全文获取类型
收费全文 | 20519篇 |
免费 | 3380篇 |
国内免费 | 4383篇 |
专业分类
测绘学 | 1753篇 |
大气科学 | 2364篇 |
地球物理 | 3419篇 |
地质学 | 10115篇 |
海洋学 | 3136篇 |
天文学 | 1875篇 |
综合类 | 1415篇 |
自然地理 | 4205篇 |
出版年
2024年 | 60篇 |
2023年 | 211篇 |
2022年 | 611篇 |
2021年 | 732篇 |
2020年 | 721篇 |
2019年 | 909篇 |
2018年 | 714篇 |
2017年 | 782篇 |
2016年 | 855篇 |
2015年 | 950篇 |
2014年 | 1167篇 |
2013年 | 1034篇 |
2012年 | 1254篇 |
2011年 | 1277篇 |
2010年 | 1086篇 |
2009年 | 1426篇 |
2008年 | 1268篇 |
2007年 | 1563篇 |
2006年 | 1535篇 |
2005年 | 1358篇 |
2004年 | 1233篇 |
2003年 | 1174篇 |
2002年 | 1020篇 |
2001年 | 812篇 |
2000年 | 662篇 |
1999年 | 608篇 |
1998年 | 551篇 |
1997年 | 409篇 |
1996年 | 406篇 |
1995年 | 333篇 |
1994年 | 297篇 |
1993年 | 269篇 |
1992年 | 235篇 |
1991年 | 191篇 |
1990年 | 129篇 |
1989年 | 163篇 |
1988年 | 79篇 |
1987年 | 56篇 |
1986年 | 37篇 |
1985年 | 25篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1954年 | 5篇 |
1877年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
21.
22.
MEN Wu ) WEI Hao ) and LIU Guangshan ) * ) College of Oceanography Environmental Science Xiamen University State Key Laboratory of Marine Environmental Science 《中国海洋大学学报(英文版)》2006,5(3):228-234
1Introduction Therearefourradiumisotopesinnature,ofwhich224Ra,226Raand228Rahavebeenwidelyinvestigated inmarineresearch,butthespecificactivityof223Rais toolowtobemeasured,therefore,itsapplicationsarelimited(Moore,1969;Broeckeretal.,1976;KuandLin,1976;Nozak… 相似文献
23.
24.
A. S. Oliveira J. E. Steiner D. Cieslinski 《Monthly notices of the Royal Astronomical Society》2003,346(3):963-967
The star WR 7a, also known as SPH 2, has a spectrum that resembles that of V Sagittae stars although no O vi emission has been reported. The Temporal Variance Spectrum – TVS – analysis of our data shows weak but strongly variable emission of O vi lines which is below the noise level in the intensity spectrum.
Contrary to what is seen in V Sagittae stars, optical photometric monitoring shows very little, if any, flickering. We found evidence of periodic variability. The most likely photometric period is Pphot = 0.227(±14) d , while radial velocities suggest a period of P spec = 0.204(±13) d . One-day aliases of these periods can not be ruled out. We call attention to similarities with HD 45166 and DI Cru (= WR 46), where multiple periods are present. They may be associated to the binary motion or to non-radial oscillations.
In contrast to a previous conclusion by Pereira et al., we show that WR 7a contains hydrogen. The spectrum of the primary star seems to be detectable as the N v 4604 Å absorption line is visible. If so, it means that the wind is optically thin in the continuum and that it is likely to be a helium main sequence star.
Given the similarity to HD 45166, we suggests that WR 7a may be a qWR – quasi Wolf–Rayet – star. Its classification is WN4h/CE in the Smith, Shara & Moffat three-dimensional classification system. 相似文献
Contrary to what is seen in V Sagittae stars, optical photometric monitoring shows very little, if any, flickering. We found evidence of periodic variability. The most likely photometric period is P
In contrast to a previous conclusion by Pereira et al., we show that WR 7a contains hydrogen. The spectrum of the primary star seems to be detectable as the N v 4604 Å absorption line is visible. If so, it means that the wind is optically thin in the continuum and that it is likely to be a helium main sequence star.
Given the similarity to HD 45166, we suggests that WR 7a may be a qWR – quasi Wolf–Rayet – star. Its classification is WN4h/CE in the Smith, Shara & Moffat three-dimensional classification system. 相似文献
25.
26.
J.-M. Carquillat N. Ginestet J.-L. Prieur Y. Debernardi 《Monthly notices of the Royal Astronomical Society》2003,346(2):555-564
We present the results of a radial-velocity study of eight Am stars (HD 341, 55822, 61250, 67317, 93991, 162950, 224890 and 225137) observed at Observatoire de Haute-Provence with the CORAVEL instrument. We find that these systems are single-line spectroscopic binaries whose orbital elements are determined for the first time. 相似文献
27.
28.
Piero F. Spinnato Michael Fellhauer Simon F. Portegies Zwart 《Monthly notices of the Royal Astronomical Society》2003,344(1):22-32
We study the efficiency at which a black hole or dense star cluster spirals in to the Galactic Centre. This process takes place on a dynamical friction time-scale, which depends on the value of the Coulomb logarithm (ln Λ). We determine the accurate value of this parameter using the direct N -body method, a tree algorithm and a particle-mesh technique with up to two million plus one particles. The three different techniques are in excellent agreement. Our measurement for the Coulomb logarithm appears to be independent of the number of particles. We conclude that ln Λ= 6.6 ± 0.6 for a massive point particle in the inner few parsec of the Galactic bulge. For an extended object, such as a dense star cluster, ln Λ is smaller, with a value of the logarithm argument Λ inversely proportional to the object size. 相似文献
29.
从地形地貌特征、重磁地球物理场、深部构造与断裂构造以及地震活动、震源机制等多方面论述北东东向构造是南海北缘的主要活动构造。对于长期以来认为新华夏系北东向构造是该区主要活动构造的观点来说,这是一种新的学术思想。 相似文献
30.
用直接数值积分方法通过模拟不同的行星构型探讨了HD82943行星系统(由两颗共振的巨行星组成)的长期动力演化,同时,还研究了在相空间的轨道运动.在对系统长达107年数值积分中,发现所有的稳定轨道均与2:1共振相关联.典型地,在相同的时标内,两个共振幅角θ1和θ2同时(或其中之一)存在秤动.由于共振幅角在一定范围内的秤动,因而使两颗行星轨道半长径被约束而表现为规则运动模式.另外,利用分析模型(包含了外行星偏心率e2的因素),还讨论了对于不同取值的e2和相对近星点经度θ时,内行星在相空间的运动,并发现2:1轨道共振对于相对较小的e2以及当θ=0°时易于保持.此外,适中的e2将导致系统的两颗行星进入深度共振状态.再者,分析模型和数值计算的结果吻合得很好,两者都揭示了行星系统的2:1共振结构. 相似文献