Usor:Sinister Petrus/Mea arena
|Publii Vergilii Maronis Aeneis|
|Di: Venus - Iuno - Iuppiter|
A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year.
Solar calendars usually aim to predict the seasons, but because the length of individual seasonal years varies significantly, they instead use an astronomical year as a surrogate. For example, the ancient Egyptians used the heliacal rising of Sirius to predict the flooding of the Nile.
Among solar calendars in wide use today, the Persian calendar is one of the most precise. Rather than being based on numerical rules, the Persian year begins on the day (for the time zone of Tehran) on which the vernal equinox actually falls, as determined by precise astronomical computations.
In the formerly used Julian calendar, the average length of a year was 365.25 days. This is still used as a convenient time unit in astronomy, see below.
Annus siderealis is the time for the Earth to complete one revolution of its orbit, as measured in a fixed frame of reference (such as the fixed stars, Latin sidus). Its duration in SI days of 86,400 SI seconds each is on average:
A tropical year is the time for the Earth to complete one revolution with respect to the framework provided by the intersection of the ecliptic (the plane of the orbit of the Earth) and the plane of the equator (the plane perpendicular to the rotation axis of the Earth). Because of the precession of the equinoxes, this framework moves slowly westward along the ecliptic with respect to the fixed stars (with a period of about 26,000 tropical years); as a consequence, the Earth completes this year before it completes a full orbit as measured in a fixed reference frame. Therefore a tropical year is shorter than the sidereal year. The exact length of a tropical year depends on the chosen starting point: for example the vernal equinox year is the time between successive vernal equinoxes. The mean tropical year (averaged over all ecliptic points) is:
- 365.242 189 67 days (365 d 5 h 48 min 45 s) (at the epoch J2000.0).
The Besselian year is a tropical year that starts when the fictitious mean Sun reaches an ecliptic longitude of 280°. This is currently on or close to 1 January. It is named after the 19th century German astronomer and mathematician Friedrich Bessel. An approximate formula to compute the current time in Besselian years from the Julian day is:
- B = 2,000 + (JD - 2,451,544.53)/365.242189
Variation in the length of the year and the dayRecensere
The exact length of an astronomical year changes over time. The main sources of this change are:
- The precession of the equinoxes changes the position of astronomical events with respect to the apsides of Earth's orbit. An event moving toward perihelion recurs with a decreasing period from year to year; an event moving toward aphelion recurs with an increasing period from year to year.
- The gravitational influence of the Moon and planets changes the shape of the Earth's orbit.
It is also suspected that changes in the effective mass of the sun, caused by nuclear fusion, could have a significant impact on the earth year over time.
Index obiectorum systematis solaris secundum radium, arranged in descending order of mean volumetric radius. Hic index haud completus est; solem, planetas, nonullas satellites, et alios obiectos notos habet.
Hic index ordinem dissimilis indici obiectorum systematis solaris secundum massam habet quod nonnullae obiectes densitatem maiorem quam alterae habent. Exempla gratia: Uranus maior diametro quam Neptunus sed minor massa est. Quamquam Ganymedes et Titan sunt maiores quam Mercurius, minus quam massam dimidiam eius habent.
Several new trans-Neptunian objects have been discovered of significant size. While their radius remains provisional due to the recency of discovery, and is often expressed as a range, the approximate locations in this list are shown.