Theoretical astronomy
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.
Astronomy was early to adopt computational techniques to model stellar and galactic formation and celestial mechanics.
Most of theoretical astronomy uses Newtonian theory of gravitation, considering that the effects of general relativity are weak for most celestial objects.
"Contrary to the belief generally held by laboratory physicists, astronomy has contributed to the growth of our understanding of physics.
[1] 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.
Infrared astronomy, for example, has revealed that the interstellar medium contains a suite of complex gas-phase carbon compounds called aromatic hydrocarbons, often abbreviated (PAHs or PACs).
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.
Gravitational pull toward the center of a star creates massive amounts of heat and pressure, which cause nuclear fusion.
Theoretical astronomers use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations.
[citation needed] Topics studied by theoretical astronomers include: 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 (astro)physics and the study of gravitational waves.
[6] A general form of the first law of thermodynamics for stationary black holes can be derived from the microcanonical functional integral for the gravitational field.
[8] Theoretical astrochemistry offers the prospect of being able to place constraints on the inventory of organics for exogenous delivery to the early Earth.
[9] The organic inventories of primitive meteorites display large and variable enrichments in deuterium, carbon-13 (13C), and nitrogen-15 (15N), which is indicative of their retention of an interstellar heritage.
[10] While comets retain a strong signature of their ultimate interstellar origins, significant processing must have occurred in the protosolar nebula.
[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.
Theoretical chemistry as applied to astronomy seeks to find new ways to observe chemicals in celestial objects, 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.
From the 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 (133Cs).
The weighted mean of many clocks distributed over the whole Earth defines the Temps Atomique International; i.e., the Atomic Time TAI.
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.
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.
