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In [[astrodynamics]] or [[celestial dynamics]] '''mean longitude''' is the [[longitude]] at which an orbiting body could be found if its orbit were [[circular orbit|circular]] and its [[inclination]] were zero. The mean longitude changes at a constant rate over time. The only times when it is equal to the [[true longitude]] are at [[periapsis]] and [[apoapsis]].
'''Mean longitude''' is the [[Ecliptic coordinate system#Spherical coordinates|ecliptic longitude]] at which an [[orbit]]ing body could be found if its orbit were [[circular orbit|circular]] and free of [[perturbation (astronomy)|perturbations]]. While nominally a simple longitude, in practice the mean longitude does not correspond to any one physical angle.<ref name=Meeus1>
{{cite book
| last = Meeus
| first = Jean
| title = Astronomical Algorithms
| url = https://archive.org/details/astronomicalalgo00meeu_597
| url-access = limited
| publisher = Willmann-Bell, Inc., Richmond, VA
| year = 1991
|isbn=0-943396-35-2
|pages=[https://archive.org/details/astronomicalalgo00meeu_597/page/n201 197]–198}}
</ref>


==Calculation==
==Definition==
[[File:Orbit1-mean.png|thumb|400px|right|An orbiting body's ''mean longitude'' is calculated {{nowrap|''L'' {{=}} ''Ω'' + ''ω'' + ''M''}}, where ''Ω'' is the [[longitude of the ascending node]], ''ω'' is the [[argument of periapsis|argument of the pericenter]] and ''M'' is the ''[[mean anomaly]]'', the body's angular distance from the [[apsis|pericenter]] as if it moved with constant speed rather than with the [[Orbital speed|variable speed]] of an [[Elliptic orbit|elliptical orbit]]. Its ''[[true longitude]]'' is calculated similarly, {{nowrap|''l'' {{=}} ''Ω'' + ''ω'' + ''ν''}}, where ''ν'' is the [[true anomaly]].]]
The mean longitude <math>L\,</math> can be calculated as follows:
* Define a reference direction, ♈︎, along the [[ecliptic]]. Typically, this is the direction of the March [[equinox]]. At this point, ecliptic longitude is 0°.
* The body's orbit is generally [[Orbital inclination|inclined]] to the ecliptic, therefore define the angular distance from ♈︎ to the place where the orbit crosses the ecliptic from south to north as the ''[[longitude of the ascending node]]'', ''Ω''.
* Define the angular distance along the plane of the orbit from the [[ascending node]] to the [[Apsis|pericenter]] as the ''[[Argument of periapsis|argument of the pericenter]]'', ''ω''.
* Define the ''[[mean anomaly]]'', ''M'', as the angular distance from the pericenter which the body would have if it moved in a circular orbit, in the same orbital period as the actual body in its elliptical orbit.
From these definitions, the ''mean longitude'', ''L'', is the angular distance the body would have from the reference direction if it moved with uniform speed,
:''L'' = ''Ω'' + ''ω'' + ''M'',
measured along the ecliptic from ♈︎ to the ascending node, then up along the plane of the body's orbit to its mean position.<ref name=Smart1>
{{cite book
| last = Smart
| first = W. M.
| title = Textbook on Spherical Astronomy
| publisher = Cambridge University Press, Cambridge
| year = 1977
|edition=sixth
|isbn=0-521-29180-1
|page=122}}
</ref>


Sometimes the value defined in this way is called the "mean mean longitude", and the term "mean longitude" is used for a value that does have short-term variations (such as over a synodic month or a year in the case of the moon) but does not include the correction due to the difference between true anomaly and mean anomaly.<ref name=Simon>{{cite journal|display-authors=etal |last1=Jean-Louis Simon |title=Numerical expressions for precession formulae and mean elements for the Moon and the planets |journal=Astronomy and Astrophysics |date=1994 |volume=282 |page=663 |url=https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?db_key=AST&bibcode=1994A%26A...282..663S&letter=0&classic=YES&defaultprint=YES&whole_paper=YES&page=663&epage=663&send=Send+PDF&filetype=.pdf |bibcode=1994A&A...282..663S}}</ref><ref>{{cite web |title=Comprendre - Glossaire |url=https://promenade.imcce.fr/fr/pages2/221.html#longmoymoy |website=Promenade dans le système solaire |publisher=The FP7 ESPaCE Program |access-date=26 March 2024}}</ref>
:<math>L=M + \bar{\omega} = M + \Omega + \omega\,</math>
Also, sometimes the mean longitude (or mean mean longitude) is considered to be a slowly varying function, modeled with a [[Maclaurin series]], rather than a simple linear function of time.<ref name=Simon/>


==Discussion==
where:
*<math>M\, </math> is orbit's [[mean anomaly]],
⚫
*<math>\bar{\omega}\,</math> is [[longitude of the periapsis | longitude of the orbit's periapsis]],
*<math>\Omega\,</math> is the [[longitude of ascending node]] and
*<math>\omega\,</math> is the [[argument of periapsis]].


Mean longitude, like [[mean anomaly]], does not measure an angle between any physical objects. It is simply a convenient uniform measure of how far around its orbit a body has progressed since passing the reference direction. While mean longitude measures a mean position and assumes constant speed, [[true longitude]] measures the actual longitude and assumes the body has moved with its [[Orbital speed|actual speed]], which varies around its [[Elliptic orbit|elliptical orbit]]. The difference between the two is known as the [[equation of the center]].<ref>Meeus, Jean (1991). p. 222</ref>


==Formulae==
{{orbits}}
⚫
From the above definitions, define the ''[[longitude of the periapsis|longitude of the pericenter]]''
:''ϖ'' = ''Ω'' + ''ω''.
Then mean longitude is also<ref name=Meeus1/>
:''L'' = ''ϖ'' + ''M''.


Another form often seen is the ''mean longitude at epoch'', ''ε''. This is simply the mean longitude at a reference time ''t''<sub>0</sub>, known as the [[Epoch (astronomy)|epoch]]. Mean longitude can then be expressed,<ref name=Smart1/>
⚫
[[Category:Astrodynamics]]
:''L'' = ''ε'' + ''n''(''t'' − ''t''<sub>0</sub>), or
[[Category:Celestial mechanics]]
:''L'' = ''ε'' + ''nt'', since ''t'' = 0 at the epoch ''t''<sub>0</sub>.
{{astronomy-stub}}
where ''n'' is the ''[[mean motion|mean angular motion]]'' and ''t'' is any arbitrary time. In some sets of [[orbital elements]], ''ε'' is one of the six elements.<ref name=Smart1/>


==See also==
[[es:Longitud media]]
* [[Mean motion]]
[[sl:Srednja dolžina]]
* [[Orbital elements]]
[[zh:平黃經]]
* [[True longitude]]

==References==
{{reflist}}

{{Orbits}}

{{DEFAULTSORT:Mean Longitude}}
⚫
[[Category:Orbits]]

Latest revision as of 20:50, 23 October 2024

Mean longitude is the ecliptic longitude at which an orbiting body could be found if its orbit were circular and free of perturbations. While nominally a simple longitude, in practice the mean longitude does not correspond to any one physical angle.[1]

Definition

[edit]
An orbiting body's mean longitude is calculated L = Ω + ω + M, where Ω is the longitude of the ascending node, ω is the argument of the pericenter and M is the mean anomaly, the body's angular distance from the pericenter as if it moved with constant speed rather than with the variable speed of an elliptical orbit. Its true longitude is calculated similarly, l = Ω + ω + ν, where ν is the true anomaly.
  • Define a reference direction, ♈︎, along the ecliptic. Typically, this is the direction of the March equinox. At this point, ecliptic longitude is 0°.
  • The body's orbit is generally inclined to the ecliptic, therefore define the angular distance from ♈︎ to the place where the orbit crosses the ecliptic from south to north as the longitude of the ascending node, Ω.
  • Define the angular distance along the plane of the orbit from the ascending node to the pericenter as the argument of the pericenter, ω.
  • Define the mean anomaly, M, as the angular distance from the pericenter which the body would have if it moved in a circular orbit, in the same orbital period as the actual body in its elliptical orbit.

From these definitions, the mean longitude, L, is the angular distance the body would have from the reference direction if it moved with uniform speed,

L = Ω + ω + M,

measured along the ecliptic from ♈︎ to the ascending node, then up along the plane of the body's orbit to its mean position.[2]

Sometimes the value defined in this way is called the "mean mean longitude", and the term "mean longitude" is used for a value that does have short-term variations (such as over a synodic month or a year in the case of the moon) but does not include the correction due to the difference between true anomaly and mean anomaly.[3][4] Also, sometimes the mean longitude (or mean mean longitude) is considered to be a slowly varying function, modeled with a Maclaurin series, rather than a simple linear function of time.[3]

Discussion

[edit]

Mean longitude, like mean anomaly, does not measure an angle between any physical objects. It is simply a convenient uniform measure of how far around its orbit a body has progressed since passing the reference direction. While mean longitude measures a mean position and assumes constant speed, true longitude measures the actual longitude and assumes the body has moved with its actual speed, which varies around its elliptical orbit. The difference between the two is known as the equation of the center.[5]

Formulae

[edit]

From the above definitions, define the longitude of the pericenter

ϖ = Ω + ω.

Then mean longitude is also[1]

L = ϖ + M.

Another form often seen is the mean longitude at epoch, ε. This is simply the mean longitude at a reference time t0, known as the epoch. Mean longitude can then be expressed,[2]

L = ε + n(t − t0), or
L = ε + nt, since t = 0 at the epoch t0.

where n is the mean angular motion and t is any arbitrary time. In some sets of orbital elements, ε is one of the six elements.[2]

See also

[edit]

References

[edit]
  1. ^ a b Meeus, Jean (1991). Astronomical Algorithms. Willmann-Bell, Inc., Richmond, VA. pp. 197–198. ISBN 0-943396-35-2.
  2. ^ a b c Smart, W. M. (1977). Textbook on Spherical Astronomy (sixth ed.). Cambridge University Press, Cambridge. p. 122. ISBN 0-521-29180-1.
  3. ^ a b Jean-Louis Simon; et al. (1994). "Numerical expressions for precession formulae and mean elements for the Moon and the planets" (PDF). Astronomy and Astrophysics. 282: 663. Bibcode:1994A&A...282..663S.
  4. ^ "Comprendre - Glossaire". Promenade dans le système solaire. The FP7 ESPaCE Program. Retrieved 26 March 2024.
  5. ^ Meeus, Jean (1991). p. 222
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