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   72749篇
  免费   861篇
  国内免费   392篇
测绘学   1938篇
大气科学   5522篇
地球物理   13175篇
地质学   27596篇
海洋学   5760篇
天文学   15797篇
综合类   248篇
自然地理   3966篇
  2020年   364篇
  2019年   356篇
  2018年   3725篇
  2017年   3552篇
  2016年   2626篇
  2015年   840篇
  2014年   1234篇
  2013年   2545篇
  2012年   2223篇
  2011年   4246篇
  2010年   4012篇
  2009年   4515篇
  2008年   3783篇
  2007年   4390篇
  2006年   1787篇
  2005年   2070篇
  2004年   1915篇
  2003年   1893篇
  2002年   1655篇
  2001年   1295篇
  2000年   1225篇
  1999年   1162篇
  1998年   1094篇
  1997年   1081篇
  1996年   864篇
  1995年   854篇
  1994年   817篇
  1993年   761篇
  1992年   726篇
  1991年   696篇
  1990年   799篇
  1989年   699篇
  1988年   662篇
  1987年   751篇
  1986年   634篇
  1985年   846篇
  1984年   971篇
  1983年   930篇
  1982年   876篇
  1981年   850篇
  1980年   760篇
  1979年   714篇
  1978年   699篇
  1977年   637篇
  1976年   605篇
  1975年   519篇
  1974年   599篇
  1973年   588篇
  1972年   362篇
  1971年   334篇
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
31.
32.
Long-term changes in growth of Norwegian spring-spawning herring   总被引:3,自引:1,他引:2  
  相似文献   
33.
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   
34.
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   
35.
36.
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   
37.
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   
38.
39.
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   
40.
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