Please answer the following question: Information: - In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element on a manifold such as Euclidean space. The order of the coordinates is significant, and they are sometimes identified by their position in an ordered tuple and sometimes by a letter, as in "the "x"-coordinate". The coordinates are taken to be real numbers in elementary mathematics, but may be complex numbers or elements of a more abstract system such as a commutative ring. The use of a coordinate system allows problems in geometry to be translated into problems about numbers and "vice versa"; this is the basis of analytic geometry. - Geodesy, also known as geodetics, geodetic engineering or geodetics engineering a branch of applied mathematics and earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth (or any planet), including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal motion, tides, and polar motion. For this they design global and national control networks, using space and terrestrial techniques while relying on datums and coordinate systems. - Earth, otherwise known as the world, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets. - Polar motion of the Earth is the motion of the Earth's rotational axis relative to its crust. This is measured with respect to a reference frame in which the solid Earth is fixed (a so-called "Earth-centered, Earth-fixed" or ECEF reference frame). This variation is only a few meters. - Earth science or geoscience is a widely embraced term for the fields of science related to the planet Earth. Earth science can be considered to be a branch of planetary science, but with a much older history. There are both reductionist and holistic approaches to Earth sciences. The Earth sciences can include the study of geology, the lithosphere, and the large-scale structure of the Earth's interior, as well as the atmosphere, hydrosphere, and biosphere. Typically, Earth scientists use tools from geography, chronology, physics, chemistry, biology, and mathematics to build a quantitative understanding of how the Earth system works and evolves. - In physics, a gravitational field is a model used to explain the influence that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenomena, and is measured in newtons per kilogram (N/kg). In its original concept, gravity was a force between point masses. Following Newton, Laplace attempted to model gravity as some kind of radiation field or fluid, and since the 19th century explanations for gravity have usually been taught in terms of a field model, rather than a point attraction. - A planet is an astronomical body orbiting a star or stellar remnant that The term "planet" is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such. - Applied mathematics is a branch of mathematics that deals with mathematical methods that find use in science, engineering, business, computer science, and industry. Thus, applied mathematics is a combination of mathematical science and specialized knowledge. The term "applied mathematics" also describes the professional specialty in which mathematicians work on practical problems by formulating and studying mathematical models. In the past, practical applications have motivated the development of mathematical theories, which then became the subject of study in pure mathematics where abstract concepts are studied for their own sake. The activity of applied mathematics is thus intimately connected with research in pure mathematics. - Geodetic astronomy is the application of astronomical methods into networks and technical projects of geodesy . The most important topics are : Establishment of geodetic datum systems ( e.g. ED50 ) or at expeditions apparent places of stars , and their proper motions precise astronomical navigation astro - geodetic geoid determination modelling the rock densities of the topography and of geological layers in the subsurface Satellite geodesy using the background of stars Monitoring of the Earth rotation and polar wandering Contribution to the time system of physics and geosciences Important measuring techniques are : Latitude and longitude determination by theodolites , tacheometers , astrolabes or zenith cameras time and star positions by observation of star transits , e.g. by meridian circles ( visual , photographic or CCD ) Azimuth measurements for the exact orientation of geodetic networks for mutual transformations between terrestrial and space methods for improved accuracy by means of '' Laplace points '' at special fixed points Vertical deflection measurements and their use in geoid determination in mathematical reduction of very precise networks for geophysical and geological purposes ( see above ) Modern spatial methods VLBI with radio sources ( quasars ) Astrometry of stars by scanning satellites like Hipparcos or the future Gaia . The accuracy of these methods depends on the instrument and its spectral wavelength , the measuring or scanning method , the time amount ( versus economy ) , the atmospheric situation , the stability of the surface resp. the satellite , on mechanical and temperature effects to the instrument , on the experience and skill of the observer , and on the accuracy of the physical - mathematical models . Therefore the accuracy reaches from 60 '' ( navigation , ~ 1 mile ) to 0,001 '' and better ( a few cm ; satellites , VLBI ) , e.g. : angles ( vertical deflections and azimuths ) 1 '' up to 0,1 '' geoid determination & height systems ca . 5 cm up to 0,2 cm astronomical lat /... - Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. What object entity has the relation of 'instance of' with the subject 'geodetic astronomy'? Choices: - activity - atmosphere - belt - branch - century - combination - community - coordinate system - definition - design - eight - engineering - field - fluid - four - human - industry - letter - mathematics - model - motion - perception - physics - planet - point - position - reference - religion - science - sea - space - star - study - system - three - time - understanding - variation - work
A:
combination