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   8981篇
  免费   1450篇
  国内免费   1588篇
测绘学   2968篇
大气科学   909篇
地球物理   1824篇
地质学   3610篇
海洋学   982篇
天文学   414篇
综合类   662篇
自然地理   650篇
  2024年   28篇
  2023年   81篇
  2022年   280篇
  2021年   359篇
  2020年   396篇
  2019年   429篇
  2018年   298篇
  2017年   476篇
  2016年   458篇
  2015年   468篇
  2014年   615篇
  2013年   658篇
  2012年   665篇
  2011年   645篇
  2010年   492篇
  2009年   555篇
  2008年   600篇
  2007年   577篇
  2006年   590篇
  2005年   562篇
  2004年   444篇
  2003年   347篇
  2002年   338篇
  2001年   245篇
  2000年   207篇
  1999年   184篇
  1998年   174篇
  1997年   145篇
  1996年   116篇
  1995年   109篇
  1994年   114篇
  1993年   89篇
  1992年   75篇
  1991年   42篇
  1990年   38篇
  1989年   27篇
  1988年   34篇
  1987年   18篇
  1986年   11篇
  1985年   6篇
  1984年   4篇
  1983年   4篇
  1982年   1篇
  1980年   1篇
  1979年   6篇
  1978年   2篇
  1977年   1篇
  1974年   1篇
  1954年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
嫦娥工程获得了大量月球探测数据。针对让公众用户通过网络获取月球信息,更好了解月球知识的需求,地面应用系统提出了建立基于WebGIS的月球三维可视化系统的解决方案。首先介绍了国内外三维月球的发展现状,详细分析了开发WebGIS系统的客户端和服务器端技术,重点阐述了月球空间数据模型和三维场景可视化渲染等三维GIS可视化关键技术,为建立我国的三维月球可视化系统提供了理论支持。  相似文献   
992.
From a historical point of view, it was only through the advent of the CCD as a linear, high dynamic range panoramic detector that it became possible to overcome the source confusion problem for stellar photometry, e.g., in star clusters or nearby galaxies. The ability of accurately sampling the point-spread-function (PSF) in two dimensions and to use it as a template for fitting severely overlapping stellar images is of fundamental importance for crowded-field photometry, and has thus become the foundation for the determination of accurate color-magnitude diagrams of globular clusters and the study of resolved stellar populations in nearby galaxies. Analogous to CCDs, the introduction of integral field spectrographs has opened a new avenue for crowded-field 3D spectroscopy, which benefits in the same way from PSF-fitting techniques as CCD photometry does. This paper presents first experience with sampling the PSF in 3D spectroscopy, reviews the effects of atmospheric refraction, discusses background subtraction problems, and presents several science applications as obtained from observations with the PMAS instrument at Calar Alto Observatory.  相似文献   
993.
Phase-referenced observations of 13 star-forming regions in the  2Π1/2, J = 1/2  transition of rotationally excited OH at 4765 MHz have been carried out using MERLIN. Two of the regions were also observed at 4750 MHz and one at 4660 MHz. There were 10 maser detections at 4765 MHz and three non-detections. There were no detections at 4750 and 4660 MHz. The 4765-MHz masers have brightness temperatures of  ∼107 K  at MERLIN resolution (∼50 mas). Several cases of 4765-MHz masers overlapping in position and velocity with 1720- and 1665-MHz masers are reported. There are also isolated 4765-MHz masers with peak flux densities ≥30 times that of any ground-state counterpart. Most of the 4.7-GHz maser spots are unresolved at 50-mas angular resolution, but in four of the nearest sources the maser spots are resolved, indicating a characteristic size for 4765-MHz maser regions of ∼100 au. In W3(OH) we discovered that 20 per cent of the 4765-MHz emission comes from a narrow low-brightness filament that stretches north–south for ∼1.0 arcec (∼2200 au) between two previously known 4765-MHz maser spots. The filament appears in projection against the H  ii region and has a brightness temperature of  ∼4 × 105 K  . There are matching absorption features in mainline transitions of highly excited OH. The filament may trace a shock front in a rotating disc.  相似文献   
994.
Radio frequency interference (RFI) test observations were carried out at one of the candidate Square Kilometre Array (SKA) sites in Guizhou province, following the “RFI Measurement Protocol for Candidate SKA Sites” (hereafter RFI protocol). All data (raw and calibrated) are preserved in some suitable format, such as that set by the international RFI working group of the Site Evaluation and Selection Committee (SESC). An RFI test in December 2003 was performed according to Mode 1 of the RFI Protocol, in order to identify technical difficulties which might arise during a co-ordinated RFI measurement campaign over a period of 1 year. In this paper we describe the current equipment, observational technique and data presentation. The preliminary results demonstrate that the RFI situation at Dawodang depression in Guizhou province makes it quite a promising location for the proposed SKA. Furthermore, the first session of the RFI monitoring program, which was made in May 2004, showed that a complete RFI measurement including both modes 1 and 2 of the RFI Protocol would take about 2 weeks. The possible ways to minimize some limitations of the current equipment are also discussed, which will enable us to meet the RFI protocol.  相似文献   
995.
This expository paper gathers some of the results obtained by the author in recent works in collaboration with Davide Ferrario and Vivina Barutello, focusing on the periodic n-body problem from the perspective of the calculus of variations and minimax theory. These researches were aimed at developing a systematic variational approach to the equivariant periodic n-body problem in the two and three-dimensional space. The purpose of this paper is to expose the main problems and achievements of this approach. The material here was exposed in the talk that given at the Meeting CELMEC IV promoted by SIMCA (Società italiana di Meccanica Celeste).  相似文献   
996.
Microfossils in isolation basin sediments are frequently used to reconstruct sea‐level change, but preservation problems and non‐analogue situations can limit their usefulness. Here we investigate the potential of stable carbon isotopes (δ13C) and C/N ratios from bulk organic matter, as an alternative proxy of salinity within isolation basin sediments from a basin in northwest Scotland. Within the Holocene sediment δ13C and C/N are determined largely by the mean weighted values of the predominant source of the organic material. Analysis of modern materials and comparison with the diatom record shows that the marine parts of the sequence are dominated by high δ13C and variable C/N. In the fresh water sequences the organic material is a mixture of both freshwater aquatic and terrestrial plant input that have relatively low δ13C and high C/N. The application of δ13C and C/N ratios in the studied basin in general follow the environmental change recorded by the diatoms and shows the potential of bulk organic matter in the investigation of salinity change in isolation basins. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
997.
We present a preliminary analysis of medium resolution optical spectra of comet C/2000 WM1 (LINEAR) obtained on 22 November 2001. Theemission lines of the molecules C2, C3, CN, NH2,H2O+ and presumably CO (Asundi and triplet bands) and C2 -were identified in these spectra. By analysing the brightnessdistributions of the C2, C3, CN emission lines along theslit of the spectrograph we determined some physical parameters of theseneutrals, such as their lifetimes and expansion velocities inthe coma. The Franck–Condon factors for the CO Asundi bands and C2 - bands were calculated using a Morse potential model.  相似文献   
998.
999.
1000.
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