Glossary

A paleoclimatology puzzle referring to the transition in the Earth climate cycles that occurred approximately 1 million years ago. Before this transition, ice core data from Antarctica shows climate was driven by regular ~41,000-year cycles (matching the obliquity cycle). After this point, dominant climate cycles shifted to ~100,000-year periodicities, which corresponds to the inclination cycle in the Holistic Universe Model. This shift suggests a fundamental change in how the Earth orbital parameters influenced climate, marking the beginning of the current balanced solar system configuration described in the Holistic Universe Model.

**1 296 000 arcseconds** = **1440 minutes** (our celestial sphere) = **24 hours** = **100%** of 1 circle or revolution. The sky can be divided into a number of degrees, arcseconds, minutes or hours of RA.

The figure-eight curve from plotting the Sun's position at the same time each day over a year. Shaped by eccentricity, obliquity, and perihelion timing. In the Holistic Universe Model, the analemma shifts through the perihelion precession cycle of 20,957 years, with width modulated by eccentricity and length by obliquity (22.21° – 24.72°).

The time between successive passages of the Moon through its perigee (closest point to Earth), approximately ~27.5545542147 days. This period is slightly longer than the sidereal month because the Moon's line of apsides (connecting perigee and apogee) precesses forward in the same direction as the Moon's orbit due to apsidal precession.

The time for Earth to return to perihelion. A MEAN value of 365.2596324 days (~31,558,432.12 seconds). Directly related to the 20,957-year Perihelion precession cycle. The J2000 value is ~31,558,432.59 seconds.

The point in an orbit where a celestial body is furthest from the Sun. For Earth, this currently occurs around July 4th. From Greek: ap (away from) + helios (Sun).

Furthest transit point of the Moon with respect to Earth.

The gradual rotation of the Moon's line of apsides (the line connecting perigee and apogee) within its orbital plane. The Moon's apsidal precession has a duration of ~8.85 years. Unlike nodal precession, the apsidal precession runs in the same direction as the Sun's orbit and Moon's orbit around Earth, which explains why the duration experienced on Earth is shorter than against ICRF (opposite to nodal precession).

The two extreme points in an elliptical orbit where a celestial body is closest to and farthest from the object it orbits. For Earth orbiting the Sun, the apsides are the perihelion (closest point) and aphelion (farthest point). The line connecting these two points is called the line of apsides. The term comes from Greek "hapsis" meaning "arch" or "vault."

An orbital element that defines the orientation of the closest approach within the orbital plane. It is the angle (starting from the center of the orbit) between an orbiting body's ascending node and its periapsis, measured in the direction of orbital motion. Also known as the argument of perihelion or the argument of perifocus.

The point where an orbiting object crosses the reference plane (typically the ecliptic) moving from south to north. It is one of two places where the orbit intersects the reference plane; the other is the descending node. Together they define the line of nodes.

The average distance from Earth to the Sun with a value of 149,597,870.698828 km which is the distance of Earth to the PERIHELION-OF-EARTH (close to the Sun). This value is fixed.

The slow, continuous change in the orientation of Earth's rotational axis. Historically called "precession of the equinoxes" because the equinoxes move westward along the ecliptic. In the Holistic model, this is caused by Earth's clockwise orbit around the EARTH-WOBBLE-CENTER over approximately 25,794 years (currently experienced ~25,771 years).

The clockwise circular path Earth travels in ~25,794 years around the EARTH-WOBBLE-CENTER. This orbit results in the visible precession "wobble" (Axial Precession), moving Earth's axial tilt relative to the Ecliptic up and down by ~0.63603°. In year 2000 AD Earth's axis points to Polaris, but e.g. in year ~2900 BC it was pointing to Thuban.

Current experienced value on 21 June 12:00 UTC was 23°26′21″ (~23.4393°). The real MEAN value of the Axial Tilt is ~23.41354° and oscillating by ~0.63603° (above and below). Since the Axial Tilt and Inclination Tilt work upon each other, this currently experienced ~23.44° does not only contain the axial tilt factor but also contains the inclination-tilt factor.

The common center of mass around which two or more celestial bodies orbit. For Earth-Moon, it lies about 4,671 km from Earth's center (inside Earth). The solar system's barycenter moves in a complex pattern around the Sun due to the combined influence of all planets, particularly Jupiter and Saturn.

An orbit path that forms a perfect circle, with eccentricity = 0. In the Holistic Universe Model, the Sun's orbit around the Earth is modelled circular.

When a circle rolls around another circle of equal size, it completes one more rotation than expected. This geometric principle explains why Earth rotates one more time than the number of days in a year, and why there is one fewer sidereal year than solar years in a Great Year.

A cognitive bias that causes people to favor information that confirms their existing beliefs while ignoring or dismissing contradicting evidence. In scientific methodology, confirmation bias can lead researchers to unconsciously interpret data in ways that support their hypotheses.

When two or more celestial bodies appear close together in the sky. Inferior conjunction: inner planet passes between Earth and Sun. Superior conjunction: planet is on the far side of the Sun. The "Great Conjunction" of Jupiter and Saturn occurs every ~19.86 years.

Recognized patterns of stars in the night sky. The 88 official constellations divide the celestial sphere into distinct regions. Twelve along the ecliptic form the zodiac. Due to axial precession, the constellation behind the Sun on any given date shifts over the ~25,794-year precession cycle.

The moment when the Sun reaches its southernmost position in the sky, around December 21-22. Marks the shortest day in the Northern Hemisphere and longest in the Southern. The hemisphere-neutral term "December solstice" is preferred. In the Holistic Universe Model, the start of 1246.03125 AD marks the epoch when year and day lengths matched their mean values.

Used in astronomy as the celestial equivalent of terrestrial latitude.

The time difference ΔT = TT − UT between Terrestrial Time (fixed at 86,400 SI seconds/day) and Universal Time (tracking Earth's actual rotation). When Earth rotates slower, ΔT increases. Was ~0 around 1900 AD; ~69 seconds by 2020. Essential for converting between astronomical and observed time.

The point where an orbiting body crosses the reference plane moving from north to south. Together with the ascending node, these two points define the line of nodes.

The time between successive passages of the Moon through the same node (the points where the Moon's orbit crosses the ecliptic plane), approximately ~27.2122208886 days. This is shorter than the sidereal month because the lunar nodes precess westward (opposite to the Moon's orbital direction) due to nodal precession. The name comes from the ancient belief that eclipses were caused by a dragon eating the Sun or Moon.

The time for the Sun (as seen from Earth) to complete one revolution with respect to the same lunar node, approximately 346.6071969322 days. This is shorter than a solar year because the lunar nodes precess westward. Eclipse seasons occur when the Sun is near a lunar node, making the draconic year crucial for predicting eclipses.

The longitude of perihelion as measured from the EARTH-WOBBLE-CENTER, rather than from Earth (PERIHELION-OF-EARTH). Because Earth orbits the EARTH-WOBBLE-CENTER at ~0.001356 AU, the perihelion angle differs between the two perspectives — e.g. at J2000, ~102.95° from Earth vs ~103° from the EARTH-WOBBLE-CENTER. This difference explains why the currently experienced axial precession (~25,771 years) is longer than the mean (~25,794 years).

A mathematical reference point that Earth orbits around. This point is used to simulate Axial precession (the "wobble" of Earth). Earth moves clockwise around this center on its Axial Precession Orbit (APO) in a period of approximately 25,794 years (currently experienced ~25,771 years)

A measure of how elliptical an orbit is. A value of 0 indicates a perfect circle, while values closer to 1 indicate increasingly elongated ellipses. In the Holistic Universe Model, the PERIHELION-OF-EARTH's orbit around the Sun combined with Earth's movement around the EARTH-WOBBLE-CENTER determines how Earth's effective eccentricity changes over time. The eccentricity therefore has a cycle similar to the 20,957-year Perihelion precession cycle.

A recurring period after which eclipses repeat in a predictable pattern, due to periodic alignment of the Sun, Earth, and Moon. The most well-known is the Saros cycle (~18 years and 11 days).

The plane of Earth's orbit around the Sun (in heliocentric view) or the apparent path of the Sun across the sky as seen from Earth (in geocentric view). The ecliptic is tilted relative to Earth's equator by the obliquity angle.

The precession of Earth's orbital plane (the ecliptic) around the invariable plane, with a period of H/5 = 67,063 years (currently experienced ~70,000 years). This matches the s₃ secular eigenfrequency from Laskar's La2004 solution (68,750 yr) to ~3%. In the Fibonacci framework, H/5 also equals Jupiter's ecliptic perihelion precession period — a dual role. The Ecliptic Precession appears in Earth's ecliptic-frame apsidal dynamics but does not affect Earth's inclination oscillation, which operates in the ICRF.

An orbit path that forms an ellipse (oval shape) with eccentricity > 0. In elliptical orbits, the orbiting body moves faster when closer to the focus (perihelion) and slower when farther away (aphelion).

A unit of time equal to 1/86,400 of a mean solar day. One Ephemeris Second equals one SI second. The 86,400-second standard corresponds to the solar day length around 1820 AD. The actual solar day fluctuates around this value.

A specific moment in time used as a reference point for astronomical measurements. Orbital elements and positions are specified relative to a particular epoch (like J2000) and then calculated forward or backward in time.

The difference between apparent solar time (sundial) and mean solar time (clock), ranging from about −14 to +16 minutes throughout the year. Caused by orbital eccentricity and obliquity. Directly related to the analemma.

The imaginary great circle around Earth equidistant from both poles. The celestial equator is its projection onto the celestial sphere, used as the fundamental plane for the equatorial coordinate system (Right Ascension along it, Declination perpendicular to it).

The two times per year (around March 21 and September 22) when Earth's equatorial plane passes through the center of the Sun, resulting in approximately equal day and night lengths. The equinoxes move westward along the ecliptic due to Axial precession.

The mathematical relationship between the durations of Axial Precession and Inclination Precession cycles, expressed as a Fibonacci ratio of 3:13. This means that 13 Axial Precession cycles (13 × ~25,794 = 335,317 years) occur in the same time as 3 Inclination Precession cycles (3 × 111,772 = 335,317 years), resulting in a repeating pattern of 16 Perihelion Precession cycles within one Holistic-Year. This Fibonacci connection reveals the elegant mathematical structure underlying all precession movements in the solar system.

A cycle related to the Moon's phases and orbital mechanics. In the Holistic Universe Model, the Full Moon cycle ICRF is ~411.7658213165 days and the Full Moon cycle Earth is 411.78 days. This represents the time between alignments of the lunar phases with specific orbital configurations.

A theory of gravitation developed by Albert Einstein between 1907 and 1915, describing gravity not as a force but as a curvature of spacetime caused by mass and energy. One of its key proofs was explaining the anomalous precession of Mercury's perihelion (~43 arc-seconds per century).

A frame of reference of the solar system that puts Earth at the center, and has Sun and Moon orbiting the Earth with other planets revolving around the Sun

The long-term increase in Earth's average surface temperature observed since the late 19th century. The Holistic Universe Model emphasizes that climate has always changed naturally due to orbital mechanics over precession cycles of tens of thousands of years.

A mathematical ratio approximately equal to 1.618 (represented by the Greek letter Phi, φ). The ratio of consecutive Fibonacci numbers approaches φ. In the Holistic Universe Model, the solar system's precession cycles follow Fibonacci ratios on their timescales.

Eight Holistic-Years (8H = 2,682,536 years) — the master period of the Holistic Universe Model. All ICRF orbital periods of the planets divide evenly into this single timescale, so after one Grand Holistic Octave every planet, every precession cycle, and every wobble layer returns to the same configuration. It is the smallest common period over which the entire solar system completes an integer number of cycles.

The fundamental force of attraction between objects with mass

~25,794 years = the full time period for the Earth to complete one APO. This old term of the Axial precession (precession of the equinoxes) is related to Plato and can be seen as a popular term for Axial Precession.

Introduced by Pope Gregory XIII in 1582. Refines the Julian calendar by omitting 3 leap days every 400 years, giving an average year of 365.2425 days. Since the actual solar year (~365.2422 days) differs slightly, the calendar gradually drifts relative to the seasons.

A model or frame of reference where the Sun is at the center and planets orbit around it. The standard modern astronomical model. From Greek: helios (sun) + centric (center).

The solar system is a remarkably complex yet balanced system. Despite this complexity, its primary movements can be modelled by simulating just two interacting motions: Axial precession, which moves clockwise, and Inclination precession, which moves counter clockwise. The Holistic Universe Model explores these motions in detail, offering insight into the dynamic equilibrium governing our solar system and its impact on our Earth.

The complete cycle period of 13 APO (Axial Precession Orbit) or 3 IPO (Inclination Precession Orbit) to meet each other again. This period is 335,317 years and shows on historic temperature/CO2 graphs as measured on e.g. Antarctica.

International Celestial Reference Frame - A system of distant quasars used as fixed reference points for measuring positions and motions of celestial objects. Provides an inertial (non-rotating) reference frame for astronomical measurements.

The counterclockwise movement of the PERIHELION-OF-EARTH around the Sun, caused by the Inclination Precession Orbit (IPO). Also known as "apsidal precession". This motion takes approximately 111,772 years to revolve once relative to the ICRF. When measured against the ecliptic, the period is ~67,063 years — see Ecliptic Precession.

The counterclockwise circular path the PERIHELION-OF-EARTH travels in 111,772 years around the Sun resulting in the "Inclination Precession" which will move Earth inclination tilt compared to the Ecliptic up and down by ~0.63603°.

Inclination to the invariable plane. Current experienced value on 01 January 2000 AD 12:00 UTC was ~1.57869°. The real MEAN value of the Inclination Tilt is ~1.48113° and oscillating by ~0.63603° (above and below) so from ~0.845° to ~2.117°.

When a body (e.g. Venus) is aligned with the Sun while transiting closest to Earth.

The plane through the Solar System barycenter perpendicular to its total angular momentum vector. Unlike the ecliptic, it remains essentially fixed in space. Earth's orbital plane is tilted relative to the invariable plane by a mean of ~1.48113° with an amplitude of ~0.63603°, creating the inclination precession cycle of 111,772 years.

Julian epoch 2000.0 - A standard reference epoch used in astronomy, corresponding to January 1, 2000, 12:00 TT (Terrestrial Time). Used as a baseline for measuring positions and motions of celestial objects.

A calendar introduced in 46 BC by Julius Caesar with a year length of 365.25 days (leap day every 4 years). Slightly too long compared to the actual solar year (~365.2422 days), causing a drift of ~1 day per 128 years. Replaced by the Gregorian calendar in 1582 AD.

Continuous count of days since January 1, 4713 BC at noon UT. Provides an unambiguous date reference across calendar systems. Example: June 21, 2000 AD 00:00 UTC = JD 2,451,716.5. Fractional part = time of day (.0 = noon, .5 = midnight).

The moment when the Sun reaches its northernmost position in the sky, around June 20-21. Marks the longest day in the Northern Hemisphere and shortest in the Southern. The hemisphere-neutral term "June solstice" is preferred over "summer solstice". In the Holistic Universe Model, the June Solstice of 2000 AD is a key reference point.

Three laws of planetary motion by Johannes Kepler (early 17th century): (1) Planets orbit in ellipses with the Sun at one focus, (2) A line from Sun to planet sweeps equal areas in equal times, (3) The square of the orbital period is proportional to the cube of the semi-major axis (T² ∝ a³).

A principle describing the attenuation of light as it passes through a substance. In climate science, it explains why the greenhouse effect of CO2 is logarithmic: each additional unit of CO2 has a diminishing effect on heat absorption as infrared wavelengths become saturated.

The most recent period when ice sheets were at their greatest extent (~26,500 to 19,000 years ago, peaking ~20,000 BC). Sea levels were ~125 m lower. In the Holistic Universe Model, the end of the LGM correlates with maximum inclination tilt combined with rising obliquity.

A correction day (February 29th) added to keep the calendar synchronized with the solar year (~365.242 days). In the Gregorian calendar: every 4 years, except centuries not divisible by 400, giving an average of 365.2425 days.

An orbital element that defines where an orbit crosses the reference plane (typically the ecliptic). It is the angle measured eastward from a specified reference direction (the vernal equinox) to the direction of the ascending node.

An orbital element (symbol: ϖ) that measures the angular position of the perihelion point along the ecliptic, measured from a reference direction (typically the March equinox). Calculated as ϖ = Ω + ω (longitude of ascending node + argument of periapsis). Specifically in the Holistic Universe Model, this value indicates the position of the PERIHELION-OF-EARTH as seen from Earth and changes over time due to Perihelion Precession, completing a full 360° cycle in 20,957 years (currently experienced ~21,000 years).

A calendar based on Moon phases (~29.5 days per month). A lunar year of 12 months is ~354 days, drifting ~11 days per year relative to the seasons. Example: Islamic Hijri calendar.

A ~16.88-year cycle over which the net angular displacement of the Moon's nodal precession (~~18.6 yr, retrograde) and apsidal precession (~~8.85 yr, prograde) completes a full 360° revolution. Needed to correctly model the Moon's 3D position.

The time between successive same lunar phases (new moon to new moon), averaging ~29.53 days. Longer than the sidereal month (~27.32 days) because Earth is also moving along its orbit.

A phenomenon every ~~18.6 years when the Moon's declination range reaches its maximum (major standstill) or minimum (minor standstill) due to the nodal precession cycle. Ancient monuments like Stonehenge were aligned to observe these events.

A calendar combining lunar months with periodic adjustments to stay aligned with the solar year. An extra (intercalary) month is added every 2-3 years. Examples: Hebrew, Chinese, and Hindu calendars.

In the Northern Hemisphere, the March equinox is called the vernal or spring equinox. In the Southern Hemisphere, the reverse is true. That's why the term March equinox is preferred over vernal equinox.

The average rate at which Earth rotates over a long period, approximately 7.292116 × 10⁻⁵ radians/second. Directly related to the mean stellar day length.

Long-term variations in Earth's climate caused by changes in orbital parameters: eccentricity, obliquity, and precession. Named after Serbian scientist Milutin Milanković who first calculated their effects on Earth's climate.

Movements compared to the path of the Sun. This is how we experience all precession movements on Earth.

Movements compared to the fixed stars. This is how someone who looks at the total solar system relative to the fixed stars would observe all precession movements.

A proposed redefinition of the SI second to align with the mean length of day (86,399.999677 seconds) rather than the 1820 epoch value. This would eliminate the systematic Delta-T drift and make 86,400 seconds equal the true mean solar day.

The gradual rotation of the Moon's orbital plane around Earth. The Moon's orbital plane is tilted about 5.1° relative to the ecliptic, and the line of nodes (where the Moon's orbit intersects the ecliptic) rotates westward completing a full cycle in approximately ~18.6 years. This precession has a duration of 18.61321 years against ICRF and ~18.59979 years as experienced on Earth. The nodal precession is responsible for the Lunar Standstill phenomenon and is crucial for predicting eclipses. The nodal precession runs in the direction opposite to the Sun's apparent orbit around Earth.

The standardized Earth rotation rate for timekeeping, set at the epoch 1820. Approximately 7.2921151467064 × 10⁻⁵ radians/second, consistent with the ratio mean solar day / stellar day (1.002 737 811 911 354 48). Since the stellar day fluctuates, so does the true angular velocity. [See IERS useful constants](https://hpiers.obspm.fr/eop-pc/index.php?index=constants&lang=en).

The total result of the Axial Tilt and Inclination Tilt resulting in the actual angle of tilt of Earth's axis of rotation relative to the plane of its orbit (the ecliptic). Currently about 23.44 degrees and this will oscillate and return to the same pattern in a period of 335,317 years between 22.21° – 24.72° in cycles of 41,915 years (currently experienced ~41,000 years).

A problem-solving principle attributed to the 14th-century philosopher William of Ockham, stating that "of two competing theories, the simpler explanation of an entity is to be preferred." When multiple hypotheses explain the same observations, the one with the fewest assumptions should be selected.

When an outer planet is positioned directly opposite the Sun as seen from Earth (angular distance 180°). At opposition the planet is closest to Earth and appears brightest.

Six parameters that uniquely define an orbit: (1) semi-major axis, (2) eccentricity, (3) inclination, (4) longitude of ascending node, (5) argument of periapsis, and (6) true anomaly. Together they allow calculation of position and velocity at any point in time.

The branch of physics that studies the motions of celestial bodies under the influence of gravitational forces. Includes calculations of orbits, trajectories, and interactions between multiple bodies.

The study of past climates using ice cores, tree rings, sediment layers, and isotope ratios. Ice core data extends back ~800,000 years. Paleoclimate data showing ~100,000-year climate cycles supports the inclination precession cycle of 111,772 years as a driver of ice ages.

A unit of astronomical distance equal to approximately 30,856,775,814,671.9 km or about 3.26 light-years. One parsec is the distance at which one astronomical unit (AU) subtends an angle of one arcsecond. The term combines "parallax" and "arcsecond." The relation to a parsec is: 648000 / π × 1 AU = 206,264.806247096 AU.

The partially shadowed outer region of a shadow where only part of the light source is blocked. During eclipses, observers in the penumbra see a partial eclipse.

Closest transit point of the Moon with respect to Earth.

The point in an orbit where a celestial body is closest to the Sun. For Earth, this currently occurs around January 3rd. From Greek: peri (near) + helios (Sun).

The combined cycle where Earth (moving clockwise around the EARTH-WOBBLE-CENTER) and the PERIHELION-OF-EARTH (moving counterclockwise around the Sun) meet each other. This motion occurs over 20,957 years (currently experienced ~21,000 years).

The gradual rotation of Mercury's orbital ellipse around the Sun. Observed at ~575″/cy heliocentric. Classical Newtonian mechanics accounts for ~532″/cy, leaving ~43″/cy unexplained — historically cited as proof of General Relativity. In the Holistic Universe Model, this anomaly is explained by Earth's changing reference frame. The heliocentric rate changes from 574.63″/cy (1900) to 569.95″/cy (2000) to 566.13″/cy (2100). The anomaly was 42.71″ in Einstein's era (1900) and will decrease to 34.21″ by 2100.

A proposed calendar where year 0 corresponds to 1246.03125 AD, when perihelion aligned with the December solstice. Tracks the 20,957-year perihelion precession cycle. Under this system, 2030 AD = PY 784.

A mathematical reference point close to the Sun that serves as the center-point of Earth's yearly orbit, resulting in Earth's perihelion and aphelion. This point moves counterclockwise around the Sun in approximately 111,772 years.

A geological period (~299 to 252 million years ago) ending with the largest mass extinction ("Great Dying"), when ~96% of marine species went extinct. CO2 levels were 1,000-2,000+ PPM, far higher than today's ~425 PPM.

The time required for the PERIHELION OF A PLANET to return to the same fixed star – as viewed from the Earth. This cycle differs per planet. This value can fluctuate across time for a specific planet, but in the long run it returns to this mean value.

The North Star - currently Earth's celestial pole is pointing to the star Polaris. Due to Axial precession, different stars occupy this position over the approximately 25,794-year precession cycle (currently experienced ~25,771 years).

A unit of measurement expressing concentration as parts per million, commonly used to measure atmospheric gas concentrations. For atmospheric CO2, 1 PPM means one molecule of CO2 per million molecules of air. Current atmospheric CO2 is approximately 425 PPM (as of 2024), while during the Last Glacial Maximum it dropped to approximately 170 PPM—dangerously close to the ~150 PPM threshold below which plant life cannot survive through photosynthesis. Historically, CO2 levels have ranged from 170 PPM to over 2,000 PPM across geological time.

A slow, continuous change in the orientation of an astronomical body's axis or orbital plane. The term originally meant "backwards drift" (from the precession of the equinoxes), but in this model it covers both Axial Precession (backward/clockwise) and Inclination Precession (forward/counterclockwise) as the two drivers of all known precession movements.

A celestial body is said to be "in prograde mode" when it moves in the same direction as the Sun.

Used in astronomy as the celestial equivalent of terrestrial longitude.

A coordinate system or set of axes used to measure positions, velocities, and other properties of objects. Different reference frames can give different perspectives on the same motions. Examples include heliocentric, geocentric, and ICRF.

A celestial body is said to be "in retrograde mode" when it moves in the opposite direction of the Sun.

A calendar introduced in 1923 AD, co-designed by Milutin Milankovitch. Average year length of 365.2422222 days — more accurate than the Gregorian (365.2425 days). Will not diverge from the Gregorian until 2800 AD. Adopted by a few Orthodox churches.

An updated interpretation incorporating: (1) Inclination Precession (~111,772 years) as the primary driver of ~100,000-year climate cycles, and (2) revised values based on the Holistic model where all orbital parameters are connected through Fibonacci ratios within the 335,317-year Holistic-Year cycle.

A period of ~18 years, 11 days, and 8 hours (6585.32 days) after which eclipses repeat with nearly identical geometry. Results from the alignment of synodic, draconic, and anomalistic months. Known to ancient Babylonian astronomers.

In the Northern Hemisphere, the September equinox is called the autumnal or fall equinox. In the Southern Hemisphere, the reverse is true. That's why the term September equinox is preferred over autumnal equinox.

A base-60 numeral system inherited from ancient Babylonian mathematics. In astronomy, used to express angles and time: RA as hours:minutes:seconds (e.g., 12h34m56s) and Declination as degrees:arcminutes:arcseconds (e.g., +23°26'21").

The base unit of time in the International System of Units, defined as 9,192,631,770 periods of the caesium-133 atom transition. One SI second equals one Ephemeris Second. The fixed standard of 86,400 SI seconds per day is used for astronomical calculations, though the actual solar day length fluctuates around this value.

Earth's rotation period measured relative to the moving vernal equinox. The J2000 value is ~86,164.0905332 SI seconds. This fluctuates across time in the same way the solar day fluctuates. The MEAN value is 86,164.0902196 seconds.

The time it takes for the Moon to complete one orbit around Earth relative to the fixed stars (ICRF), approximately 27.3216624124 days. This is shorter than the synodic month (~29.5305969755 days) because it measures the Moon's position against the stellar background rather than relative to the Sun. As Earth moves along its orbit, additional time is needed for the Moon to return to the same phase relative to the Sun.

A celestial body completes a "sidereal" period each time it aligns again with a given star.

The time for Earth to complete one orbit relative to the fixed stars. A FIXED period of ~31,558,149.76 seconds (MEAN 365.2563644 days). This is ~1,224.5 seconds longer than the Solar year due to the coin rotation paradox.

A calendar that keeps a solar event (equinox or solstice) fixed on the same date each year, based on the tropical year (~365.2422 days). Examples: Julian, Gregorian, and Revised Julian calendars. Background constellations gradually shift due to axial precession.

The time for the Sun to return to the same position in the sky, from one noon to the next. Computed as Sidereal Year (seconds) / Sidereal Year (days). The J2000 value is 86,400.0001 SI seconds. This fluctuates both within a year and over millennia as the sidereal year in days changes. The difference between clock time (TT) and observed time (UT) is known as [Delta T (ΔT)](https://en.wikipedia.org/wiki/%CE%94T_%28timekeeping%29). The MEAN value is 86,399.999677 SI seconds.

The angle subtended by Earth's equatorial radius as seen from the center of the Sun, approximately 8.794143836 arcseconds. Solar parallax is used to determine the Earth-Sun distance: the distance to the Sun equals the radius of Earth divided by the solar parallax, times the conversion factor from arcseconds to radians (206,264.806247096).

Also known as "Tropical year". The time between two successive March equinoxes. A MEAN value of 365.2422 days. The J2000 value is 31,556,925.30 seconds (calculated against the fixed 86,400 seconds/day). The actual length fluctuates because the length of day varies both within a year and across the ages.

The two times per year (around June 21 and December 21) when the Sun reaches its highest or lowest point in the sky at noon, marking the longest and shortest days of the year. These dates are determined by Earth's tilt relative to the ecliptic.

A theory developed by Albert Einstein in 1905 describing the relationship between space and time. It is based on two postulates: the laws of physics are the same for all observers in uniform motion, and the speed of light in a vacuum is constant for all observers. Along with General Relativity, it forms the foundation of modern physics.

A theoretical calendar that fixes dates to star positions rather than solar events. Follows the sidereal year, so equinoxes would drift through the calendar over the ~25,794-year axial precession cycle. No major civilization has adopted one.

Earth's rotation period relative to the fixed stars (ICRF), measured as the Earth Rotation Angle (ERA). A 360° increase in ERA is one full rotation. The stellar day is directly connected to the Nominal Angular Velocity (ΩN), which is the reference angular velocity set at the epoch 1820. Most sources list 86,164.0996618 seconds. Like the solar day, this value [fluctuates over time](https://en.wikipedia.org/wiki/Day_length_fluctuations). The mean value is 86,164.0993407 seconds — about 9.12 ms longer than the MEAN sidereal day.

When a body (e.g. Venus) is aligned with the Sun while transiting furthest from Earth.

A celestial body completes a "synodic" period each time it aligns again with the Sun.

A uniform time scale independent of Earth's rotation, fixed at exactly 86,400 SI seconds per day. Coincidentally aligned with the mean solar day around 1820 AD. Provides a stable reference for predicting astronomical events. The difference TT − UT is Delta-T (ΔT).

The starting point of the 335,317-year Holistic-Year cycle, showing as 3 times ~100k year cycles as identified on historical temperature graphs from ice core data. The Balanced Year represents the moment when Earth's maximum Axial tilt and minimum Inclination tilt are in exact opposite position and therefore level out on the mean Axial tilt of Earth's movement in the ecliptic plane. The current cycle began in 302,635 BC, and the next Balanced Year will occur in 32,682 AD.

A JavaScript library for creating 3D graphics in a web browser using WebGL. The Interactive 3D Solar System Simulation is built in Three.js.

Differential gravitational forces caused by the variation in gravitational pull across an extended object. In standard astronomy, tidal forces from the Sun and Moon on Earth's equatorial bulge are said to cause Axial precession.

The passage of a celestial body directly between a larger body and the observer. Mercury transits occur ~13-14 times per century; Venus transits occur in pairs separated by 8 years, with over a century between pairs.

The time for the Moon to return to the same ecliptic longitude relative to the vernal equinox, approximately 27.3215892720 days. This is very close to the sidereal month but slightly shorter due to the precession of the equinoxes.

The darkest central part of a shadow where the light source is completely blocked. During eclipses, only observers within the umbra see a total eclipse.

Constant velocity - moving at the same speed without acceleration or deceleration. In the Holistic model, the Sun moves at uniform speed in its circular orbit around the PERIHELION-OF-EARTH, unlike the variable speed in elliptical orbits.

A time standard based on Earth's actual rotation. Unlike Terrestrial Time (TT), UT varies because Earth's rotation speed is not constant. Corresponds to the actual day-night cycle as experienced on Earth. The difference TT − UT is Delta-T (ΔT).

Coordinated Universal Time — the primary time standard for civil timekeeping worldwide. Based on atomic clocks (TAI) with leap seconds added to stay close to mean solar time. The acronym "UTC" is a compromise between the English ordering "Coordinated Universal Time" (CUT) and the French "Temps Universel Coordonné" (TUC).

Variations Séculaires des Orbites Planétaires — an analytical solution for computing planetary positions and orbital elements, developed by Bretagnon and Francou (1987). Valid for several thousand years around J2000 and widely used in astronomical software.

The terminology used by Copernicus to describe the apparent movement of the Earth in the phenomenon known as the "Precession of the Equinoxes". This movement is simulated by Earth orbiting the EARTH-WOBBLE-CENTER in its Axial Precession Orbit (APO).

A sudden return to near-glacial conditions that occurred approximately 12,900 to 11,700 years ago (around 10,900 to 9,700 BC), interrupting the warming trend following the Last Glacial Maximum. Named after a cold-tolerant wildflower (Dryas octopetala) that became common during this period. It coincides with the max value of the obliquity cycle.

The apparent path our Sun travels across the celestial sphere over the course of the year through the 12 constellations.

The twelve 30-degree divisions of the ecliptic used in astronomy and astrology (Aries, Taurus, Gemini, etc.). Due to Axial precession, the equinoxes move westward through these signs over approximately 25,794 years (currently experienced ~25,771 years).