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   5520篇
  免费   455篇
  国内免费   575篇
测绘学   260篇
大气科学   278篇
地球物理   549篇
地质学   1463篇
海洋学   343篇
天文学   2777篇
综合类   137篇
自然地理   743篇
  2024年   27篇
  2023年   53篇
  2022年   115篇
  2021年   123篇
  2020年   121篇
  2019年   194篇
  2018年   105篇
  2017年   119篇
  2016年   116篇
  2015年   144篇
  2014年   204篇
  2013年   234篇
  2012年   201篇
  2011年   234篇
  2010年   174篇
  2009年   469篇
  2008年   401篇
  2007年   474篇
  2006年   473篇
  2005年   382篇
  2004年   363篇
  2003年   336篇
  2002年   256篇
  2001年   228篇
  2000年   177篇
  1999年   178篇
  1998年   201篇
  1997年   93篇
  1996年   71篇
  1995年   46篇
  1994年   41篇
  1993年   41篇
  1992年   35篇
  1991年   18篇
  1990年   22篇
  1989年   16篇
  1988年   14篇
  1987年   12篇
  1986年   12篇
  1985年   8篇
  1984年   6篇
  1982年   6篇
  1981年   1篇
  1980年   1篇
  1979年   2篇
  1977年   1篇
  1971年   2篇
排序方式: 共有6550条查询结果,搜索用时 15 毫秒
1.
The Bear Brook Watershed in Maine (BBWM) is a long-term research site established to study the response of forest ecosystem function to environmental disturbances of chronic acidic deposition and ecosystem nitrogen enrichment. Starting in 1989, the West Bear (treated) watershed received bimonthly applications of ammonium sulfate [(NH4)2SO4] fertilizer from above the canopy, whereas East Bear (reference) received ambient deposition. The treatments were stopped in 2016, marking the beginning of the recovery phase. Research at the site has focused on soils, streams, and vegetation. Here, we describe data collected over three decades at the BBWM—input and stream output nutrient fluxes, quantitative soil pits and soil chemistry, and soil temperature and moisture.  相似文献   
2.
3.
4.
5.
6.
7.
8.
We describe a procedure for the numerical modelling of astronomical interferometers, with particular relevance to far-infrared and submillimetre wavelengths. The scheme is based on identifying a set of modes that carry power from the sky to the detector. The procedure is extremely general, and can be used to model scalar or vector fields, in any state of coherence and polarization, the only limitation being that the propagation of a coherent field through the system be described by an integral transform, a constraint that is in practise always met.
We present simulations of ideal, multimode two-dimensional interferometers, and show that the modal theory reproduces the correct behaviour of both Michelson and Fizeau interferometers. We calculate simulated visibility data for a multimode bolometric Michelson interferometer, with a synthesized source, and produce a dirty map, recovering the original source with the usual artefacts associated with interferometers.  相似文献   
9.
10.
In the first paper of this series, we presented EBAS – Eclipsing Binary Automated Solver, a new fully automated algorithm to analyse the light curves of eclipsing binaries, based on the ebop code. Here, we apply the new algorithm to the whole sample of 2580 binaries found in the Optical Gravitational Lensing Experiment (OGLE) Large Magellanic Cloud (LMC) photometric survey and derive the orbital elements for 1931 systems. To obtain the statistical properties of the short-period binaries of the LMC, we construct a well-defined subsample of 938 eclipsing binaries with main-sequence B-type primaries. Correcting for observational selection effects, we derive the distributions of the fractional radii of the two components and their sum, the brightness ratios and the periods of the short-period binaries. Somewhat surprisingly, the results are consistent with a flat distribution in log P between 2 and 10 d. We also estimate the total number of binaries in the LMC with the same characteristics, and not only the eclipsing binaries, to be about 5000. This figure leads us to suggest that  (0.7 ± 0.4)  per cent of the main-sequence B-type stars in the LMC are found in binaries with periods shorter than 10 d. This frequency is substantially smaller than the fraction of binaries found by small Galactic radial-velocity surveys of B stars. On the other hand, the binary frequency found by Hubble Space Telescope ( HST ) photometric searches within the late main-sequence stars of 47 Tuc is only slightly higher and still consistent with the frequency we deduced for the B stars in the LMC.  相似文献   
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