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   54299篇
  免费   716篇
  国内免费   1186篇
测绘学   2195篇
大气科学   4280篇
地球物理   10371篇
地质学   22467篇
海洋学   3676篇
天文学   8015篇
综合类   2228篇
自然地理   2969篇
  2021年   291篇
  2020年   316篇
  2019年   316篇
  2018年   5374篇
  2017年   4655篇
  2016年   3395篇
  2015年   722篇
  2014年   924篇
  2013年   1678篇
  2012年   1906篇
  2011年   3869篇
  2010年   3032篇
  2009年   3614篇
  2008年   3029篇
  2007年   3494篇
  2006年   1262篇
  2005年   1069篇
  2004年   1257篇
  2003年   1182篇
  2002年   1038篇
  2001年   791篇
  2000年   784篇
  1999年   602篇
  1998年   564篇
  1997年   622篇
  1996年   493篇
  1995年   486篇
  1994年   487篇
  1993年   396篇
  1992年   404篇
  1991年   372篇
  1990年   398篇
  1989年   358篇
  1988年   364篇
  1987年   389篇
  1986年   345篇
  1985年   441篇
  1984年   438篇
  1983年   464篇
  1982年   434篇
  1981年   401篇
  1980年   439篇
  1979年   343篇
  1978年   318篇
  1977年   304篇
  1976年   281篇
  1975年   278篇
  1974年   279篇
  1973年   269篇
  1971年   189篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
41.
Anomalies of some tidal waves of UT1   总被引:1,自引:0,他引:1  
Summary. The M f and M m waves of UT1 have been analysed from the BIH data during the period 1967.0 to 1984.0 in order to derive the Love number k .
These analyses performed during successive intervals of this period show some anomalies in the values of the Love number k derived from the M f wave. Large variations with time of the amplitude and the phase appear for this wave while the values derived from the M m wave present a good stability during the whole period.
The spectrum of the UT1-residuals (as obtained by removing the theoretical zonal tidal UT1 terms) shows the existence of a perturbation wave near the M f period. This wave lies in the range 13.5–13.9 day according to the analysed interval; it could be related with the perturbation wave noticed by some authors near the M f gravimetric wave.  相似文献   
42.
43.
bdAbstract The open cluster NGC 2818 containing a planetary nebula has been observed inVRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope. The study extending to starsV ∼ 21 magnitude establishes the distance modulus as(m-M) 0 = 12.9 ±0.1 for the cluster. Based on the fitting of theoretical isochrones computed for solar metallicity, an age of 5(±1) × 108 years has been assigned to the cluster. Association of the planetary nebula with the cluster indicates that the progenitor mass of the planetary nebula on the main sequence is ≥2.5M Based on observations obtained with the Vainu Bappu Telescope.  相似文献   
44.
A new procedure to separate the instrumental and atmospheric components of stray light is presented. It is based on the dependence of the aureole's atmospheric component on the air mass and is applied to measurements taken with the Vacuum Newton Telescope (VNT) at the Observatorio del Teide (Tenerife). The resulting instrumental part is independent of the air mass.The variation of both components with wavelength is also studied. The instrumental component shows no dependence on wavelength, in contrast to the atmospheric one which is greater in the blue than in the red.It is concluded that observations with air masses larger than two will probably be strongly affected by stray light.  相似文献   
45.
46.
1 Introduction Inrecentyearstherehasbeengrowingresearchin terestinthetwo dimensional (2 D )systemtheory .The 2 Dsystemsmayfindapplicationsinareassuchasmarineseismicdataprocessingandimageprocessing .Althoughmoreandmorevaluableresultshavebeengained ,mos…  相似文献   
47.
Two spectrophotometric scans of comet Levy (1990c) have been analysed to estimate Haser model column densities and production rates of CN andC 2 molecules.  相似文献   
48.
An updated Lagrangian finite-element formulation has been developed for time-dependent problems of soil consolidation involving finite deformations. Large plastic strains as well as rotations occur in such problems and nominal stress measures are introduced in the formulation to redefine stresses. This leads to corrective terms for equilibrium and yield violations in addition to geometric stiffening terms in the governing integral equations. The soil is considered to be either a linear elastic or an elastoplastic, critical-state material. Some simple numerical examples are studied to validate the formulation, followed by a detailed analysis of the problem of penetration of a pile into soil. The results of this problem are viewed with emphasis on the physical interpretation and practical significance.  相似文献   
49.
50.
Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non-coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens.  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