Theoretical astronomy is the use ofanalytical andcomputational models based on principles from physics and chemistry to describe and explainastronomical objects and astronomical phenomena. Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena.

Ptolemy'sAlmagest, although a brilliant treatise on theoreticalastronomy combined with a practical handbook for computation, nevertheless includes compromises to reconcile discordant observations with ageocentric model. Modern theoretical astronomy is usually assumed to have begun with the work ofJohannes Kepler (1571–1630), particularly withKepler's laws. The history of the descriptive and theoretical aspects of theSolar System mostly spans from the latesixteenth century to the end of the nineteenth century.

Theoretical astronomy is built on the work ofobservational astronomy,astrometry,astrochemistry, andastrophysics. Astronomy was early to adopt computational techniques to model stellar and galactic formation and celestial mechanics. From the point of view of theoretical astronomy, not only must the mathematical expression be reasonably accurate but it should preferably exist in a form which is amenable to further mathematical analysis when used in specific problems. Most of theoretical astronomy usesNewtonian theory of gravitation, considering that the effects ofgeneral relativity are weak for most celestial objects. Theoretical astronomy does not attempt to predict the position, size and temperature of every object in theuniverse, but by and large has concentrated upon analyzing the apparently complex but periodic motions of celestial objects.

"Contrary to the belief generally held by laboratory physicists, astronomy has contributed to the growth of our understanding of physics."^[1] Physics has helped in the elucidation of astronomical phenomena, and astronomy has helped in the elucidation of physical phenomena:

1. discovery of the law of gravitation came from the information provided by the motion of theMoon and the planets,
2. viability of nuclear fusion as demonstrated in theSun and stars and yet to be reproduced on earth in a controlled form.^[1]

Integrating astronomy with physics involves:

The aim of astronomy is to understand the physics and chemistry from the laboratory that is behind cosmic events so as to enrich our understanding of the cosmos and of these sciences as well.^[1]

Astrochemistry, the overlap of the disciplines ofastronomy andchemistry, is the study of the abundance and reactions ofchemical elements and molecules in space, and their interaction with radiation. The formation, atomic and chemical composition, evolution and fate ofmolecular gas clouds, is of special interest because it is from these clouds that solar systems form.

Infrared astronomy, for example, has revealed that theinterstellar medium contains a suite of complex gas-phase carbon compounds called aromatic hydrocarbons, often abbreviated (PAHs or PACs). These molecules composed primarily of fused rings of carbon (either neutral or in an ionized state) are said to be the most common class of carbon compound in the galaxy. They are also the most common class of carbon molecule inmeteorites and in cometary and asteroidal dust (cosmic dust). These compounds, as well as the amino acids, nucleobases, and many other compounds in meteorites, carrydeuterium (²H) andisotopes of carbon, nitrogen, and oxygen that are very rare on earth, attesting to their extraterrestrial origin. The PAHs are thought to form in hot circumstellar environments (around dying carbon richred giant stars).

The sparseness of interstellar and interplanetary space results in some unusual chemistry, since symmetry-forbidden reactions cannot occur except on the longest of timescales. For this reason, molecules and molecular ions which are unstable on earth can be highly abundant in space, for example theH₃⁺ ion. Astrochemistry overlaps withastrophysics andnuclear physics in characterizing the nuclear reactions which occur in stars, the consequences forstellar evolution, as well as stellar 'generations'. Indeed, the nuclear reactions in stars produce every naturally occurringchemical element. As the stellar 'generations' advance, the mass of the newly formed elements increases. A first-generation star uses elementalhydrogen (H) as a fuel source and produceshelium (He). Hydrogen is the most abundant element, and it is the basic building block for all other elements as its nucleus has only oneproton. Gravitational pull toward the center of a star creates massive amounts of heat and pressure, which causenuclear fusion. Through this process of merging nuclear mass, heavier elements are formed.Lithium,carbon,nitrogen andoxygen are examples of elements that form in stellar fusion. After many stellar generations, very heavy elements are formed (e.g.iron andlead).

Theoretical astronomers use a wide variety of tools which includeanalytical models (for example,polytropes to approximate the behaviors of astar) andcomputationalnumerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.^[2][3]

Astronomy theorists endeavor to create theoretical models and figure out the observational consequences of those models. This helps observers look for data that can refute a model or help in choosing between several alternate or conflicting models.^{[citation needed]}

Theorists also try to generate or modify models to take into account new data. Consistent with the general scientific approach, in the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.^{[citation needed]}

Topics studied by theoretical astronomers include:

1. stellar dynamics andevolution;
2. galaxy formation;
3. large-scale structure ofmatter in theUniverse;
4. origin ofcosmic rays;
5. general relativity andphysical cosmology, includingstring cosmology andastroparticle physics.

Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis forblack hole (astro)physics and the study ofgravitational waves.

Some widely accepted and studied theories and models in astronomy, now included in theLambda-CDM model are theBig Bang,Cosmic inflation,dark matter, and fundamental theories ofphysics.

A few examples of this process:

Dark matter anddark energy are the current leading topics in astronomy,^[4] as their discovery and controversy originated during the study of the galaxies.

Of the topics approached with the tools of theoretical physics, particular consideration is often given to stellar photospheres, stellar atmospheres, the solar atmosphere, planetary atmospheres, gaseous nebulae, nonstationary stars, and the interstellar medium. Special attention is given to the internal structure of stars.^[5]

The observation of a neutrino burst within 3 h of the associated optical burst fromSupernova 1987A in theLarge Magellanic Cloud (LMC) gave theoretical astrophysicists an opportunity to test that neutrinos and photons follow the same trajectories in the gravitational field of the galaxy.^[6]

A general form of the first law of thermodynamics for stationaryblack holes can be derived from the microcanonical functional integral for the gravitational field.^[7] The boundary data

1. the gravitational field as described with a microcanonical system in a spatially finite region and
2. the density of states expressed formally as a functional integral over Lorentzian metrics and as a functional of the geometrical boundary data that are fixed in the corresponding action,

are the thermodynamical extensive variables, including the energy and angular momentum of the system.^[7] For the simpler case of nonrelativistic mechanics as is often observed in astrophysical phenomena associated with a black hole event horizon, the density of states can be expressed as a real-time functional integral and subsequently used to deduce Feynman's imaginary-time functional integral for the canonical partition function.^[7]

Reaction equations and large reaction networks are an important tool in theoretical astrochemistry, especially as applied to the gas-grain chemistry of the interstellar medium.^[8] Theoretical astrochemistry offers the prospect of being able to place constraints on the inventory of organics for exogenous delivery to the early Earth.

"An important goal for theoretical astrochemistry is to elucidate which organics are of true interstellar origin, and to identify possible interstellar precursors and reaction pathways for those molecules which are the result of aqueous alterations."^[9] One of the ways this goal can be achieved is through the study of carbonaceous material as found in some meteorites. Carbonaceous chondrites (such as C1 and C2) include organic compounds such as amines and amides; alcohols, aldehydes, and ketones; aliphatic and aromatic hydrocarbons; sulfonic and phosphonic acids; amino, hydroxycarboxylic, and carboxylic acids; purines and pyrimidines; andkerogen-type material.^[9] The organic inventories of primitive meteorites display large and variable enrichments in deuterium,carbon-13 (¹³C), andnitrogen-15 (¹⁵N), which is indicative of their retention of an interstellar heritage.^[9]

The chemical composition of comets should reflect both the conditions in the outer solar nebula some 4.5 × 10⁹ ayr, and the nature of the natal interstellar cloud from which theSolar System was formed.^[10] While comets retain a strong signature of their ultimate interstellar origins, significant processing must have occurred in the protosolar nebula.^[10] Early models of coma chemistry showed that reactions can occur rapidly in the inner coma, where the most important reactions are proton transfer reactions.^[10] Such reactions can potentially cycle deuterium between the different coma molecules, altering the initial D/H ratios released from the nuclear ice, and necessitating the construction of accurate models of cometary deuterium chemistry, so that gas-phase coma observations can be safely extrapolated to give nuclear D/H ratios.^[10]

While the lines of conceptual understanding between theoretical astrochemistry and theoretical chemical astronomy often become blurred so that the goals and tools are the same, there are subtle differences between the two sciences. Theoretical chemistry as applied to astronomy seeks to find new ways to observe chemicals in celestial objects, for example. This often leads to theoretical astrochemistry having to seek new ways to describe or explain those same observations.

The new era of chemical astronomy had to await the clear enunciation of the chemical principles of spectroscopy and the applicable theory.^[11]

Supernova radioactivity dominates light curves and the chemistry of dust condensation is also dominated by radioactivity.^[12] Dust is usually either carbon or oxides depending on which is more abundant, but Compton electrons dissociate the CO molecule in about one month.^[12] The new chemical astronomy of supernova solids depends on the supernova radioactivity:

1. the radiogenesis of⁴⁴Ca from⁴⁴Ti decay after carbon condensation establishes their supernova source,
2. their opacity suffices to shift emission lines blueward after 500 d and emits significant infrared luminosity,
3. parallel kinetic rates determine trace isotopes in meteoritic supernova graphites,
4. the chemistry is kinetic rather than due to thermal equilibrium and
5. is made possible by radiodeactivation of the CO trap for carbon.^[12]

Like theoretical chemical astronomy, the lines of conceptual understanding between theoretical astrophysics and theoretical physical astronomy are often blurred, but, again, there are subtle differences between these two sciences. Theoretical physics as applied to astronomy seeks to find new ways to observe physical phenomena in celestial objects and what to look for, for example. This often leads to theoretical astrophysics having to seek new ways to describe or explain those same observations, with hopefully a convergence to improve our understanding of the local environment of Earth and the physicalUniverse.

Nuclear matrix elements of relevant operators as extracted from data and from a shell-model and theoretical approximations both for the two-neutrino and neutrinoless modes of decay are used to explain the weak interaction and nuclear structure aspects of nuclear double beta decay.^[13]

New neutron-rich isotopes,³⁴Ne,³⁷Na, and⁴³Si have been produced unambiguously for the first time, and convincing evidence for the particle instability of three others,³³Ne,³⁶Na, and³⁹Mg has been obtained.^[14] These experimental findings compare with recent theoretical predictions.^[14]

Until recently all the time units that appear natural to us are caused by astronomical phenomena:

1. Earth's orbit around the Sun => the year, and the seasons,
2. Moon's orbit around the Earth => the month,
3. Earth's rotation and the succession of brightness and darkness => the day (and night).

High precision appears problematic:

1. ambiguities arise in the exact definition of a rotation or revolution,
2. some astronomical processes are uneven and irregular, such as the noncommensurability of year, month, and day,
3. there are a multitude of time scales and calendars to solve the first two problems.^[15]

Some of thesetime standard scales aresidereal time,solar time, anduniversal time.

From theSysteme Internationale (SI) comes the second as defined by the duration of 9 192 631 770 cycles of a particular hyperfine structure transition in the ground state ofcaesium-133 (¹³³Cs).^[15] For practical usability a device is required that attempts to produce the SI second (s) such as anatomic clock. But not all such clocks agree. The weighted mean of many clocks distributed over the whole Earth defines theTemps Atomique International; i.e., the Atomic Time TAI.^[15] From theGeneral theory of relativity the time measured depends on the altitude on earth and the spatial velocity of the clock so that TAI refers to a location on sea level that rotates with the Earth.^[15]

Since the Earth's rotation is irregular, any time scale derived from it such asGreenwich Mean Time led to recurring problems in predicting theEphemerides for the positions of theMoon,Sun,planets and theirnatural satellites.^[15] In 1976 theInternational Astronomical Union (IAU) resolved that the theoretical basis for ephemeris time (ET) was wholly non-relativistic, and therefore, beginning in 1984 ephemeris time would be replaced by two further time scales with allowance for relativistic corrections. Their names, assigned in 1979,^[16] emphasized their dynamical nature or origin,Barycentric Dynamical Time (TDB) andTerrestrial Dynamical Time (TDT). Both were defined for continuity with ET and were based on what had become the standard SI second, which in turn had been derived from the measured second of ET.

During the period 1991–2006, the TDB and TDT time scales were both redefined and replaced, owing to difficulties or inconsistencies in their original definitions. The current fundamental relativistic time scales areGeocentric Coordinate Time (TCG) andBarycentric Coordinate Time (TCB). Both of these have rates that are based on the SI second in respective reference frames (and hypothetically outside the relevant gravity well), but due to relativistic effects, their rates would appear slightly faster when observed at the Earth's surface, and therefore diverge from local Earth-based time scales using the SI second at the Earth's surface.^[17]

The currently defined IAU time scales also includeTerrestrial Time (TT) (replacing TDT, and now defined as a re-scaling of TCG, chosen to give TT a rate that matches the SI second when observed at the Earth's surface),^[18] and a redefined Barycentric Dynamical Time (TDB), a re-scaling of TCB to give TDB a rate that matches the SI second at the Earth's surface.

For astar, the dynamical time scale is defined as the time that would be taken for a test particle released at the surface to fall under thestar's potential to the centre point, if pressure forces were negligible. In other words, the dynamical time scale measures the amount of time it would take a certainstar to collapse in the absence of anyinternal pressure. By appropriate manipulation of the equations of stellar structure this can be found to be

${\displaystyle {\tau }_{dynamical}\simeq \frac{R}{v}=\sqrt{\frac{R^3}{2GM}}\sim 1/\sqrt{G\rho }}$ 

where R is theradius of the star, G is thegravitational constant, M is themass of the star, ρ the star gasdensity (assumed constant here) and v is theescape velocity. As an example, theSun dynamical time scale is approximately 1133 seconds. Note that the actual time it would take a star like the Sun to collapse is greater because internal pressure is present.

The 'fundamental' oscillatory mode of a star will be at approximately the dynamical time scale. Oscillations at this frequency are seen inCepheid variables.

The basic characteristics of applied astronomical navigation are

1. usable in all areas of sailing around the Earth,
2. applicable autonomously (does not depend on others – persons or states) and passively (does not emit energy),
3. conditional usage via optical visibility (of horizon and celestial bodies), or state of cloudiness,
4. precisional measurement, sextant is 0.1', altitude and position is between 1.5' and 3.0'.
5. temporal determination takes a couple of minutes (using the most modern equipment) and ≤ 30 min (using classical equipment).^[19]

The superiority of satellite navigation systems to astronomical navigation are currently undeniable, especially with the development and use of GPS/NAVSTAR.^[19] This global satellite system

1. enables automated three-dimensional positioning at any moment,
2. automatically determines position continuously (every second or even more often),
3. determines position independent of weather conditions (visibility and cloudiness),
4. determines position in real time to a few meters (two carrying frequencies) and 100 m (modest commercial receivers), which is two to three orders of magnitude better than by astronomical observation,
5. is simple even without expert knowledge,
6. is relatively cheap, comparable to equipment for astronomical navigation, and
7. allows incorporation into integrated and automated systems of control and ship steering.^[19] The use of astronomical or celestial navigation is disappearing from the surface and beneath or above the surface of the Earth.

Geodetic astronomy is the application ofastronomical methods intonetworks and technical projects ofgeodesy for

• apparent places of stars, and theirproper motions
• precise astronomicalnavigation
• astro-geodeticgeoid determination and
• modelling the rockdensities of the topography and ofgeological layers in thesubsurface
• Satellite geodesy using the stellar background (see alsoastrometry and cosmic triangulation)
• Monitoring of theEarth rotation and polar wandering
• Contribution to the time system of physics andgeosciences

Astronomical algorithms are thealgorithms used to calculateephemerides,calendars, and positions (as incelestial navigation orsatellite navigation).

Many astronomical and navigational computations use theFigure of the Earth as a surface representing the Earth.

TheInternational Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time andreference frame standards, notably through its Earth Orientation Parameter (EOP) andInternational Celestial Reference System (ICRS) groups.

TheDeep Space Network, orDSN, is an internationalnetwork of largeantennas and communication facilities that supportsinterplanetaryspacecraft missions, andradio andradar astronomy observations for the exploration of theSolar System and theuniverse. The network also supports selected Earth-orbiting missions. DSN is part of theNASAJet Propulsion Laboratory (JPL).

An observer becomes a deep space explorer upon escaping Earth's orbit.^[20] While theDeep Space Network maintains communication and enables data download from an exploratory vessel, any local probing performed by sensors or active systems aboard usually require astronomical navigation, since the enclosing network of satellites to ensure accurate positioning is absent.

• Astrochemistry
• Astrometry
• Astrophysics
• Celestial mechanics
• Celestial navigation
• Celestial sphere
• Orbital mechanics

• Introduction to Cataclysmic Variables (CVs)
• L. Sidoli, 2008Transient outburst mechanisms
• Commentary on "The Compendium of Plain Astronomy" is a manuscript from 1665 about theoretical astronomy