox (20th of March, 23rd of September) the Sun is rising exactly in the east and setting exactly in the west, resulting in exactly 12 hours of daylight and 12 hours of darkness. In the summer, while the Sun in Europe rises in the northeast and sets in the northwest, there are more than 12 hours of daylight and less than 12 hours of darkness. In the winter, the opposite is true: The Sun rises in the southeast and sets in the southwest, resulting in less than 12 hours of daylight and more than 12 hours of darkness.
