全文获取类型
收费全文 | 11421篇 |
免费 | 2020篇 |
国内免费 | 2616篇 |
专业分类
测绘学 | 709篇 |
大气科学 | 2302篇 |
地球物理 | 3023篇 |
地质学 | 5549篇 |
海洋学 | 1280篇 |
天文学 | 671篇 |
综合类 | 1049篇 |
自然地理 | 1474篇 |
出版年
2024年 | 57篇 |
2023年 | 161篇 |
2022年 | 486篇 |
2021年 | 573篇 |
2020年 | 480篇 |
2019年 | 607篇 |
2018年 | 665篇 |
2017年 | 612篇 |
2016年 | 694篇 |
2015年 | 613篇 |
2014年 | 721篇 |
2013年 | 695篇 |
2012年 | 689篇 |
2011年 | 785篇 |
2010年 | 708篇 |
2009年 | 677篇 |
2008年 | 631篇 |
2007年 | 591篇 |
2006年 | 479篇 |
2005年 | 405篇 |
2004年 | 345篇 |
2003年 | 342篇 |
2002年 | 334篇 |
2001年 | 298篇 |
2000年 | 331篇 |
1999年 | 409篇 |
1998年 | 341篇 |
1997年 | 346篇 |
1996年 | 286篇 |
1995年 | 253篇 |
1994年 | 298篇 |
1993年 | 217篇 |
1992年 | 173篇 |
1991年 | 138篇 |
1990年 | 110篇 |
1989年 | 79篇 |
1988年 | 99篇 |
1987年 | 53篇 |
1986年 | 49篇 |
1985年 | 42篇 |
1984年 | 37篇 |
1983年 | 34篇 |
1982年 | 35篇 |
1981年 | 28篇 |
1980年 | 9篇 |
1979年 | 14篇 |
1978年 | 6篇 |
1975年 | 4篇 |
1958年 | 7篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
We discuss the determination of membership of 42 open clusters. Our analysis shows that Vasilevskis' mathematical model can be reasonably applied to this case. Our improved version of Sanders' method and our definition of cluster member based on the principles of discriminatory analysis effectively exclude stars of low probabilities. It is important in the study of open cluster to use only those with high probabilities. The effectiveness of the statistical method is closely related to the velocity distributions of the member and field stars. For fields where the error rate is high, it is better to combine other data than proper motion in determining membership. 相似文献
23.
24.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献
25.
26.
Andrés E. Piatti Juan J. Clariá Andrea V. Ahumada 《Monthly notices of the Royal Astronomical Society》2003,346(2):390-402
Charge-coupled device (CCD) photometry in the Johnson V , Kron–Cousins I and Washington CMT 1 systems is presented in the field of the poorly known open cluster NGC 2627. Four independent Washington abundance indices yield a mean cluster metallicity of [Fe/H]=−0.12 ± 0.08 , which is compatible with the existence of a radial gradient in the Galactic disc. The resultant colour–magnitude diagrams indicate that the cluster is an intermediate-age object of 1.4 Gyr. Based on the best fits of the Geneva group's isochrones to the ( V , V − I ) and ( T 1 , C − T 1 ) diagrams, we estimate E ( V − I ) = 0.25 ± 0.05 and V − MV = 11.80 ± 0.25 for log t = 9.15 , and E ( C − T 1 ) = 0.23 ± 0.07 and T 1 − M T 1 = 11.85 ± 0.25 for log t = 9.10 , respectively, assuming solar metal content. The derived reddening value E ( C − T 1 ) implies E ( B − V ) = 0.12 ± 0.07 and a distance from the Sun of 2.0 ± 0.4 kpc . Using the WEBDA data base and the available literature, we re-examined the overall properties of all the open clusters with ages between 0.6 and 2.5 Gyr. We identified peaks of cluster formation at 0.7–0.8, 1.0–1.1, 1.6–1.7 and 2.0–2.1 Gyr, separated by relative quiescent epochs of ∼0.2–0.3 Gyr. We also estimated a radial abundance gradient of −0.08 ± 0.02 , which is consistent with the most recent determinations for the Galactic disc, but no clear evidence for a gradient perpendicular to the Galactic plane is found. 相似文献
27.
Dong-Mei Qin Ping Guo Zhan-Yi Hu Yong-Heng ZhaoNational Laboratory of Pattern Recognition Laboratory Institute of Automation Chinese Academy of Sciences Beijing dmqin@nlpr.ia.ac.cnDepartment of Computer Sciences Beijing Normal University Beijing National Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2003,3(3)
For LAMOST, the largest sky survey program in China, the solution of the problem of automatic discrimination of stars from galaxies by spectra has shown that the results of the PSF test can be significantly refined. However, the problem is made worse when the redshifts of galaxies are not available. We present a new automatic method of star/(normal) galaxy separation, which is based on Statistical Mixture Modeling with Radial Basis Function Neural Networks (SMM-RBFNN). This work is a continuation of our previous one, where active and non-active celestial objects were successfully segregated. By combining the method in this paper and the previous one, stars can now be effectively separated from galaxies and AGNs by their spectra-a major goal of LAMOST, and an indispensable step in any automatic spectrum classification system. In our work, the training set includes standard stellar spectra from Jacoby's spectrum library and simulated galaxy spectra of EO, SO, Sa, Sb types with redshift ranging from 0 to 1 相似文献
28.
Josep M. Trigo‐Rodriguez Jordi Llorca Jiri Borovi
ka Juan Fabregat 《Meteoritics & planetary science》2003,38(8):1283-1294
Abstract— Relative chemical abundances of 13 meteoroids were determined by averaging the composition of the radiating gas along the fireball path that originated during their penetration into the Earth's atmosphere. Mg, Fe, Ni, Cr, Mn, and Co abundances, relative to Si, are similar to those reported for CI and CM carbonaceous chondrites and interplanetary dust particles. In contrast, relative abundances of Ca and Ti in meteor spectra indicate that these elements suffer incomplete evaporation processes. The chemical composition of all meteoroids studied in this work differs from that of 1P/Halley dust. 相似文献
29.
30.