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   65722篇
  免费   1556篇
  国内免费   1102篇
测绘学   2083篇
大气科学   5059篇
地球物理   13133篇
地质学   23947篇
海洋学   5526篇
天文学   14586篇
综合类   390篇
自然地理   3656篇
  2022年   463篇
  2021年   754篇
  2020年   763篇
  2019年   845篇
  2018年   1634篇
  2017年   1552篇
  2016年   1942篇
  2015年   1256篇
  2014年   1893篇
  2013年   3368篇
  2012年   2225篇
  2011年   2885篇
  2010年   2481篇
  2009年   3163篇
  2008年   2721篇
  2007年   2723篇
  2006年   2680篇
  2005年   2043篇
  2004年   1875篇
  2003年   1782篇
  2002年   1721篇
  2001年   1498篇
  2000年   1465篇
  1999年   1252篇
  1998年   1247篇
  1997年   1222篇
  1996年   1070篇
  1995年   1031篇
  1994年   902篇
  1993年   816篇
  1992年   764篇
  1991年   758篇
  1990年   811篇
  1989年   731篇
  1988年   671篇
  1987年   822篇
  1986年   678篇
  1985年   824篇
  1984年   962篇
  1983年   864篇
  1982年   846篇
  1981年   732篇
  1980年   741篇
  1979年   647篇
  1978年   620篇
  1977年   604篇
  1976年   541篇
  1975年   528篇
  1974年   562篇
  1973年   623篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Occultations and eclipses from ancient times down to the present are analysed to determine changes in the length of the day. By subtracting the expected tidal contribution from the observed changes, the non-tidal variations are obtained. The non-tidal varations are shown to occur on time-scales of decades and millennia.  相似文献   
142.
143.
Abstract

The superconducting-gravimeter data of Melchior and Ducarme (1986) has been interpreted as internal motion in the Earth's core by Aldridge and Lumb (1987) using a Poincaré model. Several low-order modes with periods of 13–16 hours have been tentatively identified in the core which is taken to be an incompressible, homogeneous fluid within a rigid, rotating container. The identification is based on asymptotic values of the frequencies which change slowly with time while the modes decay with an e-folding time of about 280 days. The slow change in frequency with time implies a small temporal variation in the rotation rate of the core. This mean flow is a nonlinear effect often observed in laboratory experiments designed to excite Poincaré modes. Interaction among modes during free ringdown is also observed in those experiments and apparently in the data of Melchior et al. (1988) as well. Laboratory work thus provides the link to extend the Poincaré model to include viscous and nonlinear effects in order to interpret the gravimetric observations as core modes.  相似文献   
144.
Human-driven changes in the global environment pose an increasingly urgent challenge for the management of ecosystems that is made all the more difficult by the uncertain future of both environmental conditions and ecological responses. Land managers need strategies to increase regional adaptive capacity, but relevant and rapid assessment approaches are lacking. To address this need, we developed a method to assess regional protected area networks across biophysically important climatic gradients often linked to biodiversity and ecosystem function. We plot the land of the southwestern United States across axes of historical climate space, and identify landscapes that may serve as strategic additions to current protected area portfolios. Considering climate space is straightforward, and it can be applied using a variety of relevant climate parameters across differing levels of land protection status. The resulting maps identify lands that are climatically distinct from existing protected areas, and may be utilized in combination with other ecological and socio-economic information essential to collaborative landscape-scale decision-making. Alongside other strategies intended to protect species of special concern, natural resources, and other ecosystem services, the methods presented herein provide another important hedging strategy intended to increase the adaptive capacity of protected area networks.  相似文献   
145.
146.
Adaptation to climate change is already being delivered by public and private actors, yet there has been little analysis of the relationships between the providers and beneficiaries of adaptation. This paper reviews the type of actors that are supplying adaptation services and their motivations. We then focus on a specific, under-explored case of adaptation: that of privately provided adaptation public goods and services, the realization of which is contingent on the individual management of private goods and private risks. Following the work of Olson (1965) we find that the benefits of the privately provided adaptation public good do not necessarily accrue back to the (same) individuals who are the providers. The characteristics of this particular form of public good pose specific institutional challenges. In this paper we: 1) explore the characteristics and defining features of these privately provided adaptation public goods; 2) argue that this form of adaptation provisioning is increasingly recognised as a feature in climate change adaptation (and/or social transformation) problems; 3) review existing cases of effective/ineffective management of these public goods; and 4) outline the institutions that may be required to facilitate the management of these public goods for adaptation.  相似文献   
147.
148.
The angular sizes of compact steep spectrum sources in the Peacock and Wall sample were found to decrease steeply with redshift, similar to the angular size-redshift variation for extended sources. This, however, could be an artefact if the compact sources are not all of the same nature.Models such as those of Subramanian and Swarup with a beam power of 1037 W were found to be compatible with compact sources being either young sources at an early stage of development or relatively weak sources in dense, active media where star formation is continuing.  相似文献   
149.
Results of three-colour VRI CCD photometry are presented obtained during 11 nights in 2001. Twelve new minima timings are derived. The eclipse depth decreases with wavelength, being equal to 1.28, 1.13 and 1.02 for V, R and I, respectively. The instrumental magnitudes and colours are tabulated for the mid-eclipse, out-of-eclipse part and the eclipsed component. The VR colour of the eclipsed component of emission is by 0.22 mag bluer than that for mid-eclipse, indicating a much higher temperature of the eclipsed region.  相似文献   
150.
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