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   42950篇
  免费   578篇
  国内免费   233篇
测绘学   916篇
大气科学   2145篇
地球物理   9357篇
地质学   16528篇
海洋学   3721篇
天文学   9675篇
综合类   129篇
自然地理   1290篇
  2022年   531篇
  2021年   806篇
  2020年   863篇
  2019年   910篇
  2018年   1957篇
  2017年   1786篇
  2016年   1997篇
  2015年   882篇
  2014年   1706篇
  2013年   2399篇
  2012年   1913篇
  2011年   2132篇
  2010年   2030篇
  2009年   2237篇
  2008年   1958篇
  2007年   2130篇
  2006年   1882篇
  2005年   1010篇
  2004年   929篇
  2003年   932篇
  2002年   844篇
  2001年   853篇
  2000年   721篇
  1999年   475篇
  1998年   524篇
  1997年   546篇
  1996年   397篇
  1995年   431篇
  1994年   424篇
  1993年   336篇
  1992年   345篇
  1991年   333篇
  1990年   395篇
  1989年   318篇
  1988年   303篇
  1987年   302篇
  1986年   247篇
  1985年   339篇
  1984年   346篇
  1983年   350篇
  1982年   333篇
  1981年   295篇
  1980年   309篇
  1979年   254篇
  1978年   289篇
  1977年   244篇
  1976年   204篇
  1975年   231篇
  1974年   208篇
  1973年   256篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
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   
72.
73.
74.
75.
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   
76.
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   
77.
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   
78.
A very long series of photographic observations of the comet Hale-Bopp has been made during January–April 1997 at the double astrograph (400/2000) of the Main Astronomical Observatory (Kyiv, Ukraine). Some of the cometary photos were obtained with two wide-band filter combinations. One of these combinations isolates C2 emission, another — the nearby dust continuum. The images were digitized by means of AMDPH-XY machine and then calibrated following the standard procedure. After subtraction of the dust continuum the distribution of surface brightness in the C2 emission coma of comet Hale-Bopp was studied. We found an asymmetric brightness distribution both pre- and post-perihelion. On 21.77 April 1997 a secondary brightness peak is found at the distance of 1.03 × 105 km from the nucleus. It is possible that this peak is related to the extended source of the C2 molecules. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
79.
The distribution of perihelion distances of nearly periodic comets was studied using different assumptions about comet discovery probabilities. It was shown that the concentration of perihelia at small heliocentric distances cannot be explained solely by the influence of visibility conditions.  相似文献   
80.
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