再论银道坐标系的定义

Rediscussion on the Definition of Galactic Coordinate System

作者曾在《陕西天文台台刊》第10卷(1987年)第1、2期合刊上发表《关于银道坐标系的定义》,本文是这一文章的继续,给出银道坐标系的转换公式、矩阵和程序,并进一步论述4参数定义的缺点。

The author has reviewed the background and definition of galactic coordinate system, discovered shortcomings on the definition in domestic authoritative books and a mistaken datum in a few books ( see'Publications of Shaanxi Observatory,vol.10 (1987), p.62 ) .Here is a further discussion concerned with opinions of some readers. The galactic system is needed by the research on stellar distribution.Its physical background is the tendency of stars and interstellar matter towards a great circle(the galactic circle)on the sky and towards a point(the galactic centre)at the circle.The traditional astronomical measurements depend on the plumb line and the axis of the earth's rotation therefore equatorial coordinate system is adopted as the basic astronomical system. The mathematical definition of galactic system depends on equatorial system. A celestial coordinate system can be determined by three parameters, usually by a pole(two parameters)and a zero point of longitude(one parameter)oThe definition of IAU(l958)galactic system is as follows.the coordinates of the northern galactic pole and the coordinate of the position angle of zero point of longitude in the 1950.0 equatorial coordinate system are(A,D) and(θ), especticely.It is easy to get transformation formulas (l) and matrixes (2), (3). (1).l,b stand for galactic longitude and latitude: α,δ for 1950.0 right ascension and declination. (2), (3).Triad x_g, y_g, z_g for galactic and x_e,y_e,z_e for equatorial coordinate.A computer program is given in note(4). The four-parameter definition which appears in many books has following shortcomings: 1.It is not standard.IAU(l958)have given the formal definition of galactic system. Which is not only strict, but also easy to be understood and convenient to be used.It should not be replaced by other form in textbooks and scientific books. 2.It is inconvenient. In practical usage,it can not be avoided to get the position angle 8. The process is like(4), A,D,8 are definite parameters while other values are taken from deduced values by IAU (1958) definition. For a practical user, to reduce θ from l^I,b^I is far from easy.Some books did give mistaken result. 3.It is not strict. To define a three-dimension quantity with four parameters is self-contradictory.Definite parameters are infinite accurate while reduced ones are not.In fact we can not compute accurate 0 from L^I', b^I. Three parameters are enough for the definition but 8 (A,D,l^I) is not exactly equal to θ (A,D,b^I). In a word,the four-parameter can be used to describe the new galactic system from one aspect (say,its relation with old system). Formal definition should be given in textbooks and scientific books.