全文获取类型
收费全文 | 104302篇 |
免费 | 1469篇 |
国内免费 | 634篇 |
专业分类
测绘学 | 2466篇 |
大气科学 | 6991篇 |
地球物理 | 19458篇 |
地质学 | 39508篇 |
海洋学 | 9052篇 |
天文学 | 23112篇 |
综合类 | 370篇 |
自然地理 | 5448篇 |
出版年
2022年 | 660篇 |
2021年 | 1112篇 |
2020年 | 1174篇 |
2019年 | 1323篇 |
2018年 | 4416篇 |
2017年 | 3986篇 |
2016年 | 3785篇 |
2015年 | 1529篇 |
2014年 | 2833篇 |
2013年 | 4804篇 |
2012年 | 3665篇 |
2011年 | 5555篇 |
2010年 | 4966篇 |
2009年 | 6068篇 |
2008年 | 5184篇 |
2007年 | 5622篇 |
2006年 | 3823篇 |
2005年 | 2932篇 |
2004年 | 2840篇 |
2003年 | 2734篇 |
2002年 | 2614篇 |
2001年 | 2141篇 |
2000年 | 2128篇 |
1999年 | 1645篇 |
1998年 | 1716篇 |
1997年 | 1571篇 |
1996年 | 1324篇 |
1995年 | 1331篇 |
1994年 | 1122篇 |
1993年 | 1068篇 |
1992年 | 983篇 |
1991年 | 1035篇 |
1990年 | 1009篇 |
1989年 | 866篇 |
1988年 | 801篇 |
1987年 | 962篇 |
1986年 | 824篇 |
1985年 | 1024篇 |
1984年 | 1142篇 |
1983年 | 1118篇 |
1982年 | 1015篇 |
1981年 | 965篇 |
1980年 | 884篇 |
1979年 | 814篇 |
1978年 | 790篇 |
1977年 | 681篇 |
1976年 | 675篇 |
1975年 | 659篇 |
1974年 | 653篇 |
1973年 | 708篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Summary. A method of synthetic seismogram computation for teleseismic SV -waves is developed in order to treat quantitatively SV -waves in problems of body wave source inversion and source—receiver structure studies. The method employs WKBJ theory for a generalized ray in a vertically inhomogeneous half-space and the propagator matrix technique for waves in near-surface homogeneous layers. Wavenumber integration is done along the real axis of the wavenumber plane and anelasticity is included by using complex velocity in all regions of the earth model. The near-surface source structure is taken into account in the computation for the case of the shallow source by allowing a point source to be located in the homogeneous layers. Source and receiver area structures are also allowed to differ. A general moment tensor point source is considered. 相似文献
62.
Carlos J.A.P. Martins 《Astrophysics and Space Science》2003,283(4):439-444
I briefly present the Organizing Committee's and my own motivation for organizing this workshop, and I suggest a few key questions
for which we will try to find possible answers in the coming days.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
63.
64.
65.
66.
67.
68.
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
69.
We have investigated the central regions of the galaxies in the NGC 3169/NGC 3166/NGC 3156 group with the multipupil fiber spectrograph of the 6-m telescope; the first (central) galaxy in the group is a spiral (Sa) one and the other two galaxies are lenticular ones. The group is known to have an extended HI cloud with a size of more than 100 kpc that is associated in its position, orientation, and rotation with the central galaxy NGC 3169. The mean age of the stellar populations in the centers of all three galaxies has been found to be approximately the same, ~1 Gyr. Since the galaxies are early-type ones and since NGC 3166 and NGC 3156 show no global star formation, we are dealing here with a synchronous star formation burst in the centers of all three galaxies. 相似文献
70.
Surface morphology and related issues for nuclei of three comets: Halley, Borrelly and Wild 2, are considered in the paper. Joint consideration of publications and results of our analysis of the comets’ images led to conclusions, partly new, partly repeating conclusions published by other researchers. It was found that typical for all three nuclei is the presence of rather flat areas: floors of craters and other depressions, mesas and terraces. This implies that flattening surfaces or planation is a process typical for the comet nuclei. Planation seems to work through the sublimation-driven slope collapse and retreat. This requires effective sublimation so this process should work only when a comet is close to the Sun and if on the nucleus there are starting slopes, steep and high enough to support the “long-distance” avalanching of the collapsing material. If the surface had no starting slopes, then instead of planation, the formation of pitted-and-hilly surfaces should occur. An example of this could be the mottled terrain of the Borelly nucleus. Both ways of the sublimational evolution on the nucleus surface should lead to accumulation of cometary regolith. The thickness of the degassed regolith is not known, but it is obvious that in surface depressions, including the flat-floor ones, it should be larger compared with nondepression areas. This may have implications for the in situ study of comets by the Deep Impact and Rosetta missions.Our morphological analysis puts constraints on the applicability of the popular “rubble-pile comet nucleus” hypothesis (Weissman, 1986. Are cometery nuclei primordial rubble piles? Nature 320, 242-244.). We believe that the rubble pile hypothesis can be applicable to the blocky Halley nucleus. The Borelly and Wild 2 nuclei also could be rubble piles. But in these cases the “rubbles” have to be either smaller than 30-50 m (a requirement to keep lineament geometry close to ideal), or larger than 1-2 km (a requirement to form the rather extended smooth, flat surfaces of mesa tops and crater floors). Another option is that the Borelly and Wild 2 nuclei are not rubble piles.In relation to surface morphology we suggest that three end-member types of the comet nuclei may exist: (1) impact cratered “pristine” bodies, (2) non-cratered fragments of catastrophic disruption, and (3) highly Sun-ablated bodies. In this threefold classification, the Wild 2 nucleus is partially ablated primarily cratered body. Borrelly is significantly ablated and could be either primarily cratered or not-cratered fragment. Halley is certainly partially ablated but with the available images it is difficult to say if remnants of impact craters do exist on it.Recently published observations and early results of analysis of the Tempel 1 nucleus images taken by Deep Impact mission are in agreement with our conclusions on the processes responsible for the Halley, Borrelly and Wild 2 nuclei morphologies. In particular, we have now more grounds to suggest that decrease in crater numbers and increase of the role of smooth flat surfaces in the sequence Wild 2?Tempel 1?Borelli reflects a progress in the sublimational degradation of the nucleus surface during comet passages close to the Sun. 相似文献