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   260542篇
  免费   5903篇
  国内免费   3374篇
测绘学   7005篇
大气科学   19232篇
地球物理   55743篇
地质学   90521篇
海洋学   21608篇
天文学   56579篇
综合类   1021篇
自然地理   18110篇
  2021年   2288篇
  2020年   2704篇
  2019年   3012篇
  2018年   3665篇
  2017年   3416篇
  2016年   5985篇
  2015年   4469篇
  2014年   7229篇
  2013年   14552篇
  2012年   6755篇
  2011年   8066篇
  2010年   7035篇
  2009年   9657篇
  2008年   8469篇
  2007年   7883篇
  2006年   9765篇
  2005年   7779篇
  2004年   7697篇
  2003年   7186篇
  2002年   6782篇
  2001年   6055篇
  2000年   6012篇
  1999年   5233篇
  1998年   5231篇
  1997年   5048篇
  1996年   4693篇
  1995年   4417篇
  1994年   4125篇
  1993年   3872篇
  1992年   3630篇
  1991年   3597篇
  1990年   3769篇
  1989年   3515篇
  1988年   3303篇
  1987年   3847篇
  1986年   3406篇
  1985年   4223篇
  1984年   4732篇
  1983年   4414篇
  1982年   4312篇
  1981年   3924篇
  1980年   3634篇
  1979年   3511篇
  1978年   3491篇
  1977年   3280篇
  1976年   3048篇
  1975年   2958篇
  1974年   2916篇
  1973年   3096篇
  1972年   2028篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
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   
142.
143.
144.
145.
146.
147.
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   
148.
Long-term changes in growth of Norwegian spring-spawning herring   总被引:3,自引:1,他引:2  
  相似文献   
149.
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   
150.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during 1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles. The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed.  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