The Earth, like other planets of the Solar System, moves on an elliptical orbit around the Sun counter-clockwise (when viewed from the north celestial pole) with an average speed of 30 km/s.
The image above provides an animated depiction of Earth circling the Sun, showing the projection of the Sun onto the background stars, forming the ecliptic (copyright and source: Tfr000 User / Wikimedia Commons).
The Earth revolution is subjected to Kepler’s laws
Kepler's First law states that the orbit described by a planet is an ellipse, where the Sun occupies one focus. The whole orbital circumference is about 940 million km. For any object orbiting around the Sun along their orbit there are two key points:
- Aphelion: the point where the object in its elliptical orbit is at greatest distance from the Sun.
- Perihelion: the point of the orbit at the minimum distance from the Sun.
Kepler's Second Law states that the orbital speed is not constant but varies along the orbit. Near the perihelion, where the radius vector is shorter than at the aphelion, the arc of the ellipse is correspondingly longer. So, the orbital velocity is maximum at the perihelion and minimum at the aphelion, consequently the solar day at the perihelion is longer than the average day and the opposite at the aphelion, hence the inconvenience to use the solar day as a measure of time.
Like the day, there are even several types of years:
The sidereal year, or astral year, is the time the Earth needs to make a complete revolution around the Sun relative to the stars. So, it is the interval for a fixed observer on the Earth when the Sun occupies the same position among the stars after one year. The duration of the sidereal year is 365 days, 6 hours, 9 minutes and 10 seconds equal to 365.256 average solar days. This period was defined by the ancient Egyptians observing the star Sirius, one of the brightest in the sky, but the use of fixed stars made it increasingly difficult to measure in ancient times.
The solar year, or tropical year, is the time interval between two successive spring equinoxes, or the time the Sun needs to return to the same location, after a complete cycle of the seasons. However, the revolution of the Earth is not constant, but it is subjected to variations due to the different speed along the orbit (Kepler's Second law), because of gravitational interference by the presence of other planets, and because of the precessing of the equinoxes (see the next paragraphs for further details). The solar year is therefore better to observe than the sidereal year, yet all these changes do not make the solar year to be used as a precise reference. So, it was defined a mean solar year, equal to 365.2422 days or 365 days, 5 hours, 48 minutes and 46 seconds, i.e. 20 minutes and 24.6 seconds shorter than the sidereal year.
The current calendar is based on the civil year, that is exactly 365 days, less than about a quarter of a day than the tropical year, and derives from the Julian and the Gregorian calendars (see also the dedicated pages within this web section). The Julian calendar, was introduced by Julius Caesar (100 BC - 44 BC, ancient Roman politician and military leader) in 46 BC as a reform of the previous and complicated Roman lunar calendar based on the Moon phases. The Julian calendar, a solar calendar based entirely on Earth's revolutions around the Sun (tropical year) was the main calendar in the Roman world, most of Europe, as well as in the European settlements worldwide until it was refined and replaced by the Gregorian calendar, which was promulgated in 1582 by Pope Gregory XIII.
Every 4 years the Julian calendar accumulates an additional day of delay. To correct this shift to the "normal" years of 365 days, every 4 years one "leap" year of 366 is added: the extra day is added in the month of February in leap years, which therefore consists of 29 days instead of the usual 28. With a leap year the difference compared to the tropical year is reduced from minus 5.8128 hours to just 11 minutes and 14 seconds in excess. The later Gregorian calendar further reduced this difference by eliminating 3 leap years every 400 years from the calendar compared to the Julian calendar, i.e. in 1600 and 2000: thus, the average length of 365.2425 days of the year becomes (365 + 97 / 400), reducing the difference at only 26 seconds in excess.
The image here below shows the Earth at the start of the four astronomical seasons as seen from the north and ignoring the atmosphere, e.g. no clouds, no twilight (copyright and source: By Tfr000 (talk) 16:54, 15 March 2012 (UTC) (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons).