Specifies the orbit of an object in space
The
argument of periapsis
(also called
argument of perifocus
or
argument of pericenter
), symbolized as
ω (
omega
)
, is one of the
orbital elements
of an
orbiting
body. Parametrically,
ω
is the angle from the body's
ascending node
to its
periapsis
, measured in the direction of motion.
For specific types of orbits, terms such as
argument of perihelion
(for
heliocentric orbits
),
argument of perigee
(for
geocentric orbits
),
argument of periastron
(for orbits around stars), and so on, may be used (see
apsis
for more information).
An argument of periapsis of 0° means that the orbiting body will be at its closest approach to the central body at the same moment that it crosses the plane of reference from South to North. An argument of periapsis of 90° means that the orbiting body will reach periapsis at its northmost distance from the plane of reference.
Adding the argument of periapsis to the
longitude of the ascending node
gives the
longitude of the periapsis
. However, especially in discussions of binary stars and exoplanets, the terms "longitude of periapsis" or "longitude of periastron" are often used synonymously with "argument of periapsis".
Calculation
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In
astrodynamics
the
argument of periapsis
ω
can be calculated as follows:
-
- If
e
z
< 0 then
ω
→ 2
π
?
ω
.
where:
- n
is a vector pointing towards the ascending node (i.e. the
z
-component of
n
is zero),
- e
is the
eccentricity vector
(a vector pointing towards the periapsis).
In the case of
equatorial orbits
(which have no ascending node), the argument is strictly undefined. However, if the convention of setting the longitude of the ascending node Ω to 0 is followed, then the value of
ω
follows from the two-dimensional case:
- If the orbit is clockwise (i.e. (
r
×
v
)
z
< 0) then
ω
→ 2
π
?
ω
.
where:
- e
x
and
e
y
are the
x
- and
y
-components of the eccentricity vector
e
.
In the case of circular orbits it is often assumed that the periapsis is placed at the ascending node and therefore
ω
= 0. However, in the professional exoplanet community,
ω
= 90° is more often assumed for circular orbits, which has the advantage that the time of a planet's inferior conjunction (which would be the time the planet would transit if the geometry were favorable) is equal to the time of its periastron.
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[2]
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See also
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References
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External links
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