Independent · Assigned · Finite

This class contains time units that exist independently of human systems and are defined by recurring natural cycles, not by fixed duration. Each unit is bounded because the underlying process completes, but the exact length may vary.

These units are not fundamentally about counting seconds. They are about regular return—rotation, orbit, or phase—used to structure time through repetition rather than precision.

Rotation-Return Units

stellar day
sidereal day
apparent solar day
planetary sidereal day
planetary solar day
Mercury sidereal day
Mercury solar day
Venus sidereal day
Venus solar day
Earth sidereal day
Earth apparent solar day
Martian sol
Jupiter rotation period
Saturn rotation period
Uranus rotation period
Neptune rotation period
lunar day
solar sidereal rotation
solar synodic rotation
Carrington rotation, weak / technical
pulsar rotation period
magnetar rotation period
stellar rotation period

Earth-Year / Orbit-Return Units

sidereal year
tropical year
equinox year
solstice year
anomalistic year
draconic year
eclipse year
planetary year
Mercury year
Venus year
Earth year, if astronomical, not calendar
Mars year
Jupiter year
Saturn year
Uranus year
Neptune year
exoplanet year
moon orbital period
asteroid orbital period
cometary period
binary-star orbital period
compact-binary orbital period
galactic year, when treated as one orbit around the galactic center

Lunar Cycle Units

synodic month
lunation
sidereal month
tropical month
anomalistic month
draconic month
nodical month
lunar fortnight
lunar phase quarter
lunar apsidal cycle
lunar nodal cycle

Synodic / Alignment Units

planetary synodic period
Mars synodic period
Venus synodic period
Mercury synodic period
opposition cycle
conjunction cycle
inferior conjunction cycle
superior conjunction cycle
quadrature cycle
greatest elongation cycle
planetary transit recurrence interval

Eclipse / Node-Return Units

eclipse year
eclipse season
Saros
Exeligmos
Inex
Tritos
semester eclipse cycle
node fortnight

Precession / Orientation Units

axial precession cycle
precessional year
astronomical Great Year
apsidal precession cycle
nodal precession cycle
obliquity cycle
eccentricity cycle
orbital inclination cycle
Milankovitch precession cycle
Milankovitch obliquity cycle
Milankovitch eccentricity cycle

Stellar / Compact-Object Cycle Units

Cepheid pulsation period
RR Lyrae pulsation period
Mira variable period
eclipsing binary period
binary-star orbital period
pulsar period
millisecond pulsar period
magnetar rotation period
stellar rotation period
starspot modulation period
solar magnetic cycle
Schwabe cycle
Hale cycle

Seasonal Astronomical Units

astronomical spring
astronomical summer
astronomical autumn
astronomical winter
equinox-to-equinox interval
solstice-to-solstice interval
heliacal year
Sothic year

Resonance / Multi-Body Recurrence Units

orbital resonance cycle
Laplace resonance cycle
near-resonant exoplanet super-period
orbital beat period
apsidal-alignment cycle
nodal-alignment cycle

Best Examples to Use

Example	Why it is strong
sidereal day	One Earth rotation relative to celestial reference.
apparent solar day	Sun-return cycle; natural, finite, variable.
Martian sol	Planetary solar day grounded in Mars’s rotation and orbit.
sidereal year	Earth orbit-return relative to stars.
tropical year	Seasonal-return cycle.
anomalistic year	Perihelion-return cycle.
synodic month / lunation	Lunar phase-return cycle.
sidereal month	Moon orbit-return relative to stars.
draconic month	Moon node-return cycle.
anomalistic month	Moon perigee-return cycle.
planetary synodic period	Alignment-return cycle between planet, Earth, and Sun.
eclipse year	Sun’s return to a lunar node.
Saros	Eclipse-geometry recurrence cycle.
axial precession cycle	Natural orientation-return cycle.
binary-star orbital period	Gravitational orbit-return cycle.
Cepheid pulsation period	Stellar pulsation-return cycle.
pulsar period	Compact-object rotation-return cycle.

These are grounded in natural celestial cycles. They are finite because the cycle completes, but they are assigned by recurrence rather than defined as fixed counted duration.

Prompt:

A clean vector brand logo for the time class “Independent · Assigned · Finite,” representing astronomical time. Use a circular time-ring built from deep cosmic blue, violet, and ember red. The icon should combine a natural orbital path, a deliberate violet marker placed on the orbit like an assigned celestial point, and a closed eclipse-like boundary. Minimal flat vector logo, centered icon, consistent stroke weight, ivory background, no text, no letters, no numbers, premium astronomical identity-system design.

Boundary of This Class

Included:

Units defined by natural repetition (rotation, orbit, phase)
Units with clear cycle completion
Units independent of human agreement

Excluded:

Units defined by exact duration (seconds, hours)
Units defined by convention (weeks, calendar months)
Units defined by role or interpretation (deadlines, eras)

Independent · Assigned · Finite

Astronomical Cycle Units

This class contains natural recurrence-units. They exist because some astronomical configuration completes and returns: a rotation closes, an orbit closes, a phase repeats, a conjunction recurs, a node is crossed again, or an axis/precessional orientation returns.

They are not defined by a fixed number of seconds. Their measured length may be given in seconds, days, or years afterward, but that measurement is secondary. The unit itself is defined by cycle completion.

A unit belongs here only when it passes this test:

Test	Requirement
Independent	The underlying cycle exists without human law, calendars, institutions, or narrative interpretation.
Assigned	The unit is assigned to a natural recurrence: one rotation, one orbit, one phase cycle, one conjunction cycle, one node return, etc.
Finite	The cycle closes. There is a recognizable completion condition.
Astronomical	The grounding process is celestial: rotation, orbit, phase, conjunction, precession, eclipse geometry, stellar pulsation, or similar.

The clean name for this class is:

Astronomical Cycle Units

Not “astronomical durations” in general. The decisive feature is return.

The Core Difference from Physical or Fundamental Time Units and Cosmological or Deep Time Units

Class	What defines the unit	Example
Physical / Fundamental	Fixed counted duration	nanosecond, second, Planck time
Cosmological / Deep Time	Large-scale physical timescale	Hubble time, stellar lifetime, galaxy merger timescale
Astronomical	Natural cycle completion	solar day, sidereal day, synodic month, tropical year

So:

Item	Class
86,400 seconds	Physical or Fundamental Time Units, counted physical duration
civil day	Calendar or Civil Time Units, declared counted finite
apparent solar day	Astronomical Time Units, natural Sun-return cycle
365.25 days	Physical or Fundamental Time Units or Calendar or Civil Time Units depending use; counted convention
Julian year	Calendar or Civil Time Units, declared counted finite
sidereal year	Astronomical Time Units, one orbit relative to stars
calendar month	Calendar or Civil Time Units, declared counted finite
synodic month / lunation	Astronomical Time Units, lunar phase-return cycle

That is the spine of the distinction.

A. Rotation-Return Units

These are units defined by a body completing a rotation relative to some natural reference: distant stars, the Sun, or an observer’s sky. JPL defines a sidereal rotation period as the time required for a planet to complete one full rotation relative to fixed stars, and a sidereal orbital period as the time required for a planet to complete one orbit around the Sun relative to fixed stars.

Unit	Completion condition	Why it fits Astronomical Time Units
stellar day	One rotation of Earth relative to distant stars	Natural rotation-return unit; not defined by fixed seconds.
sidereal day	Equinox returns across a meridian	Astronomical rotation unit; USNO gives it as about 23h 56m 04s of solar civil time, but that measurement is not the definition.
apparent solar day	Sun returns to local meridian	Natural Sun-return cycle; length varies through the year.
planetary sidereal day	A planet completes one rotation relative to stars	Applies to Mars, Mercury, Venus, Jupiter, etc.; body-specific and natural.
planetary solar day	The Sun returns to the same local sky position on a planet	Natural day-night cycle for that body.
Martian sol	Sun returns in the Martian sky	Natural Martian solar day; mission sol-numbering is a declared use layered on top.
Mercury solar day	Sun returns in Mercury’s sky	Natural but very different from Mercury’s sidereal rotation because of orbital-rotation resonance.
Venus solar day	Sun returns in Venus’s sky	Natural cycle, complicated by retrograde rotation.
lunar day	Sun returns in the lunar sky	Natural lunar illumination cycle.
solar sidereal rotation	A region of the Sun rotates relative to stars	Natural solar rotation, though the Sun’s differential rotation makes this less clean than a solid planet.
solar synodic rotation	A solar feature returns to the same apparent position as seen from Earth	Natural Earth-Sun viewing cycle.
pulsar rotation period	A neutron star’s beam returns once per rotation	Astronomical rotation cycle; not fixed because pulsars spin down and can glitch.

The Carrington rotation is weaker because it imposes a uniform reference frame on a differentially rotating Sun; NOAA defines Carrington longitude using a fixed synodic period of 27.2753 days. That makes it partly technical/conventional, even though it is grounded in solar rotation.

B. Earth-Year and Orbit-Return Units

These are units defined by one orbital return, but “return” can mean different things: return to the stars, return of seasons, return to perihelion, or return to a node.

The IAU Office of Astronomy for Education defines a year as Earth going once around the Sun, but notes that “once around” has multiple astronomical meanings: the tropical year returns the Sun to the same apparent place in the sky, the sidereal year returns Earth to the same orbit relative to distant stars, and the anomalistic year is measured between perihelion passages.

Unit	Completion condition	Why it fits Astronomical Time Units
sidereal year	Earth returns to same orbital position relative to distant stars	Natural orbit-return cycle.
tropical year	Sun returns to same ecliptic longitude / seasonal position	Natural seasonal-return cycle.
equinox year	One equinox returns to the corresponding equinox	Natural seasonal boundary cycle.
solstice year	One solstice returns to the corresponding solstice	Natural seasonal boundary cycle.
anomalistic year	Earth returns from perihelion to perihelion	Natural apsidal-return cycle.
draconic year / eclipse year	Sun returns to the same lunar node	Natural node-return cycle; relevant to eclipses.
planetary year	A planet completes one orbit around its primary	Natural orbit-return cycle.
Mercury year	Mercury completes one orbit around the Sun	Body-specific orbital cycle.
Venus year	Venus completes one orbit around the Sun	Body-specific orbital cycle.
Mars year	Mars completes one orbit around the Sun	Body-specific orbital cycle.
Jupiter year	Jupiter completes one orbit around the Sun	Body-specific orbital cycle.
Saturn year	Saturn completes one orbit around the Sun	Body-specific orbital cycle.
Uranus year	Uranus completes one orbit around the Sun	Body-specific orbital cycle.
Neptune year	Neptune completes one orbit around the Sun	Body-specific orbital cycle.
exoplanet year	An exoplanet completes one orbit around its star	Natural orbit-return cycle outside the Solar System.
cometary period	A comet completes one orbit	Natural return cycle, though perturbations can change it.
asteroid orbital period	An asteroid completes one orbit	Natural orbit-return cycle.
moon orbital period	A moon completes one orbit around its planet	Natural orbit-return cycle.
binary-star orbital period	Two stars complete one mutual orbit	Natural gravitational cycle.
compact-binary orbital period	Neutron stars, white dwarfs, or black holes complete an orbit	Natural orbital cycle; can shrink through radiation or mass transfer.
galactic year	A star system completes one orbit around the galactic center	Natural orbit-return cycle at galactic scale. NASA says the Solar System takes about 230 million years to orbit the Milky Way’s center.

Important distinction:

Item	Class
sidereal year	Astronomical Time Units
tropical year	Astronomical Time Units
anomalistic year	Astronomical Time Units
calendar year	Calendar or Civil Time Units
Julian year of exactly 365.25 days	Calendar or Civil Time Units or Physical or Fundamental Time Units-style counted convention, not Astronomical Time Units
1 billion years	counted duration expression, not an astronomical cycle unit

C. Lunar Cycle Units

These are among the best examples of Astronomical Time Units because they show exactly what “assigned” means. A “month” is not one thing. It depends on which natural lunar return is being used.

Wolfram’s World of Astronomy lists the major astronomical month-types: synodic, anomalistic, draconic, sidereal, and tropical, and defines them by natural return conditions such as phase conjunction, perigee passage, node passage, star reference, or lunar equinox.

Unit	Completion condition	Why it fits Astronomical Time Units
synodic month / lunation	Moon returns to same phase; new Moon to new Moon or full Moon to full Moon	Natural phase-return cycle.
sidereal month	Moon returns to same position relative to stars	Natural orbit-return cycle.
tropical month	Moon returns to same ecliptic longitude reference	Natural equinox/ecliptic-return cycle.
anomalistic month	Moon returns from perigee to perigee	Natural apsidal-return cycle.
draconic month / nodical month	Moon returns to same orbital node	Natural node-return cycle.
lunar phase quarter	New Moon to first quarter, first quarter to full, etc.	Natural phase interval; weaker as a “unit” but still cycle-assigned.
lunar fortnight	New Moon to full Moon, or full Moon to new Moon	Half of the synodic phase cycle.
lunar apsidal cycle	Moon’s line of apsides completes a precession cycle	Natural orbital-orientation cycle.
lunar nodal cycle	Moon’s orbital nodes complete a precession cycle	Natural orbital-plane cycle.

These are stronger than generic “month,” because calendar month belongs to Calendar or Civil Time Units.

D. Synodic Alignment Units

A synodic unit is defined by relative configuration: two bodies return to the same alignment as seen from a third body or within a system.

Unit	Completion condition	Why it fits Astronomical Time Units
planetary synodic period	A planet returns to the same apparent Sun-Earth-planet configuration	Natural alignment-return cycle.
Mars synodic period	Mars returns from opposition to opposition, or equivalent Earth-Mars-Sun geometry	Natural alignment cycle.
Venus synodic period	Venus returns to equivalent Earth-Venus-Sun geometry	Natural alignment cycle.
Mercury synodic period	Mercury returns to equivalent elongation/conjunction geometry	Natural alignment cycle.
opposition cycle	A superior planet returns to opposition	Natural Sun-Earth-planet alignment.
conjunction cycle	Two bodies return to conjunction	Natural apparent-alignment cycle.
inferior conjunction cycle	Inferior planet returns between Earth and Sun	Natural alignment cycle.
superior conjunction cycle	Inferior planet returns to far side of Sun from Earth	Natural alignment cycle.
quadrature cycle	A planet returns to 90° elongation from the Sun	Natural angular-configuration cycle.
greatest elongation cycle	Mercury or Venus returns to greatest apparent separation from the Sun	Natural viewing-geometry cycle.
transit recurrence interval	A planet returns to transit geometry across the Sun	Natural but less regular; depends on orbital node alignment too.

These are very clean Astronomical Time Units units because their edge is a natural configuration, not a clock duration.

E. Eclipse and Node-Return Units

These are astronomical recurrence units based on the geometry of the Sun, Earth, and Moon. NASA describes the Saros as an eclipse recurrence cycle of about 6,585.3 days, arising from a natural harmony between the synodic, anomalistic, and draconic months; NASA also notes that eclipses separated by one Saros have very similar geometries.

Unit	Completion condition	Why it fits Astronomical Time Units
eclipse year / draconic year	Sun returns to the same lunar node	Natural eclipse-geometry cycle.
eclipse season	Sun is close enough to a lunar node for eclipses to occur	Natural finite window, though threshold definitions can be technical.
Saros	Eclipse geometry approximately repeats after 223 synodic months	Natural recurrence of phase-node-distance geometry.
Exeligmos	Three Saros cycles; eclipse recurs at roughly similar local time	Natural recurrence built from Saros repetition.
Inex	Long eclipse recurrence interval involving node alternation	Astronomical recurrence cycle.
Tritos	Eclipse recurrence related to Saros/Inex combinations	Astronomical recurrence cycle.
semester eclipse cycle	Roughly half-year recurrence of eclipse possibility	Node-alignment recurrence.
node fortnight	Short interval around node alignment relevant to eclipse conditions	Natural but weak as a general unit.

The eclipse year is especially clean: the Sun returns to a lunar orbital node about every 346.62 mean solar days, making it shorter than the common calendar year.

Borderline note: the Saros is partly described by counting lunar months, but it belongs here when treated as an eclipse-geometry recurrence, not as a mere counted duration.

F. Precession, Orientation, and Long Astronomical Cycles

These are still Astronomical Time Units when they are defined by a completed astronomical orientation cycle. They may be long, but they are not Cosmological or Deep Time Units unless they are being used as vague deep-time process scales rather than recurrence units.

NASA describes Milankovitch cycles as involving changes in Earth’s orbital eccentricity, axial tilt/obliquity, and the direction of Earth’s spin axis/precession.

Unit	Completion condition	Why it fits Astronomical Time Units
axial precession cycle	Earth’s spin axis completes a precessional circuit	Natural orientation-return cycle.
precessional year / Great Year	Equinox direction completes a full precessional circuit	Natural celestial-coordinate cycle; term can become historical/mythic if used narratively.
apsidal precession cycle	Line of apsides completes one rotation	Natural orbital-orientation cycle.
nodal precession cycle	Orbital nodes complete one full circuit	Natural plane-orientation cycle.
obliquity cycle	Axial tilt oscillates through its cycle	Natural axial-tilt recurrence.
eccentricity cycle	Orbital shape cycles between more circular and more elliptical	Natural orbital-shape recurrence.
orbital inclination cycle	Orbital plane cycles relative to a reference plane	Natural orbital-plane recurrence.
Milankovitch precession cycle	Seasonal timing relative to perihelion cycles	Natural climate-relevant astronomical cycle.
Milankovitch obliquity cycle	Earth’s axial tilt variation cycle	Natural orbital/axial cycle.
Milankovitch eccentricity cycle	Earth’s orbital eccentricity variation cycle	Natural orbital-shape cycle.

Important distinction:

Item	Class
axial precession cycle	Astronomical Time Units
“the Age of Aquarius” as cultural-historical label	Historical or Narrative Time Units
26,000 years as a counted duration	Physical or Fundamental Time Units-style counted expression
precessional Great Year as completed astronomical cycle	Astronomical Time Units
Great Year as mythic/historical age system	Historical or Narrative Time Units

G. Stellar and Compact-Object Cycle Units

These belong here when they are defined by a recurring astronomical cycle: pulsation, rotation, eclipse, brightness variation, or magnetic activity.

Unit	Completion condition	Why it fits Astronomical Time Units
Cepheid pulsation period	Star completes one brightness/radius pulsation	Natural stellar cycle.
RR Lyrae pulsation period	Star completes one pulsation cycle	Natural stellar cycle.
Mira variable period	Star completes one long-period brightness cycle	Natural stellar cycle.
eclipsing binary period	Binary stars return to eclipse geometry	Natural orbit/alignment cycle.
binary-star orbital period	Two stars complete one mutual orbit	Natural gravitational cycle.
pulsar period	Pulsar beam returns once per rotation	Natural compact-object rotation cycle.
millisecond pulsar period	Millisecond-scale pulsar rotation cycle	Natural but extremely precise astronomical cycle. NASA describes millisecond pulsars as having rotational periods between 1 and 10 ms.
magnetar rotation period	Magnetar completes one rotation	Natural compact-object rotation cycle.
stellar rotation period	Star completes one rotation	Natural stellar rotation cycle.
starspot modulation period	Starspot pattern returns through rotation	Natural but observationally weaker.
solar magnetic cycle / Schwabe cycle	Sun returns from low to high to low magnetic activity	Natural solar activity cycle; NASA describes the solar cycle as a natural cycle of low-to-high magnetic activity, roughly 11 years.
Hale magnetic cycle	Solar magnetic polarity returns to original orientation after two solar cycles	Natural magnetic-orientation cycle.

Borderline note: a pulsar can be used as an extremely precise clock-like signal, but the unit is still a natural rotation cycle of a particular object, not a fixed duration like the SI second.

H. Seasonal Astronomical Units

These are natural if defined by actual celestial events, not by a calendar rule.

Unit	Completion condition	Why it fits Astronomical Time Units
astronomical spring	Vernal equinox to summer solstice	Natural solar/seasonal interval.
astronomical summer	Summer solstice to autumnal equinox	Natural solar/seasonal interval.
astronomical autumn	Autumnal equinox to winter solstice	Natural solar/seasonal interval.
astronomical winter	Winter solstice to vernal equinox	Natural solar/seasonal interval.
equinox-to-equinox interval	Same equinox recurs	Natural seasonal cycle.
solstice-to-solstice interval	Same solstice recurs	Natural seasonal cycle.
heliacal year	A star returns to heliacal rising	Natural observational sky-return cycle, though local visibility makes it weaker.
Sothic year	Sirius returns to heliacal rising	Natural star-Sun-observer recurrence, historically used but astronomically grounded.

Important distinction:

Item	Class
astronomical season: equinox to solstice	Astronomical Time Units
meteorological season: March-April-May	Calendar or Civil Time Units
liturgical season	Social or Institutional Time Units or Historical or Narrative Time Units depending use
“the season of empire” / narrative season	Historical or Narrative Time Units

I. Resonance and Multi-Body Recurrence Units

These are less common as everyday “time units,” but they are valid Astronomical Time Units members when the unit is defined by a completed recurring celestial configuration.

Unit	Completion condition	Why it fits Astronomical Time Units
orbital resonance cycle	Resonant bodies return to similar relative orbital configuration	Natural multi-body recurrence.
Laplace resonance cycle	Resonant moons return to a repeated orbital phase relation	Natural gravitational recurrence.
super-period of near-resonant exoplanets	Planetary configuration repeats approximately	Natural orbital alignment cycle.
beat period of two orbital cycles	Two cycles return to same relative phase	Natural recurrence interval.
apsidal-alignment cycle	Periapses of bodies return to alignment	Natural orbital-orientation cycle.
nodal-alignment cycle	Orbital nodes return to alignment	Natural orbital-plane recurrence.

These are weaker as ordinary named units, but structurally they fit the class.

Things That Do Not Belong in Astronomical Time Units

Do not put in Astronomical Time Units	Better class	Why
second	Physical or Fundamental Time Units	Fixed counted duration.
millisecond	Physical or Fundamental Time Units	Fixed subdivision of second.
86,400 seconds	Physical or Fundamental Time Units	Counted duration expression.
minute	Calendar or Civil Time Units	Declared/civil counted unit.
hour	Calendar or Civil Time Units	Declared/civil counted unit.
civil day	Calendar or Civil Time Units	Declared midnight-to-midnight boundary.
calendar week	Calendar or Civil Time Units	Declared seven-day coordination unit.
calendar month	Calendar or Civil Time Units	Rule-defined civil unit.
calendar year	Calendar or Civil Time Units	Rule-defined civil unit.
Julian year	Calendar or Civil Time Units / technical counted convention	Exactly 365.25 days, not an orbit-return unit.
fiscal year	Social or Institutional Time Units	Institutional period.
academic year	Social or Institutional Time Units	Institutional/social period.
Renaissance	Historical or Narrative Time Units	Historical/narrative period.
Age of Aquarius, as cultural age	Historical or Narrative Time Units	Interpretive/narrative framing.
stellar lifetime	Cosmological or Deep Time Units	Physical timescale, not cycle-return unit.
Hubble time	Cosmological or Deep Time Units	Cosmological expansion timescale, not recurrence cycle.
gestation period	Biological Time Units	Biological process duration.
heartbeat	Biological Time Units	Biological rhythm, not astronomical cycle.

The clean rule:

If the unit is “one return of a natural celestial configuration,” it belongs in Astronomical Time Units. If it is merely a fixed number of seconds, a calendar rule, an institutional period, a biological rhythm, or a historical interpretation, it does not.