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   47337篇
  免费   857篇
  国内免费   276篇
测绘学   1107篇
大气科学   2560篇
地球物理   10477篇
地质学   18142篇
海洋学   4148篇
天文学   10219篇
综合类   149篇
自然地理   1668篇
  2022年   574篇
  2021年   907篇
  2020年   999篇
  2019年   1018篇
  2018年   2134篇
  2017年   1977篇
  2016年   2251篇
  2015年   1072篇
  2014年   1928篇
  2013年   2718篇
  2012年   2189篇
  2011年   2447篇
  2010年   2303篇
  2009年   2485篇
  2008年   2205篇
  2007年   2314篇
  2006年   2050篇
  2005年   1159篇
  2004年   1073篇
  2003年   1065篇
  2002年   937篇
  2001年   947篇
  2000年   786篇
  1999年   532篇
  1998年   588篇
  1997年   608篇
  1996年   433篇
  1995年   479篇
  1994年   443篇
  1993年   361篇
  1992年   374篇
  1991年   364篇
  1990年   412篇
  1989年   324篇
  1988年   306篇
  1987年   319篇
  1986年   241篇
  1985年   367篇
  1984年   359篇
  1983年   360篇
  1982年   338篇
  1981年   312篇
  1980年   321篇
  1979年   252篇
  1978年   297篇
  1977年   253篇
  1976年   218篇
  1975年   240篇
  1974年   223篇
  1973年   264篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
101.
102.
103.
S.V.M. Clube  W.M. Napier 《Icarus》1985,62(3):384-388
The observed properties of the long-period comet system, and its periodic disturbance by galactic forces manifesting as terrestrial impact episodes, may be indicative of a comet capture/escape cycle as the Solar System orbits the Galaxy. A mean number density of comets in molecular clouds of ~10?1±1 AU?3 is implied. This is sufficient to deplete metals from the gaseous component of the interstellar medium, as observed, but leads to the problem of how stars are formed nevertheless with solar metal abundances. Formation of comets prior to stars in dense systems of near-zero energy may be indicated, and isotope signatures in cometary particles may be diagnostic of conditions in young spiral arm material.  相似文献   
104.
Abstract— Mineralogical and chemical studies of Dar al Gani 983 show that this meteorite is a eucrite. Its texture is that of an impact breccia. It contains cumulate pyroxene and feldspar megacrysts, a variety of recrystallized melt clasts, clasts of subophitic basalt, and mesostasis. These components are embedded in a matrix of fragmental pyroxene and plagioclase. In addition, the entire rock is penetrated by glassy melt veins and patches, and displays features of strong shock. The mineralogical and chemical evidence obtained for DaG 983 indicates that this meteorite experienced a complex evolutionary history. The presence of cumulate silicate crystals implies substantial, large scale cratering events on the HED asteroid. As a result of these impacts, rocks from different intrusive bodies to extrusive surface layers were laterally and vertically transported to form a thoroughly mixed megaregolith. DaG 983 represents a sample of this megabreccia.  相似文献   
105.
106.
107.
108.
Graviton may, in principle, have a small non-zero mass. In this paper the relevant theory of the massive graviton with six polarisations is developed. The drastic impact of a non-zero mass of the graviton on cosmology is also illustrated.  相似文献   
109.
Using the basic Boussinesq's equation, the expression for the vertical stress distribution (σz) underneath any point on the ground surface due to a general triangular loaded region in a preferred orientation with a linearly varied loading has been successfully derived. When the triangle is not in a preferred orientation, a simple axis transformation is required and the expression will be equally applicable. Based on this expression, σz due to an arbitrarily shaped loaded foundation can simply be determined by first triangulating the loaded area and summing up the contributions from each generated triangular region. The procedures for triangulating and calculating the stress distribution can be simply automated through computer programs.  相似文献   
110.
The intrinsic, three-dimensional shapes of small galaxy groups, containing between three and eight members, are evaluated using three different statistics: (i) the mean sum of square sines of angles in all possible triangles formed by members of the group; (ii) the variance of square paired separations in the group; (iii) the axial ratio of a rectangle containing the group. The mean values of these parameters and their rms deviations are calculated for observed galaxy groups and simulated groups with members that are distributed randomly within prolate or oblate spheroids. Comparison of observational data and simulations shows that the observed galaxy groups have shapes consistent with the projected shapes of prolate or oblate spheroids with axial ratios of 3:1, regardless of their multiplicity, but inconsistent with the projected shapes of spherical objects.  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