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   92456篇
  免费   1221篇
  国内免费   1298篇
测绘学   3093篇
大气科学   7118篇
地球物理   17444篇
地质学   35651篇
海洋学   6889篇
天文学   17301篇
综合类   2279篇
自然地理   5200篇
  2021年   354篇
  2020年   413篇
  2019年   446篇
  2018年   7271篇
  2017年   6597篇
  2016年   4682篇
  2015年   1112篇
  2014年   1416篇
  2013年   2828篇
  2012年   2982篇
  2011年   6095篇
  2010年   5433篇
  2009年   6157篇
  2008年   5175篇
  2007年   6044篇
  2006年   2004篇
  2005年   2403篇
  2004年   2399篇
  2003年   2300篇
  2002年   1950篇
  2001年   1384篇
  2000年   1363篇
  1999年   1215篇
  1998年   1179篇
  1997年   1123篇
  1996年   837篇
  1995年   893篇
  1994年   858篇
  1993年   787篇
  1992年   769篇
  1991年   723篇
  1990年   805篇
  1989年   703篇
  1988年   674篇
  1987年   770篇
  1986年   644篇
  1985年   871篇
  1984年   992篇
  1983年   962篇
  1982年   890篇
  1981年   878篇
  1980年   792篇
  1979年   745篇
  1978年   734篇
  1977年   652篇
  1976年   620篇
  1975年   541篇
  1974年   615篇
  1973年   611篇
  1972年   369篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
Long-term changes in growth of Norwegian spring-spawning herring   总被引:3,自引:1,他引:2  
  相似文献   
103.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem.  相似文献   
104.
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed.  相似文献   
105.
106.
Rostov State Pedagogical Institute; All-Union Scientific Research Institute Gradient. Translated from Astrofizika, Vol. 33, No. 3, pp. 379–393, November–December, 1990.  相似文献   
107.
A transformation of the differential equations of motion of the two-body problem in the spherical coordinates to oscillator form is derived. It is shown that the independent variable transformation dt/ds=r2 is a transformation which makes the oscillator form possible.  相似文献   
108.
109.
The results of a general theoretical investigation of three commonly used types of inductive conductivity sensors, i.e., the single transformer, the double transformer, and the double transformer with an additional loop, are presented. The resulting formulas describe the dependence of the sensor output signal not only on the conductivity of the seawater but also on the parameters of the electrical circuit, among them the permeability of the transformer core(s), which-unlike the other parameters-shifts considerably during oceanographic in situ measurements. A mathematical discussion of these formulas shows that for certain circuit configurations, the sensor output is independent of changes in permeability. Most of these configurations form the basis of existing oceanographical conductivity sensors, among them the "classical" sensors developed by H. Hinkelmann [3], [4], and by N. L. Brown [14], while some others make evident further possibilities for eliminating the unwanted effects of shifting permeability. In the era of microelectronics, the latter might lead to a reassessment, especially of the single transformer-type sensor.  相似文献   
110.
Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (~0.1 ms?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators.  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