全文获取类型
收费全文 | 123296篇 |
免费 | 1893篇 |
国内免费 | 873篇 |
专业分类
测绘学 | 3168篇 |
大气科学 | 8464篇 |
地球物理 | 23925篇 |
地质学 | 45199篇 |
海洋学 | 10594篇 |
天文学 | 27697篇 |
综合类 | 413篇 |
自然地理 | 6602篇 |
出版年
2022年 | 695篇 |
2021年 | 1189篇 |
2020年 | 1292篇 |
2019年 | 1472篇 |
2018年 | 4576篇 |
2017年 | 4177篇 |
2016年 | 4185篇 |
2015年 | 1845篇 |
2014年 | 3282篇 |
2013年 | 5749篇 |
2012年 | 4105篇 |
2011年 | 6076篇 |
2010年 | 5443篇 |
2009年 | 6768篇 |
2008年 | 5804篇 |
2007年 | 6167篇 |
2006年 | 4522篇 |
2005年 | 3511篇 |
2004年 | 3427篇 |
2003年 | 3285篇 |
2002年 | 3184篇 |
2001年 | 2676篇 |
2000年 | 2585篇 |
1999年 | 2111篇 |
1998年 | 2147篇 |
1997年 | 2049篇 |
1996年 | 1734篇 |
1995年 | 1711篇 |
1994年 | 1480篇 |
1993年 | 1376篇 |
1992年 | 1272篇 |
1991年 | 1306篇 |
1990年 | 1275篇 |
1989年 | 1185篇 |
1988年 | 1086篇 |
1987年 | 1262篇 |
1986年 | 1120篇 |
1985年 | 1348篇 |
1984年 | 1571篇 |
1983年 | 1441篇 |
1982年 | 1385篇 |
1981年 | 1277篇 |
1980年 | 1148篇 |
1979年 | 1089篇 |
1978年 | 1081篇 |
1977年 | 931篇 |
1976年 | 892篇 |
1975年 | 899篇 |
1974年 | 849篇 |
1973年 | 933篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
83.
84.
85.
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
86.
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
87.
A. Habib J. Vernin Z. Benkhaldoun H. Lanteri 《Monthly notices of the Royal Astronomical Society》2006,368(3):1456-1462
In this paper, we present a new method to estimate, for each turbulent layer labelled i , the horizontal wind speed v ( h i ) , the standard deviation of the horizontal wind speed fluctuations σ v ( hi ) and the integrated value of C 2 n over the thickness Δ hi of the turbulent layer C 2 n ( hi )Δ hi , where hi is the altitude of the turbulent layer. These parameters are extracted from single star scintillation spatiotemporal cross-correlation functions of atmospheric speckles obtained within the generalized mode. This method is based on the simulated annealing algorithm to find the optimal solution required to solve the problem. Astrophysics parameters for adaptive optics are also calculated using C 2 n ( hi ) and v ( hi ) values. The results of other techniques support this new method. 相似文献
88.
A. Quirrenbach J. Larkin M. Barczys T. Gasaway C. Iserlohe A. Krabbe M. McElwain I. Song J. Weiss S. Wright 《New Astronomy Reviews》2006,49(10-12):639
OSIRIS (OH-Suppressing Infra-Red Integral-field Spectrograph) is a new facility instrument for the Keck Observatory. After seeing first light in February 2005, OSIRIS is currently undergoing commissioning. OSIRIS provides the capability of performing three-dimensional spectroscopy in the near-infrared z, J, H, and K bands at the resolution limit of the Keck II telescope, which is equipped with adaptive optics and a laser guide star. The science case for OSIRIS is summarized, and the instrument and associated data reduction software are described. 相似文献
89.
90.
E.A. Evstigneeva R.R. de Carvalho A.L. Ribeiro H.V. Capelato 《Astrophysics and Space Science》2003,284(2):487-490
We present the preliminary results of a study of how small stellar systems merge to form larger ones. As we display the families
of galaxies in the μe - Re plane (effective surface brightness versus effective radius) we realize that different morphological types occupy different
loci, evidencing the different physical mechanisms operating in each family. As proposed by Capaccioli et al. (1992) this
diagram is the logical equivalent of the HR diagram for stars. Here we take some initial steps in understanding of how we
can establish the evolutionary tracks, solely due to dynamical processes, in the μe - Re plane, ultimately making a dwarf elliptical to turn into a normal elliptical galaxy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献