全文获取类型
收费全文 | 261450篇 |
免费 | 6363篇 |
国内免费 | 5030篇 |
专业分类
测绘学 | 7441篇 |
大气科学 | 20225篇 |
地球物理 | 54876篇 |
地质学 | 92163篇 |
海洋学 | 21754篇 |
天文学 | 56215篇 |
综合类 | 1715篇 |
自然地理 | 18454篇 |
出版年
2021年 | 2566篇 |
2020年 | 2850篇 |
2019年 | 3125篇 |
2018年 | 3732篇 |
2017年 | 3407篇 |
2016年 | 5987篇 |
2015年 | 4504篇 |
2014年 | 7255篇 |
2013年 | 14569篇 |
2012年 | 6714篇 |
2011年 | 8025篇 |
2010年 | 7130篇 |
2009年 | 9703篇 |
2008年 | 8473篇 |
2007年 | 7874篇 |
2006年 | 9812篇 |
2005年 | 7839篇 |
2004年 | 7711篇 |
2003年 | 7201篇 |
2002年 | 6801篇 |
2001年 | 6115篇 |
2000年 | 6099篇 |
1999年 | 5440篇 |
1998年 | 5465篇 |
1997年 | 5256篇 |
1996年 | 4873篇 |
1995年 | 4555篇 |
1994年 | 4243篇 |
1993年 | 3982篇 |
1992年 | 3742篇 |
1991年 | 3675篇 |
1990年 | 3812篇 |
1989年 | 3573篇 |
1988年 | 3355篇 |
1987年 | 3866篇 |
1986年 | 3438篇 |
1985年 | 4242篇 |
1984年 | 4744篇 |
1983年 | 4415篇 |
1982年 | 4317篇 |
1981年 | 3938篇 |
1980年 | 3651篇 |
1979年 | 3512篇 |
1978年 | 3490篇 |
1977年 | 3274篇 |
1976年 | 3041篇 |
1975年 | 2961篇 |
1974年 | 2914篇 |
1973年 | 3073篇 |
1972年 | 2023篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
The mass ratio of Charon to Pluto is a basic parameter describing the binary system and is necessary for determining the individual masses and densities of these two bodies. Previous measurements of the mass ratio have been made, but the solutions differ significantly (Null et al., 1993; Young et al., 1994; Null and Owen, 1996; Foust et al., 1997; Tholen and Buie, 1997). We present the first observations of Pluto and Charon with a well-calibrated astrometric instrument—the fine guidance sensors on the Hubble Space Telescope. We observed the motion of Pluto and Charon about the system barycenter over 4.4 days (69% of an orbital period) and determined the mass ratio to be 0.122±0.008 which implies a density of 1.8 to 2.1 g cm−3 for Pluto and 1.6 to 1.8 g cm−3 for Charon. The resulting rock-mass fractions for Pluto and Charon are higher than expected for bodies formed in the outer solar nebula, possibly indicating significant postaccretion loss of volatiles. 相似文献
42.
43.
44.
45.
Earth, Moon, and Planets - 相似文献
46.
47.
48.
The economic development in China and the rising of the living standard need to speed up the urbanisa-tion.The development of small towns is an important way to Chinese urbanization.Land use plays a very important role in the development of small towns.However there are many problems in the development of small towns,esp.in land use.The paper first discusses the land problems in the development of small towns.Such as much cultivated land lies idle,under-utilization and waste of land,increasing illegal use of land,unstable contractural relationship for land use.The relationship between the development of small towns and land use is also studied.Then the guidelines for the land system innovations of small towns are put forward.Namely the sustainability of social and economic development,the bal-ance between land reservation and land utilization,the provision of service to village ,agriculture and farmer,the manage-ment of land resource and land assets,the parsimonious andlegal use of land.The basic framework of land system innova-tions of small towns is put forward finally.It include the land replacement policy for small towns and the permanent tenan-cy of farmland,the overall plan for land uses and other plans in harmony,the establishment of a flexible system of land supplies,using land with payments ,the transfer of agricultural land and refining the land law related to the construction of small towns. 相似文献
49.
A CASE STUDY ON CONCENTRATION AND DECENTRALIZATION:BEHAVIOR AND DYNAMIC MECHANISM OF SPATIAL EVOLUTION IN METROPOLITAN AREA,NANJING,CHINA 总被引:1,自引:1,他引:0
In rapid socio-economic development,the process of concentration and dispersal of various elements tends to be more dramatic,tremendously influencing the shaping and transformation of the space in metropolitan area.Survey of spatial concentration and decentralization has thus become a basic method in examining metropolitan spatial evolution.In this research,three elements were selected as the essential indicators of the process:demographic density distribu-tion,employment density distribution and business office location.Performance of these elements in Nanjing City was exam-ined historically.As Nanjing City could be regarded as a representative of metropolitan areas in China,its situation large-ly suggestes the general characteristics in similar areas of China.Hence based on the investigation of Nanjing City,four general implications were highlighted.First ,metropolitan areas in China are in a violent process and shift of spatial concentra-tion and decentralization.Second,from now to at least the near future,concentration will continue to be the central fea-ture.Third,the landscape of metropolitan areas basically exhibits a dual structure character.The gap in environmental and ecological qualities among different districts will continue for a long time.Fourth,Central Business District (CBD) is playing an important role in helping to convert the traditionally single-centered city structure into a polycentric one. 相似文献
50.
本文综述了活动星系核 ,特别是blazar天体的研究现状 ,对blazar天体的多波段与多波段能谱特性研究进行了较为详细的评述。主要的研究工作包括以下内容 :(一 )γ噪blazar天体的短时标光变研究 ,通过对 1 6个γ噪blazar天体 (其中包括全部已证认和观测到VHEγ射线爆发的可能甚高能γ射线源 )自 1 998年的光学观测及光变分析研究表明 ,短时标光变 (小时量级 )是GeVγ噪blazar天体的普遍特性 ,光变幅度通常可达 0 .6mag/h ,对PKS 1 51 0 -0 89类星体的观测发现在一个小时内对象变暗 2个星等 ,对如此激烈的光变变暗目前的理论还不能很好的解释 ,但它同样反映了辐射区域的内部结构 ;而对TBLs的监测表明 ,其光学波段的短时标光变没有其他对象激烈 ,出现的频度和振幅变化都较小 ;(二 )在研究γ噪blazar天体光变时 ,研究了寄主星系对光变的影响 ,得到了 1ES 2 3 44 + 51 4的光变与PSF的FWHM的关联 ,表明随着大气视宁度的下降 (即FWHM变大 ) ,对象变暗 ,即由于寄主星系的影响从而导致假光变的产生 ;(三 )引进两个多波段复合谱指数 ,αxox=αox-αx 及αoro=αor-αo。对样本的统计研究表明 ,RBLs是能谱特性界于XBLs和OVVs之间的一类中间态 ,所得结果支持了Sambrunaetal.( 1 996)大样本多波段能谱分布特性的统计研 相似文献