全文获取类型
| 收费全文 | 261948篇 |
| 免费 | 5475篇 |
| 国内免费 | 3932篇 |
专业分类
| 测绘学 | 7238篇 |
| 大气科学 | 19541篇 |
| 地球物理 | 54879篇 |
| 地质学 | 91753篇 |
| 海洋学 | 21822篇 |
| 天文学 | 56702篇 |
| 综合类 | 1179篇 |
| 自然地理 | 18241篇 |
出版年
| 2021年 | 2307篇 |
| 2020年 | 2660篇 |
| 2019年 | 2926篇 |
| 2018年 | 3808篇 |
| 2017年 | 3437篇 |
| 2016年 | 5917篇 |
| 2015年 | 4310篇 |
| 2014年 | 7025篇 |
| 2013年 | 14371篇 |
| 2012年 | 6826篇 |
| 2011年 | 8296篇 |
| 2010年 | 7299篇 |
| 2009年 | 9970篇 |
| 2008年 | 8754篇 |
| 2007年 | 8247篇 |
| 2006年 | 9876篇 |
| 2005年 | 7900篇 |
| 2004年 | 7745篇 |
| 2003年 | 7212篇 |
| 2002年 | 6819篇 |
| 2001年 | 6076篇 |
| 2000年 | 6000篇 |
| 1999年 | 5233篇 |
| 1998年 | 5266篇 |
| 1997年 | 5059篇 |
| 1996年 | 4692篇 |
| 1995年 | 4436篇 |
| 1994年 | 4117篇 |
| 1993年 | 3864篇 |
| 1992年 | 3636篇 |
| 1991年 | 3607篇 |
| 1990年 | 3771篇 |
| 1989年 | 3526篇 |
| 1988年 | 3317篇 |
| 1987年 | 3851篇 |
| 1986年 | 3421篇 |
| 1985年 | 4228篇 |
| 1984年 | 4755篇 |
| 1983年 | 4435篇 |
| 1982年 | 4336篇 |
| 1981年 | 3945篇 |
| 1980年 | 3659篇 |
| 1979年 | 3520篇 |
| 1978年 | 3500篇 |
| 1977年 | 3289篇 |
| 1976年 | 3052篇 |
| 1975年 | 2963篇 |
| 1974年 | 2920篇 |
| 1973年 | 3080篇 |
| 1972年 | 2028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.
46.
Earth, Moon, and Planets - 相似文献
47.
48.
49.
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
50.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk. 相似文献