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   128960篇
  免费   1998篇
  国内免费   1068篇
测绘学   3241篇
大气科学   8885篇
地球物理   24599篇
地质学   47607篇
海洋学   11291篇
天文学   29035篇
综合类   453篇
自然地理   6915篇
  2022年   772篇
  2021年   1296篇
  2020年   1411篇
  2019年   1579篇
  2018年   4665篇
  2017年   4264篇
  2016年   4448篇
  2015年   2050篇
  2014年   3664篇
  2013年   6256篇
  2012年   4357篇
  2011年   6366篇
  2010年   5647篇
  2009年   7038篇
  2008年   6049篇
  2007年   6422篇
  2006年   4816篇
  2005年   3701篇
  2004年   3665篇
  2003年   3526篇
  2002年   3384篇
  2001年   2825篇
  2000年   2722篇
  1999年   2189篇
  1998年   2264篇
  1997年   2071篇
  1996年   1834篇
  1995年   1794篇
  1994年   1542篇
  1993年   1425篇
  1992年   1368篇
  1991年   1397篇
  1990年   1392篇
  1989年   1177篇
  1988年   1130篇
  1987年   1280篇
  1986年   1155篇
  1985年   1412篇
  1984年   1586篇
  1983年   1477篇
  1982年   1383篇
  1981年   1288篇
  1980年   1181篇
  1979年   1136篇
  1978年   1079篇
  1977年   909篇
  1976年   882篇
  1975年   880篇
  1974年   839篇
  1973年   914篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
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   
122.
123.
124.
We report intermediate resolution H spectroscopy of the black hole candidate Nova Muscae 1991 during quiescence. We classify the companion star as a K3-K4V which contributes 85±6 percent to the total flux from the binary. The photospheric absorption lines are broadened by 106±13 kms−1 with respect to template field stars, leading to a system mass ratio of q =M1/M2 = 7.8−2.0+3.4. Doppler imaging of the H line shows strong emission coming from the secondary star (EW=3.1±0.6Å) which we associate with chromospheric activity. However, the hot-spot is not detected and this may indicate a lower mass transfer rate than in other X-ray transients of comparable orbital periods. The surface brightness distribution of the accretion disk in H follows a relation I∝R−1.1, less steep than typically observed in cataclysmic variables. Updated system parameters are also presented.  相似文献   
125.
This work presents preliminaries spectroscopic results about ionized xenon and xenon-helium mixture using a capillary pulsed discharge under several experimental results.  相似文献   
126.
We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion. All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ) in all the images closely follows the “steady state” ρ−1 model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant implying the dust grain properties vary with coma distance. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
127.
The propagation of an ion-acoustic soliton in a collisionless plasma with adiabatic positive and negative ions (with equal ion temperature) and hot non-isothermal electrons is studied by use of the renormalization method introduced by Kodama and Taniuti in the reductive perturbation method. The basic set of fluid equations describing the system is reduced to a Korteweg-de Vries (K-dV)-type equation for the first-order perturbed potential and to a linear inhomogeneous differential equation to the second-order of the perturbed potential. A stationary solution of the coupled equations is obtained.  相似文献   
128.
The problem of clock synchronization: A relativistic approach   总被引:2,自引:0,他引:2  
The problem of synchronization of the Earth-based clocks has been discussed in the framework of General Relativity Theory. The synchronization is considered as the transformation of the observers' proper time scales to the coordinate time scale of local inertial geocentric reference system, which is single for all the observers. The formulas for the relativistic corrections occurring in some methods of Earth-based clock synchronization (transported clock, duplex communication via geostationary satellite and meteor-burst link, LASSO experiments) have been derived enabling one to attain the accuracy of 0.1 ns.  相似文献   
129.
130.
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