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   45518篇
  免费   1045篇
  国内免费   823篇
测绘学   1236篇
大气科学   3183篇
地球物理   9203篇
地质学   16188篇
海洋学   4066篇
天文学   10860篇
综合类   304篇
自然地理   2346篇
  2022年   332篇
  2021年   517篇
  2020年   550篇
  2019年   564篇
  2018年   1154篇
  2017年   1050篇
  2016年   1275篇
  2015年   756篇
  2014年   1241篇
  2013年   2287篇
  2012年   1417篇
  2011年   1835篇
  2010年   1645篇
  2009年   2281篇
  2008年   1910篇
  2007年   1908篇
  2006年   1770篇
  2005年   1326篇
  2004年   1335篇
  2003年   1236篇
  2002年   1226篇
  2001年   1115篇
  2000年   1002篇
  1999年   894篇
  1998年   876篇
  1997年   863篇
  1996年   713篇
  1995年   732篇
  1994年   651篇
  1993年   555篇
  1992年   509篇
  1991年   516篇
  1990年   591篇
  1989年   509篇
  1988年   463篇
  1987年   588篇
  1986年   495篇
  1985年   615篇
  1984年   691篇
  1983年   660篇
  1982年   575篇
  1981年   608篇
  1980年   500篇
  1979年   472篇
  1978年   477篇
  1977年   434篇
  1976年   420篇
  1975年   417篇
  1974年   396篇
  1973年   426篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
个人所得税是与每个人联系都很紧密的税种,它的完善与否,直接体现了我国法制建设的完善程度及居民素质的高低。但现行税制由于内外原因,存在各种缺陷。因此有必要在理论中找到对策,并在实践中寻求答案逐步完善个人所得税制度。  相似文献   
72.
73.
Cosmic radiation is an important problem for human interplanetary missions. The “Radiation Exposure and Mission Strategies for Interplanetary Manned Missions–REMSIM” study is summarised here. They are related to current strategies and countermeasures to ensure the protection of astronauts from radiation during interplanetary missions, with specific reference to: radiation environment and its variability; radiation effects on the crew; transfer trajectories and associated fluences; vehicle and surface habitat concepts; passive and active shielding concepts; space weather monitoring and warning systems.  相似文献   
74.
The approach proposed in the previous parts of this series of papers is used to solve the radiative transfer problem in scattering and absorbing multicomponent atmospheres. Linear recurrence relations are obtained for both the reflectance and transmittance of these kinds of atmospheres, as well as for the emerging intensities when the atmosphere contains energy sources. Spectral line formation in a one-dimensional inhomogeneous atmosphere is examined as an illustration of the possibility of generalizing our approach to the matrix case. It is shown that, in this case as well, the question reduces to solving an initial value problem for linear differential equations. Some numerical calculations are presented.  相似文献   
75.
76.
The first results of the observational program devoted to simultaneous investigation of asteroid polarimetric and photometric opposition phenomena are presented. UBVRI polarimetric and V-band photometric observations of the S-type Asteroid 20 Massalia and the E-type Asteroids 214 Aschera and 620 Drakonia were carried out in 1996-1999 down to phase angles of 0.08°, 0.7°, and 1.2°, correspondingly. The S-type Asteroid 20 Massalia is characterized by the pronounced brightness opposition surge with an amplitude larger than that observed for the E-type asteroids. A sharp peak of negative polarization at small phase angles was not observed for this asteroid. The value of polarization degree at phase angle α<1° is less than 0.5% for both S and E types. The negative polarization branches of S and especially E-asteroids have an asymmetrical shape. The phase angle at which the polarization minimum occurs is close to the angle at which non-linear increase begins in the asteroid magnitude phase curves. A relation of the observed effects to the mechanism of coherent backscattering is discussed.  相似文献   
77.
78.
79.
Moroz  L. V.  Baratta  G.  Distefano  E.  Strazzulla  G.  Starukhina  L. V.  Dotto  E.  Barucci  M. A. 《Earth, Moon, and Planets》2003,92(1-4):279-289
Trans-Neptunian Objects (TNOs) and Centaurs show remarkable colour variationsin the visual and near-infrared spectral regions. Surface alteration processes such asspace weathering (e.g., bombardment with ions) and impact resurfacingmay play an important role in the colour diversity of such bodies. Ion irradiation ofhydrocarbon ices and their mixtures with water ice transforms neutral (grey) surfacecolours of ices to red and further to grey. Along with the ices, TNOs and Centaursprobably contain complex carbonaceous compounds, in particular, complexhydrocarbons. Unlike ices, such refractory organic materials have originally lowvisual albedos and red colours in the visible and near-infrared ranges. Here wepresent the first results of ion irradiation experiments on asphaltite. Asphaltite isa natural complex hydrocarbon material. The reflectance spectra of asphaltite inthe 0.4–0.8 μm range have been recorded before irradiation and after eachirradiation step. We demonstrate that irradiation of this red dark material with30 keV H+ and 15 keV N+ ions gradually transforms its colour from redto grey as a result of carbonization. A moderate increase in the visual albedo hasbeen observed. These results may imply that the surfaces of primitive red objectsoptically dominated by complex refractory organics may show a similar spaceweathering trend. Our laboratory results were compared with published coloursof TNOs and Centaurs. A broad variety of spectral colours observed for TNOs andCentaurs may be reproduced by various spectra of irradiated organics correspondingto different ion fluences. However, such objects probably also contain ices and silicatecomponents which show different space weathering trends. This fact, together with alack of information about albedos, may explain difficulties to reveal correlations between surface colours within TNO and Centaur populations and their other properties, such as absolute magnitudes and orbital parameters.  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