首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   278447篇
  免费   5209篇
  国内免费   3450篇
测绘学   7544篇
大气科学   20529篇
地球物理   57719篇
地质学   99489篇
海洋学   23022篇
天文学   59029篇
综合类   1167篇
自然地理   18607篇
  2021年   2221篇
  2020年   2614篇
  2019年   2870篇
  2018年   6200篇
  2017年   5712篇
  2016年   7145篇
  2015年   4327篇
  2014年   6944篇
  2013年   14231篇
  2012年   7544篇
  2011年   10396篇
  2010年   9203篇
  2009年   11856篇
  2008年   10232篇
  2007年   10259篇
  2006年   9773篇
  2005年   8214篇
  2004年   8039篇
  2003年   7581篇
  2002年   7017篇
  2001年   6090篇
  2000年   6008篇
  1999年   5234篇
  1998年   5239篇
  1997年   5050篇
  1996年   4693篇
  1995年   4449篇
  1994年   4107篇
  1993年   3866篇
  1992年   3630篇
  1991年   3592篇
  1990年   3766篇
  1989年   3522篇
  1988年   3312篇
  1987年   3851篇
  1986年   3415篇
  1985年   4227篇
  1984年   4744篇
  1983年   4417篇
  1982年   4317篇
  1981年   3918篇
  1980年   3599篇
  1979年   3435篇
  1978年   3442篇
  1977年   3221篇
  1976年   2976篇
  1975年   2906篇
  1974年   2866篇
  1973年   3064篇
  1972年   1993篇
排序方式: 共有10000条查询结果,搜索用时 890 毫秒
141.
Cygnus A     
Cygnus A was the first hyper-active galaxy discovered, and it remains by far the closest of the ultra-luminous radio galaxies. As such, Cygnus A has played a fundamental role in the study of virtually all aspects of extreme activity in galaxies. We present a review of jet theory for powering the double-lobed radio emitting structures in powerful radio galaxies, followed by a review of observations of Cygnus A in the radio, optical, and X-ray relevant to testing various aspects of jet theory. Issues addressed include: jet structure from pc- to kpc-scales, jet stability, confinement, composition, and velocity, the double shock structure for the jet terminus and the origin of multiple radio hotspots, the nature of the filamentary structure in the radio lobes, and the hydrodynamic evolution of the radio lobes within a dense cluster atmosphere, including an analysis of pressure balance between the various gaseous components. Also discussed are relativistic particle acceleration and loss mechanisms in Cygnus A, as well as magnetic field strengths and geometries both within the radio source, and in the intracluster medium. We subsequently review the classification, cluster membership, and the emission components of the Cygnus A galaxy. The origin of the activity is discussed. Concentrating on the nuclear regions of the galaxy, we review the evidence for an obscured QSO, also given the constraints on the orientation of the radio source axis with respect to the sky plane. We present an overview of models of central engines in AGN and observations of Cygnus A which may be relevant to testing such models. We conclude with a brief section concerning the question of whether Cygnus A is representative of powerful high redshift radio galaxies. Received October 10, 1995  相似文献   
142.
The UBV photometric observations of RT Per, from Sanwal and Chaubey (1981), were analyzed by the Wilson and Devinney code (1971). The light curves include reflection effects that for the first time has been suggested by Dugan (1911). RT Per has a semi-detached configuration where the lower-mass component is in contact with its respective Roche surface. The higher-mass component very nearly fills its Roche lobe. It has the characteristic of an Algol type system. The absolute dimensions for the primary and secondary of this system were calculated from its spectral types and by combining the photometric solution with inferred component radial velocities (Lu, 1990).  相似文献   
143.
144.
145.
146.
147.
148.
Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them.  相似文献   
149.
Long-term changes in growth of Norwegian spring-spawning herring   总被引:3,自引:1,他引:2  
  相似文献   
150.
The growth of Glasgow from a small town into a heavily industrialised conurbation depended greatly on its local geological riches. Extensive mining and quarrying of a range of minerals took place from the 18th century onwards. The early underground stoop and room (pillar and stall) workings, and the backfilled quarries, together with variably consolidated natural superficial deposits, have bequeathed to the city a heritage of unwanted engineering problems which cannot easily be quantified. Recent work by the British Geological Survey (BGS) illustrates the scale of both geological and man-made problems.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号