View source for Theoretical astronomy

{{Short description|Applied and interdisciplinary physics}}
[[File:Kuiper belt - Oort cloud-en.svg|upright=1.5|thumb|The [[Oort cloud]], one of the most successful theoretical models about the [[Solar System]]]]

'''Theoretical astronomy''' is the use of [[Mathematical model|analytical]] and [[computational model]]s based on principles from physics and chemistry to describe and explain [[astronomical object]]s 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]]'s ''[[Almagest]]'', although a brilliant treatise on theoretical [[astronomy]] combined with a practical handbook for computation, nevertheless includes compromises to reconcile discordant observations with a [[geocentric model]]. Modern theoretical astronomy is usually assumed to have begun with the work of [[Johannes Kepler]] (1571–1630), particularly with [[Kepler's laws of planetary motion|Kepler's laws]]. The history of the descriptive and theoretical aspects of the [[Solar System]] mostly spans from the late [[History of science in the Renaissance|sixteenth century]] to the end of the nineteenth century. 

Theoretical astronomy is built on the work of [[observational astronomy]], [[astrometry]], [[astrochemistry]], and [[astrophysics]]. 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 uses [[Gravity|Newtonian theory of gravitation]], considering that the effects of [[general relativity]] are weak for most celestial objects. Theoretical astronomy does not attempt to predict the position, size and temperature of every object in the [[universe]], but by and large has concentrated upon analyzing the apparently complex but periodic motions of celestial objects.

==Integrating astronomy and physics==
{{Main|Astrophysics}}
"Contrary to the belief generally held by laboratory physicists, astronomy has contributed to the growth of our understanding of physics."<ref name=Narlikar>{{Cite journal|author=Narlikar JV |title=Curriculum for the Training of Astronomers In: ''The Teaching of astronomy'' |journal=IAU Colloq. 105: The Teaching of Astronomy |pages=7 |editor=Pasachoff JM |editor2=Percy JR |publisher=[[Cambridge University Press]] |location=Cambridge, England |date=1990 |bibcode=1990teas.conf....7N }}</ref> Physics has helped in the elucidation of astronomical phenomena, and astronomy has helped in the elucidation of physical phenomena:
# discovery of the law of gravitation came from the information provided by the motion of the [[Moon]] and the planets,
# viability of nuclear fusion as demonstrated in the [[Sun]] and stars and yet to be reproduced on earth in a controlled form.<ref name=Narlikar/>

Integrating astronomy with physics involves:
{| class="wikitable" border="1"
|-
| |<!-- A --> '''Physical interaction'''
| |<!-- B --> '''Astronomical phenomena'''
|-
| |<!-- A --> ''[[Electromagnetism]]'':
| |<!-- B --> observation using the [[electromagnetic spectrum]]
|-
| |<!-- A --> [[Thermal radiation|black body radiation]]
| |<!-- B --> stellar radiation
|-
| |<!-- A --> [[synchrotron radiation]]
| |<!-- B --> [[Astronomical radio source|radio]] and [[Astronomical X-ray source|X-ray source]]s
|-
| |<!-- A --> [[Compton scattering|inverse-Compton scattering]]
| |<!-- B --> [[astronomical X-ray source]]s
|-
| |<!-- A --> acceleration of charged particles
| |<!-- B --> [[pulsar]]s and [[cosmic ray]]s
|-
| |<!-- A --> [[Absorption (electromagnetic radiation)|absorption]]/[[scattering]]
| |<!-- B --> [[Interstellar medium|interstellar dust]]
|-
| |<!-- A --> ''Strong and weak interaction'':
| |<!-- B --> [[nucleosynthesis]] in [[star]]s
|-
| |<!-- A -->
| |<!-- B --> [[cosmic ray]]s
|-
| |<!-- A -->
| |<!-- B --> [[supernova]]e
|-
| |<!-- A -->
| |<!-- B --> primeval universe
|-
| |<!-- A --> ''[[Gravitation|Gravity]]'':
| |<!-- B --> motion of [[planet]]s, [[Natural satellite|satellite]]s and [[binary star]]s, stellar structure and evolution, N-body motions in [[Star cluster|clusters of stars]] and [[Galaxy groups and clusters|galaxies]], [[black hole]]s, and the [[Big Bang|expanding universe]].<ref name=Narlikar/>
|-
|}

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.<ref name=Narlikar/>

==Integrating astronomy and chemistry==
{{Main|Astrochemistry}}
'''Astrochemistry''', the overlap of the disciplines of [[astronomy]] and [[chemistry]], is the study of the abundance and reactions of [[chemical elements]] and molecules in space, and their interaction with radiation. The formation, atomic and chemical composition, evolution and fate of [[Molecular cloud|molecular gas cloud]]s, is of special interest because it is from these clouds that solar systems form.

Infrared astronomy, for example, has revealed that the [[interstellar medium]] contains a suite of complex gas-phase carbon compounds called aromatic hydrocarbons, often abbreviated ([[Polycyclic aromatic hydrocarbon|PAH]]s 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 in [[meteorite]]s and in cometary and asteroidal dust ([[cosmic dust]]). These compounds, as well as the amino acids, nucleobases, and many other compounds in meteorites, carry [[deuterium]] (<sup>2</sup>H) and [[isotope]]s 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 rich [[red 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 the [[Protonated molecular hydrogen|H<sub>3</sub><sup>+</sup>]] ion. Astrochemistry overlaps with [[astrophysics]] and [[nuclear physics]] in characterizing the nuclear reactions which occur in stars, the consequences for [[stellar evolution]], as well as stellar 'generations'. Indeed, the nuclear reactions in stars produce every naturally occurring [[chemical element]]. As the stellar 'generations' advance, the mass of the newly formed elements increases. A first-generation star uses elemental [[hydrogen]] (H) as a fuel source and produces [[helium]] (He). Hydrogen is the most abundant element, and it is the basic building block for all other elements as its nucleus has only one [[proton]]. Gravitational pull toward the center of a star creates massive amounts of heat and pressure, which cause [[nuclear fusion]]. Through this process of merging nuclear mass, heavier elements are formed. [[Lithium]], [[carbon]], [[nitrogen]] and [[oxygen]] are examples of elements that form in stellar fusion. After many stellar generations, very heavy elements are formed (e.g. [[iron]] and [[lead]]).

==Tools of theoretical astronomy==
Theoretical astronomers use a wide variety of tools which include [[mathematical model|analytical models]] (for example, [[polytrope]]s to approximate the behaviors of a [[star]]) and [[computation]]al [[Numerical analysis|numerical 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.<ref name=Roth>{{Cite journal|doi=10.1103/PhysRev.39.525|author=Roth H |title=A Slowly Contracting or Expanding Fluid Sphere and its Stability |journal=Phys. Rev. |volume=39|issue=3 |pages=525–9 |date=1932 |bibcode=1932PhRv...39..525R}}</ref><ref name=Eddington>{{Cite book|author=Eddington AS |title=Internal Constitution of the Stars |journal=Science |date=1926 |volume=52 |issue=1341 |pages=233–40 |publisher=Cambridge University Press |location=Cambridge, England |doi=10.1126/science.52.1341.233 |pmid=17747682 |bibcode=1920Sci....52..233E |isbn=978-0-521-33708-3 |url=https://zenodo.org/record/1429642 }}</ref>

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.{{cn|date=April 2023}}

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.{{cn|date=April 2023}}

==Topics of theoretical astronomy==
Topics studied by theoretical astronomers include:
# [[stellar dynamics]] and [[Stellar evolution|evolution]];
# [[Galaxy formation and evolution|galaxy formation]];
# [[large-scale structure of the universe|large-scale structure]] of [[matter]] in the [[Universe]];
# origin of [[cosmic ray]]s;
# [[general relativity]] and [[physical cosmology]], including [[string theory|string]] cosmology and [[astroparticle 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 for [[black hole]] {{Not a typo|(astro)}}[[physics]] and the study of [[gravitational waves]].

==Astronomical models==
Some widely accepted and studied theories and models in astronomy, now included in the [[Lambda-CDM model]] are the [[Big Bang]], [[Cosmic inflation]], [[dark matter]], and fundamental theories of [[physics]].

A few examples of this process:

{| class="wikitable" border="1"
|-
| |<!-- A --> '''Physical process'''
| |<!-- B --> '''Experimental tool'''
| |<!-- C --> '''Theoretical model'''
| |<!-- D --> '''Explains/predicts'''
|-
| |<!-- A -->[[Gravitation]]
| |<!-- B -->[[Radio telescope]]s
| |<!-- C -->[[Nordtvedt effect|Self-gravitating system]]
| |<!-- D -->Emergence of a [[star system]]
|-
| |<!-- A --> [[Nuclear fusion]]
| |<!-- B --> [[Spectroscopy]]
| |<!-- C --> [[Stellar evolution]]
| |<!-- D --> How the stars shine and how [[nucleosynthesis|metals formed]]
|-
| |<!-- A -->[[The Big Bang]]
| |<!-- B -->[[Hubble Space Telescope]], [[Cosmic Background Explorer|COBE]]
| |<!-- C --> [[Expanding universe]]
| |<!-- D --> [[Age of the Universe]]
|-
| |<!-- A --> [[Quantum fluctuation]]s
| |<!-- B -->
| |<!-- C --> [[Cosmic inflation]]
| |<!-- D --> [[Flatness problem]]
|-
| |<!-- A --> [[Gravitational collapse]]
| |<!-- B --> [[X-ray astronomy]]
| |<!-- C --> [[General relativity]]
| |<!-- D --> [[Black hole]]s at the center of [[Andromeda Galaxy]]
|-
| |<!-- A --> [[CNO cycle]] in [[star]]s
| |<!-- B -->
| |<!-- C -->
| |<!-- D -->
|-
|}

==Leading topics in theoretical astronomy==
[[Dark matter]] and [[dark energy]] are the current leading topics in astronomy,<ref>http://imagine.gsfc.nasa.gov/docs/science/know_l1/dark_matter.html third paragraph, "There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is". Retrieved 2009-11-02</ref> as their discovery and controversy originated during the study of the galaxies.

==Theoretical astrophysics==
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.<ref name=Sobolev>{{Cite book|author=Sobolev VV |title=Course in theoretical astrophysics |date=1985 |pages=504 |edition=3rd revised and enlarged |publisher=Izdatel'stvo Nauka |location=Moscow |bibcode=1985cta..book.....S }}</ref>

===Weak equivalence principle===
The observation of a neutrino burst within 3 h of the associated optical burst from [[Supernova 1987A]] in the [[Large 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.<ref name=Krauss>{{Cite journal|author=Krauss LM|author2=Tremaine S |title=Test of the Weak Equivalence Principle for Neutrinos and Photons |journal=Phys. Rev. Lett. |date=Mar 1988 |volume=60 |issue=3 |pages=176–7 |doi=10.1103/PhysRevLett.60.176 |bibcode=1988PhRvL..60..176K |pmid=10038467}}</ref>

===Thermodynamics for stationary black holes===
A general form of the first law of thermodynamics for stationary [[black hole]]s can be derived from the microcanonical functional integral for the gravitational field.<ref name=Brown>{{Cite journal|author=Brown JD|author2=York JW Jr. |title=Microcanonical functional integral for the gravitational field |journal=Phys. Rev. D |date=Apr 1993 |volume=47 |issue=4 |pages=1420–31 |doi=10.1103/PhysRevD.47.1420 |pmid=10015718 |arxiv = gr-qc/9209014 |bibcode = 1993PhRvD..47.1420B |s2cid=25039417 }}</ref> The boundary data
# the gravitational field as described with a microcanonical system in a spatially finite region and
# 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.<ref name=Brown/> 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.<ref name=Brown/>

==Theoretical astrochemistry==
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.<ref name=Vasyunin>{{Cite journal|author=Vasyunin AI|title=A Unified Monte Carlo Treatment of Gas-Grain Chemistry for Large Reaction Networks. I. Testing Validity of Rate Equations in Molecular Clouds|author2=Semenov DA|author3=Wiebe DS|author4=Henning T |journal=Astrophys. J. |date=Feb 2009 |volume=691 |issue=2 |pages=1459–69 |doi=10.1088/0004-637X/691/2/1459 |bibcode=2009ApJ...691.1459V|arxiv = 0810.1591 |s2cid=98364729}}</ref> Theoretical astrochemistry offers the prospect of being able to place constraints on the inventory of organics for exogenous delivery to the early Earth.

===Interstellar organics===
"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."<ref name=Ehrenfreund>{{Cite book|author=Ehrenfreund P|author2=Charnley SB|author3=Botta O |title=A voyage from dark clouds to the early Earth ''In: Astrophysics of life: proceedings of the Space Telescope Science Institute Symposium held in Baltimore, Maryland, May 6–9, 2002, Volume 16 of Space Telescope Science Institute symposium series'' |date=2005 |editor=Livio M |editor2= Reid IN |editor3=Sparks WB |pages=1–20 of 110 |publisher=Cambridge University Press |location=Cambridge, England |isbn=9780521824903 }}</ref> 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; and [[kerogen]]-type material.<ref name=Ehrenfreund/> The organic inventories of primitive meteorites display large and variable enrichments in deuterium, [[carbon-13]] (<sup>13</sup>C), and [[nitrogen-15]] (<sup>15</sup>N), which is indicative of their retention of an interstellar heritage.<ref name=Ehrenfreund/>

===Chemistry in cometary comae===
The chemical composition of comets should reflect both the conditions in the outer solar nebula some 4.5 × 10<sup>9</sup> ayr, and the nature of the natal interstellar cloud from which the [[Solar System]] was formed.<ref name=Rodgers>{{Cite journal|author=Rodgers SD|author2=Charnley SB |title=A model of the chemistry in cometary comae: deuterated molecules |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=Mar 2002 |volume=330 |issue=3 |pages=660–74 |bibcode=2002MNRAS.330..660R |doi=10.1046/j.1365-8711.2002.05165.x |doi-access=free }}</ref> While comets retain a strong signature of their ultimate interstellar origins, significant processing must have occurred in the protosolar nebula.<ref name=Rodgers/> Early models of coma chemistry showed that reactions can occur rapidly in the inner coma, where the most important reactions are proton transfer reactions.<ref name=Rodgers/> 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.<ref name=Rodgers/>

==Theoretical chemical astronomy==
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.

===Astronomical spectroscopy===
The new era of chemical astronomy had to await the clear enunciation of the chemical principles of spectroscopy and the applicable theory.<ref name=Merrill>{{Cite journal|author=Merrill PW |title=Great Epochs in Astronomy |journal=Astronomical Society of the Pacific Leaflets |date=1954 |volume=7 |issue=306 |pages=41–8 |bibcode=1954ASPL....7...41M }}</ref>

===Chemistry of dust condensation===
Supernova radioactivity dominates light curves and the chemistry of dust condensation is also dominated by radioactivity.<ref name=Clayton>{{Cite journal|author=Clayton DD|author2= Liu W |title=Radioactivity Frontier for Supernova Chemistry |journal=Bull. Am. Astron. Soc. |date=1999 |volume=31 |pages=739 |bibcode=1999HEAD....4.3602C }}</ref> Dust is usually either carbon or oxides depending on which is more abundant, but Compton electrons dissociate the CO molecule in about one month.<ref name=Clayton/> The new chemical astronomy of supernova solids depends on the supernova radioactivity:
# the radiogenesis of <sup>44</sup>Ca from <sup>44</sup>Ti decay after carbon condensation establishes their supernova source,
# their opacity suffices to shift emission lines blueward after 500 d and emits significant infrared luminosity,
# parallel kinetic rates determine trace isotopes in meteoritic supernova graphites,
# the chemistry is kinetic rather than due to thermal equilibrium and
# is made possible by radiodeactivation of the CO trap for carbon.<ref name=Clayton/>

==Theoretical physical astronomy==
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 physical [[Universe]].

===Weak interaction and nuclear double beta decay===
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.<ref name=Suhonen>{{Cite journal|doi=10.1016/S0370-1573(97)00087-2|author=Suhonen J|author2=Civitarese O |title=Weak-interaction and nuclear-structure aspects of nuclear double beta decay |journal=Phys. Rep. |date=Jul 1998 |volume=300 |issue=3 |pages=123–214 |bibcode=1998PhR...300..123S }}</ref>

===Neutron-rich isotopes===
New neutron-rich isotopes, <sup>34</sup>Ne, <sup>37</sup>Na, and <sup>43</sup>Si have been produced unambiguously for the first time, and convincing evidence for the particle instability of three others, <sup>33</sup>Ne, <sup>36</sup>Na, and <sup>39</sup>Mg has been obtained.<ref name=Notani>{{Cite journal|display-authors=6|author=Notani M|author2=Sakurai H|author3=Aoi N|author4=Yanagisawa Y|author5=Saito A|author6=Imai N|author7=Gomi T|author8=Miura M|author9=Michimasa S|author10=Iwasaki H|author11=Fukuda N|author12=Ishihara M|author13=Kubo T|author14= Kubono S|author15=Kumagai H|author16=Lukyanov SM|author17=Motobayashi T|author18=Onishi TK|author19=Penionzhkevich YE|author20=Shimoura S|author21=Teranishi T|author22=Ue K|author23=Ugryumov V|author24=Yoshida A |title=New neutron-rich isotopes, <sup>34</sup>Ne, <sup>37</sup>Na and <sup>43</sup>Si, produced by fragmentation of a 64 A MeV 48Ca beam |journal=Phys. Lett. B |date=Aug 2002 |volume=542 |issue=1 |pages=49–54 |doi=10.1016/S0370-2693(02)02337-7 |bibcode = 2002PhLB..542...49N }}</ref> These experimental findings compare with recent theoretical predictions.<ref name=Notani/>

==Theory of astronomical time keeping{{anchor|Time keeping}}==
{{main cat|Time in astronomy}}
{{see also|Time in physics}}

Until recently all the time units that appear natural to us are caused by astronomical phenomena:
# Earth's orbit around the Sun => the year, and the seasons,
# [[Moon]]'s orbit around the Earth => the month,
# Earth's rotation and the succession of brightness and darkness => the day (and night).

High precision appears problematic:
# ambiguities arise in the exact definition of a rotation or revolution,
# some astronomical processes are uneven and irregular, such as the noncommensurability of year, month, and day,
# there are a multitude of time scales and calendars to solve the first two problems.<ref name=Husfeld>{{Cite web |author=Husfeld D |author2=Kronberg C |title=Astronomical Time Keeping |url=http://www.maa.mhn.de/Scholar/times.html |access-date=2009-12-18 |archive-date=2008-12-17 |archive-url=https://web.archive.org/web/20081217090158/http://www.maa.mhn.de/Scholar/times.html |url-status=dead }}</ref>

Some of these [[time standard]] scales are [[sidereal time]], [[solar time]], and [[universal time]].

===Atomic time===
{{Main|International Atomic Time}}
[[File:Clock accuracy.jpg|thumb|right|250px|Historical accuracy of atomic clocks from [[NIST]].]]
From the [[International System of Units|Systeme 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 of [[caesium-133]] (<sup>133</sup>Cs).<ref name=Husfeld/> For practical usability a device is required that attempts to produce the SI second (s) such as an [[atomic clock]]. But not all such clocks agree. The weighted mean of many clocks distributed over the whole Earth defines the [[International Atomic Time|Temps Atomique International]]; i.e., the Atomic Time TAI.<ref name=Husfeld/> From the [[General 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.<ref name=Husfeld/>

===Ephemeris time===
{{Main|Ephemeris time}}
Since the Earth's rotation is irregular, any time scale derived from it such as [[Greenwich Mean Time]] led to recurring problems in predicting the [[Ephemeris|Ephemerides]] for the positions of the [[Moon]], [[Sun]], [[planet]]s and their [[natural satellite]]s.<ref name=Husfeld/> In 1976 the [[International 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,<ref name=Guinot>{{Cite journal|author=Guinot B|author2=Seidelmann PK |title=Time scales&nbsp;– Their history, definition and interpretation |journal=Astronomy and Astrophysics |date=Apr 1988 |volume=194 |issue=1–2 |pages=304–8 |bibcode=1988A&A...194..304G }}</ref> emphasized their dynamical nature or origin, [[Barycentric Dynamical Time]] (TDB) and [[Terrestrial 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.
{{See also|Dynamical time scale}}
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 are [[Geocentric Coordinate Time]] (TCG) and [[Barycentric 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.<ref name=Klioner>{{Cite journal|author=Klioner S |display-authors=etal |title=Units of relativistic time scales and associated quantities |journal=IAU Symposium |volume=261 |date=2009 }}</ref>

The currently defined IAU time scales also include [[Terrestrial 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),<ref name=Resol>{{Cite web|url=http://syrte.obspm.fr/IAU_resolutions/Resol-UAI.rtf |title=IAU 2000 resolutions, at Resolution B1.9. }}</ref> 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.

===Extraterrestrial time-keeping===
{{Main|Timekeeping on Mars|Martian calendar}}

===Stellar dynamical time scale===
For a [[star]], the dynamical time scale is defined as the time that would be taken for a test particle released at the surface to fall under the [[star]]'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 certain [[star]] to collapse in the absence of any [[internal pressure]]. By appropriate manipulation of the equations of stellar structure this can be found to be

<math> \tau_{dynamical} \simeq \frac{R}{v} = \sqrt{\frac{R^3}{2GM}} \sim 1/\sqrt{G\rho} </math>

where R is the [[radius]] of the star, G is the [[gravitational constant]], M is the [[mass]] of the star, ρ the star gas [[density]] (assumed constant here) and v is the [[escape velocity]]. As an example, the [[Sun]] 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 in [[Cepheid variables]].

==Theory of astronomical navigation==
===On Earth===
The basic characteristics of applied astronomical navigation are
# usable in all areas of sailing around the Earth,
# applicable autonomously (does not depend on others&nbsp;– persons or states) and passively (does not emit energy),
# conditional usage via optical visibility (of horizon and celestial bodies), or state of cloudiness,
# precisional measurement, sextant is 0.1', altitude and position is between 1.5' and 3.0'.
# temporal determination takes a couple of minutes (using the most modern equipment) and ≤ 30 min (using classical equipment).<ref name=Stajic/>

The superiority of satellite navigation systems to astronomical navigation are currently undeniable, especially with the development and use of GPS/NAVSTAR.<ref name=Stajic>{{Cite journal|author=Stajic D |title=Astronomical navigation at the beginning of the third millenium |journal=Publ Astron Obs Belgrade in: Proceedings of the XIII National Conference of Yugoslav Astronomers, Belgrade, October 17–20, 2002 |date=Oct 2003 |volume=75 |pages=209–16 |bibcode=2003POBeo..75..209S }}{{sic}}</ref> This global satellite system
# enables automated three-dimensional positioning at any moment,
# automatically determines position continuously (every second or even more often),
# determines position independent of weather conditions (visibility and cloudiness),
# 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,
# is simple even without expert knowledge,
# is relatively cheap, comparable to equipment for astronomical navigation, and
# allows incorporation into integrated and automated systems of control and ship steering.<ref name=Stajic/> The use of astronomical or celestial navigation is disappearing from the surface and beneath or above the surface of the Earth.
{{See also|Geodetic astronomy|Figure of the Earth|International Earth Rotation and Reference Systems Service}}
'''Geodetic astronomy''' is the application of [[astronomy|astronomical]] methods into [[Wiktionary:Network|network]]s and technical projects of [[geodesy]] for
* [[apparent places]] of stars, and their [[proper motion]]s
* precise astronomical [[astro navigation|navigation]]
* astro-geodetic [[geoid]] determination and
* modelling the rock [[density|densities]] of the topography and of [[geological]] layers in the [[subsurface]]
* [[Satellite geodesy]] using the stellar background (see also [[astrometry]] and cosmic triangulation)
* Monitoring of the [[Earth rotation]] and polar wandering
* Contribution to the time system of physics and [[geosciences]]

'''Astronomical algorithms''' are the [[algorithm]]s used to calculate [[ephemeris|ephemerides]], [[calendar]]s, and positions (as in [[celestial navigation]] or [[satellite navigation]]).

Many astronomical and navigational computations use the [[Figure of the Earth]] as a surface representing the Earth.

The [[International Earth Rotation and Reference Systems Service]] (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and [[Frame of reference|reference frame]] standards, notably through its Earth Orientation Parameter (EOP) and [[International Celestial Reference System]] (ICRS) groups.

===Deep space===
{{Main|Deep Space Network}}
The '''Deep Space Network''', or '''DSN''', is an international [[Wiktionary:network|network]] of large [[#Antennas|antenna]]s and communication facilities that supports [[interplanetary spaceflight|interplanetary]] [[spacecraft]] missions, and [[radio astronomy|radio]] and [[radar astronomy]] observations for the exploration of the [[Solar System]] and the [[universe]]. The network also supports selected Earth-orbiting missions. DSN is part of the [[NASA]] [[Jet Propulsion Laboratory]] (JPL).

===Aboard an exploratory vehicle===
An observer becomes a deep space explorer upon escaping Earth's orbit.<ref name=Kawakatsu>{{Cite journal|doi=10.1016/j.actaastro.2006.12.019|author=Kawakatsu Y |title=Concept study on Deep Space Orbit Transfer Vehicle |journal=Acta Astronautica |date=Dec 2007 |volume=61 |issue=11–12 |pages=1019–28 |bibcode=2007AcAau..61.1019K}}</ref> While the [[Deep 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.

==See also==
* [[Astrochemistry]]
* [[Astrometry]]
* [[Astrophysics]]
* [[Celestial mechanics]]
* [[Celestial navigation]]
* [[Celestial sphere]]
* [[Orbital mechanics]]

==References==
{{Reflist}}

==External links==
* [http://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html Introduction to Cataclysmic Variables (CVs)]
* [https://arxiv.org/abs/0809.3157 L. Sidoli, 2008 ''Transient outburst mechanisms'']
* [http://www.wdl.org/en/item/9545 Commentary on "The Compendium of Plain Astronomy"] is a manuscript from 1665 about theoretical astronomy

{{Astronomy navbox}}
{{Astronomy subfields}}

[[Category:Applied and interdisciplinary physics]]
[[Category:Astrometry]]
[[Category:Astronomical imaging]]
[[Category:Astronomical sub-disciplines]]
[[Category:Astronomical coordinate systems]]
[[Category:Observational astronomy]]
[[Category:Space science]]
[[Category:Stellar astronomy]]