For
celestial objects in general, the
orbital period typically refers to the
sidereal period, determined by a 360° revolution of
one body around its
primary relative to the
fixed stars projected in the sky. For the case of the
Earth orbiting around the
Sun, this period is referred to as the
sidereal year. This is the orbital period in an inertial (non-rotating)
frame of reference.
Orbital periods can be defined in several ways. The
tropical period is more particularly about the position of the parent star. It is the basis for the
solar year, and respectively the
calendar year. The
synodic period refers not to the orbital relation to the parent star, but to other
celestial objects, making it not a merely different approach to the orbit of an object around its parent, but a period of orbital relations with other objects, normally Earth, and their orbits around the Sun. It applies to the elapsed time where planets return to the same kind of phenomenon or location, such as 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. There are many
periods related to the orbits of objects, each of which are often used in the various fields of
astronomy and
astrophysics, particularly they must not be confused with other revolving periods like
rotational periods. Examples of some of the common orbital ones include the following: • 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. In common usage, these two objects are typically Earth and the Sun. The time between two successive
oppositions or two successive
conjunctions is also equal to the synodic period. For celestial bodies in the
Solar System, the synodic period (with respect to Earth and the Sun) differs from the tropical period owing to Earth's motion around the Sun. For example, the synodic period of the
Moon's orbit as seen from
Earth, relative to the
Sun, is 29.5 mean solar days, since the Moon's phase and position relative to the Sun and Earth repeats after this period. This is longer than the sidereal period of its orbit around Earth, which is 27.3 mean solar days, owing to the motion of Earth 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. • 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 25,772 years. Periods can be also defined under different specific astronomical definitions that are mostly caused by the small complex external gravitational influences of 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.
Synodic period One of the observable characteristics of two bodies which orbit a third body in different orbits, and thus have different orbital periods, is their
synodic period, which is the time between
conjunctions. An example of this related period description is the repeated cycles for celestial bodies as observed from the Earth's surface, the
synodic period, applying to the elapsed time where planets return to the same kind of phenomenon or 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. If the orbital periods of the two bodies around the third are called
T1 and
T2, so that
T1 2, their synodic period is given by: :\frac{1}{T_\mathrm{syn}} = \frac{1}{T_1} - \frac{1}{T_2} ==Examples of sidereal and synodic periods==