Orbital period

The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars.

For objects in the Solar System, this is often referred to as the sidereal period, determined by a 360° revolution of one celestial body around another, e.g. the Earth orbiting the Sun. The name sidereal is added as it implies that the object returns to the same position relative to the fixed stars projected in the sky. When describing orbits of binary stars, the orbital period is usually referred to as just the period. For example, Jupiter has a sidereal period of 11.86 years while the main binary star Alpha Centauri AB has a period of about 79.91 years.

Another important orbital period definition can refer to the repeated cycles for celestial bodies as observed from the Earth's surface. An example is the so-called synodic period, applying to the elapsed time where planets return to the same kind of phenomena or location. For example, when any planet returns between its consecutive observed conjunctions with or oppositions to the Sun. For example, Jupiter has a synodic period of 398.8 days from Earth; thus, Jupiter's opposition occurs once roughly every 13 months.

Periods in astronomy are conveniently expressed in various units of time, often in hours, days, or years. They can be also defined under different specific astronomical definitions that are mostly caused by small complex eternal gravitational influences by other celestial objects. Such variations also include the true placement of the centre of gravity between two astronomical bodies (barycenter), perturbations by other planets or bodies, orbital resonance, general relativity, etc. Most are investigated by detailed complex astronomical theories using celestial mechanics using precise positional observations of celestial objects via astrometry.

Other periods related to the orbital period

There are many periods related to the orbits of objects, each of which are often used in the various fields of astronomy and astrophysics. Examples of some of the common ones include the following:

  • The sidereal period is the amount of time that it takes an object to make a full orbit, relative to the stars. This is the orbital period in an inertial (non-rotating) frame of reference.
  • The synodic period is the amount of time that it takes for an object to reappear at the same point in relation to two or more other objects (e.g. the Moon's phase and its position relative to the Sun and Earth repeats every 29.5 day synodic period, longer than its 27.3 day orbit around the Earth, due to the motion of the Earth about the Sun). The time between two successive oppositions or conjunctions is also an example of the synodic period. For the planets in the solar system, the synodic period (with respect to Earth) differs from the sidereal period due to the Earth's orbiting around the Sun.
  • The draconitic period (also draconic period or nodal period), is the time that elapses between two passages of the object through its ascending node, the point of its orbit where it crosses the ecliptic from the southern to the northern hemisphere. This period differs from the sidereal period because both the orbital plane of the object and the plane of the ecliptic precess with respect to the fixed stars, so their intersection, the line of nodes, also precesses with respect to the fixed stars. Although the plane of the ecliptic is often held fixed at the position it occupied at a specific epoch, the orbital plane of the object still precesses causing the draconitic period to differ from the sidereal period.[1]
  • The anomalistic period is the time that elapses between two passages of an object at its periapsis (in the case of the planets in the Solar System, called the perihelion), the point of its closest approach to the attracting body. It differs from the sidereal period because the object's semi-major axis typically advances slowly.
  • Also, the tropical period of Earth (a tropical year) is the interval between two alignments of its rotational axis with the Sun, also viewed as two passages of the object at a right ascension of 0 hr. One Earth year is slightly shorter than the period for the Sun to complete one circuit along the ecliptic (a sidereal year) because the inclined axis and equatorial plane slowly precess (rotate with respect to reference stars), realigning with the Sun before the orbit completes. This cycle of axial precession for Earth, known as precession of the equinoxes, recurs roughly every 25,770 years.[citation needed]
Other Languages
Alemannisch: Umlaufzeit
العربية: دور مداري
asturianu: Periodu orbital
беларуская: Сідэрычны перыяд
беларуская (тарашкевіца)‎: Сыдэрычны пэрыяд
čeština: Doba oběhu
Deutsch: Umlaufzeit
Esperanto: Orbita periodo
한국어: 공전 주기
hrvatski: Ophodno vrijeme
Bahasa Indonesia: Periode orbit
עברית: זמן הקפה
Lëtzebuergesch: Ëmlafzäit
македонски: Орбитален период
Bahasa Melayu: Tempoh orbit
日本語: 公転周期
norsk: Omløpstid
norsk nynorsk: Baneperiode
Plattdüütsch: Ümlooptiet
português: Período orbital
Simple English: Orbital period
slovenščina: Orbitalna perioda
српски / srpski: Орбитални период
srpskohrvatski / српскохрватски: Orbitalni period
suomi: Kiertoaika
Tiếng Việt: Chu kỳ quỹ đạo
中文: 轨道周期