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   113671篇
  免费   1938篇
  国内免费   1190篇
测绘学   2902篇
大气科学   8345篇
地球物理   22263篇
地质学   42052篇
海洋学   9719篇
天文学   24566篇
综合类   418篇
自然地理   6534篇
  2021年   833篇
  2020年   965篇
  2019年   1012篇
  2018年   4315篇
  2017年   4120篇
  2016年   3907篇
  2015年   1848篇
  2014年   2736篇
  2013年   5181篇
  2012年   3747篇
  2011年   6094篇
  2010年   5280篇
  2009年   6546篇
  2008年   5691篇
  2007年   6014篇
  2006年   3808篇
  2005年   3564篇
  2004年   3319篇
  2003年   3213篇
  2002年   2858篇
  2001年   2457篇
  2000年   2432篇
  1999年   2010篇
  1998年   2039篇
  1997年   1954篇
  1996年   1626篇
  1995年   1582篇
  1994年   1373篇
  1993年   1270篇
  1992年   1234篇
  1991年   1142篇
  1990年   1221篇
  1989年   1046篇
  1988年   972篇
  1987年   1172篇
  1986年   1007篇
  1985年   1277篇
  1984年   1395篇
  1983年   1313篇
  1982年   1249篇
  1981年   1106篇
  1980年   1032篇
  1979年   937篇
  1978年   953篇
  1977年   871篇
  1976年   830篇
  1975年   777篇
  1974年   784篇
  1973年   767篇
  1972年   478篇
排序方式: 共有10000条查询结果,搜索用时 949 毫秒
11.
The vertical deformation rates (VDRs) and horizontal deformation rates (HDRs) of Shanghai VLBI station in China and Kashima and Kashima34 VLBI stations in Japan were re-analysed using the baseline length change rates from Shanghai to 13 global VLBI stations, and from Kashima to 27 stations and from Kashima34 to 12 stations, based on the NASA VLBI global solution glb1123 (Ma, 1999). The velocity vectors of the global VLBI stations were referred to the ITRF97 reference frame, and the Eulerian vectors of different models of plate motion were used for comparative solutions. The VDR of Shanghai station is estimated to be −1.91±0.56 mm/yr, and those of Kashima and Kashima34 stations, −3.72±0.74mm/yr and −8.81±0.84mm/yr, respectively. The difference between the last two was verified by further analysis. Similar estimates were also made for the Kokee, Kauai and MK_VLBA VLBI stations in mid-Pacific.  相似文献   
12.
Abstract— The possibility of volcanism on Mercury has been a topic of discussion since Mariner 10 returned images of half the planet's surface showing widespread plains material. These plains could be volcanic or lobate crater ejecta. An assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism, no volcanism on the surface whatsoever, or some range in between. It is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry. No definite volcanic features have been identified on Mercury. However, known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the Mariner 10 dataset. Examination of high‐resolution, low Sun angle Mariner 10 images reveals several features which are interpreted to be flow fronts; it is unclear if these are volcanic flows or ejecta flows. This analysis implies that a clear assessment of volcanism on Mercury must wait for better data. MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry, Ranging) will take images with viewing geometries and resolutions appropriate for the identification of such features.  相似文献   
13.
C.B Olkin  L.H Wasserman  O.G Franz 《Icarus》2003,164(1):254-259
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   
14.
15.
16.
17.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars.  相似文献   
18.
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   
19.
20.
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