In some cases, as in the introduction

In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject.^[2] Astronomy focuses on celestial objects, space, and the physical universe as a whole. Observations of the Sun, Moon, stars, and planets have formed the basis of timekeeping and navigation.  IBM Lenovo ThinkPad X300 Laptop Keyboard

Astronomy is a branch of science, but unlike other sciences, which have Earth-based laboratories in which controlled experiments are performed, astronomy has its labs located in the heavens far beyond the reach, let alone control, of the terrestrial observer.^[3] "So how can one be sure that what one sees out there is subject to the same rules and disciplines of science that govern the local laboratory experiments of physics and chemistry?"^[3] HP 519004-001 Laptop Keyboard

"The most incomprehensible thing about the universe is that it is comprehensible." – Albert Einstein.^[3]

Ptolemy's Almagest, although a brilliant treatise on theoretical astronomy combined with a practical handbook for computation, nevertheless includes many compromises to reconcile discordant observations.  ACER Aspire One D150-1577 Laptop Keyboard

Theoretical astronomy is usually assumed to have begun with Johannes Kepler (1571–1630), and Kepler's laws. It is co-equal with observation. The general history of astronomy deals with the history of the descriptive and theoretical astronomy of the solar system, from the late sixteenth century to the end of the nineteenth century.  FUJITSU Amilo Mini Ui 3520 Laptop Keyboard

The major categories of works on the history of modern astronomy include general histories, national and institutional histories, instrumentation, descriptive astronomy, theoretical astronomy, positional astronomy, and astrophysics. Astronomy was early to adopt computational techniques to model stellar and galactic formation and celestial mechanics.  HP G61-465SL Laptop Keyboard

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 Newtonian theory of gravitation, considering that the effects of general relativity are weak for most celestial objects. HP Pavilion ze5715ca Laptop Keyboard

The obvious fact is that theoretical astronomy cannot (and does not try) to predict the position, size and temperature of every star in the heavens. Theoretical astronomy by and large has concentrated upon analyzing the apparently complex but periodic motions of celestial objects.

Astrochemistry, the overlap of the disciplines of astronomy and chemistry,  GATEWAY MX8715 Laptop Keyboard

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 gas clouds, is of special interest because it is from these clouds that solar systems form. DELL Vostro 1015 Laptop Keyboard

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). 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. TOSHIBA Satellite L455 Laptop Keyboard

They are also the most common class of carbon molecule in meteorites 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 and isotopes of carbon, nitrogen, and oxygen that are very rare on earth, attesting to their extraterrestrial origin. Lenovo 39T7417 laptop keyboard

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. ACER TravelMate 8204WLMi laptop keyboard

For this reason, molecules and molecular ions which are unstable on Earth can be highly abundant in space, for example the H₃⁺ 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'.  HP Mini 110-3102sl laptop keyboard

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, TOSHIBA Satellite P300-1EI laptop keyboard

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.  HP Pavilion dv6-2114sa laptop keyboard

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).

Theoretical astronomers use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) HP Mini 110-1116NR laptop keyboard

andcomputational 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.^[4][5] HP Pavilion dv6-3106tu laptop keyboard

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. SONY VAIO VGN-SR290 laptop keyboard

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. SONY VAIO VGN-CS31Z/Q laptop keyboard

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. SONY VAIO VGN-CS31S/R laptop keyboard

and the New York City Department of Consumer Affairs issued a violation against ISR for engaging in a deceptive trade practice.

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. IBM 42T3297 laptop keyboard

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."^[11]  HP Pavilion dv6-2135er laptop keyboard

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; HP Pavilion G7T-1000 Series laptop keyboard

amino, hydroxycarboxylic, and carboxylic acids; purines and pyrimidines; and kerogen-type material.^[11] The organic inventories of primitive meteorites display large and variable enrichments in deuterium, ¹³C and ¹⁵N which is indicative of their retention of an interstellar heritage.

The chemical composition of comets should reflect both the conditions in the outer solar ASUS X53S laptop keyboard

nebula some 4.5 x 10⁹ ayr, and the nature of the natal interstellar cloud from which the Solar system was formed.^[12] While comets retain a strong signature of their ultimate interstellar origins, significant processing must have occurred in the protosolar nebula.^[12] Early models of coma chemistry showed that reactions can occur rapidly in the inner coma, SONY Vaio PCG-K23 laptop keyboard

where the most important reactions are proton transfer reactions.^[12] 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. SONY VGN-FE790G/N laptop keyboard

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. ACER PK1306F0200 laptop keyboard

This often leads to theoretical astrochemistry having to seek new ways to describe or explain those same observations.

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.HP G42-164LA Laptop Keyboard

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.  HP Pavilion dv6-2157sb Laptop Keyboard

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 ¹³³Cesium.^[17] 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. HP Mini 1116NR Laptop Keyboard

The weighted mean of many clocks distributed over the whole Earth defines the Temps Atomique International; i.e., the Atomic Time TAI.^[17] 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. HP Pavilion DV7-3065dx Laptop Keyboard

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 Ephemerides for the positions of the Moon, Sun, planets and their natural satellites.^[17] In 1976 the International Astronomical Union (IAU) resolved that the theoretical basis for ephemeris time (ET) was wholly non-relativistic, HP Pavilion dv5-1104tu Laptop Keyboard

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,^[18]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. ACER Aspire 7520 Laptop Keyboard

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). DELL Latitude D620 Laptop Keyboard

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.^[19] HP G62-a30SP Laptop Keyboard

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),^[20] 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. DELL FM8 Laptop Keyboard,Lenovo 45N2106 Laptop Keyboard,DELL PVDG3 Laptop Keyboard