全文获取类型
收费全文 | 742篇 |
免费 | 32篇 |
国内免费 | 11篇 |
专业分类
测绘学 | 27篇 |
大气科学 | 69篇 |
地球物理 | 153篇 |
地质学 | 250篇 |
海洋学 | 96篇 |
天文学 | 149篇 |
自然地理 | 41篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 12篇 |
2021年 | 17篇 |
2020年 | 16篇 |
2019年 | 20篇 |
2018年 | 35篇 |
2017年 | 25篇 |
2016年 | 49篇 |
2015年 | 25篇 |
2014年 | 39篇 |
2013年 | 68篇 |
2012年 | 35篇 |
2011年 | 50篇 |
2010年 | 35篇 |
2009年 | 47篇 |
2008年 | 38篇 |
2007年 | 45篇 |
2006年 | 42篇 |
2005年 | 25篇 |
2004年 | 17篇 |
2003年 | 19篇 |
2002年 | 12篇 |
2001年 | 9篇 |
2000年 | 3篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 1篇 |
1993年 | 5篇 |
1992年 | 2篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1938年 | 1篇 |
1895年 | 1篇 |
排序方式: 共有785条查询结果,搜索用时 15 毫秒
701.
V.G. Shevchenko I.N. Belskaya V.G. Chiorny J. Piironen A. Erikson G. Neukum R. Mohamed 《Planetary and Space Science》1997,45(12):1615-1623
We present observations of magnitude-phase dependences of three low-albedo asteroids down to phase angles of 0.1–0.2°. Data were obtained during 40 nights from 1994 to 1995 within the joint observational program at ESO and Kharkiv Astronomical Observatories with the aim to reach as low phase angles as possible. All three low-albedo asteroids may display a small nonlinear increase in magnitude-phase dependence at subdegree phase angles. The phase curves of 50 Virginia and 102 Miriam are poorly approximated by the HG function. Rotation periods of the asteroids were also determined: 14.310±0.010 hours for 50 Virginia, 6.030±0.001 h for 91 Aegina and 15.789± 0.003 h for 102 Miriam. 相似文献
702.
V. S. Shevchenko O. V. Ezhkova M. A. Ibrahimov M. E. van den Ancker H. R. E. Tjin A Djie 《Monthly notices of the Royal Astronomical Society》1999,310(1):210-222
A new photometric and spectroscopic survey of the star formation region (SFR) CMa R1 is described. In a sample of 165 stars brighter than 13th mag, 88 stars were found to be probable members of the SFR. They are defined as early-type stars with E ( B − V )0.16 mag, which corresponds to a distance of about 1 kpc. 74 of the probable members are B stars. 19 stars are possibly associated with an IRAS point source. We derive a most probable distance of 1050±150 pc to the association. It appears that about 80 candidate members are pre-main-sequence stars with ages lower than 6 million years, while the main sequence extends over 6.0–7.6 mag, which is consistent with star formation starting about 8 million years ago and continuing until at least half a million years ago. Two bright B stars in the association (GU CMa and FZ CMa) seem to be much older and probably do not originate from the same star formation episode. The star formation efficiency appears to increase roughly monotonically with time up to half a million years ago. From our data, we conclude that only a minor fraction of the stars has been created through the scenario suggested by Herbst & Assousa, in which the members of CMa R1 form by compression of ambient material by a supernova shock wave. An extensive search for candidate members with H α emission did not reveal new Herbig Ae/Be candidates, so that the number of stars in this class seems to be limited to four: Z CMa, LkH α 218, LkH α 220 and possibly HD 53367. 相似文献
703.
Rebekah M. Harries Linda A. Kirstein Alex C. Whittaker Mikael Attal Ian Main 《Basin Research》2019,31(5):866-891
Grain size trends in basin stratigraphy are thought to preserve a rich record of the climatic and tectonic controls on landscape evolution. Stratigraphic models assume that over geological timescales, the downstream profile of sediment deposition is in dynamic equilibrium with the spatial distribution of tectonic subsidence in the basin, sea level and the flux and calibre of sediment supplied from mountain catchments. Here, we demonstrate that this approach in modelling stratigraphic responses to environmental change is missing a key ingredient: the dynamic geomorphology of the sediment routing system. For three large alluvial fans in the Iglesia basin, Argentine Andes we measured the grain size of modern river sediment from fan apex to toe and characterise the spatial distribution of differential subsidence for each fan by constructing a 3D model of basin stratigraphy from seismic data. We find, using a self‐similar grain size fining model, that the profile of grain size fining on all three fans cannot be reproduced given the subsidence profile measured and for any sediment supply scenario. However, by adapting the self‐similar model, we demonstrate that the grain size trends on each fan can be effectively reproduced when sediment is not only sourced from a single catchment at the apex of the system, but also laterally, from tributary catchments and through fan surface recycling. Without constraint on the dynamic geomorphology of these large alluvial systems, signals of tectonic and climate forcing in grain size data are masked and would be indecipherable in the geological record. This has significant implications for our ability to make sensitive, quantitative reconstructions of external boundary conditions from the sedimentary record. 相似文献
704.
Preflow stresses in Martian rampart ejecta blankets: A means of estimating the water content 总被引:1,自引:0,他引:1
Alex Woronow 《Icarus》1981,45(2):320-330
Measurements of extents of rampart ejecta deposits as a function of the size of the parent craters support models which, for craters larger than about 6 km diameter, constrain ejecta blankets to all have a similar maximum thickness regardless of the crater size. These volatile-rich ejecta blankets may have failed under their own weights, then flowed radially outward. Assuming this to be so, we can then determine some of the physicomechanical properties of the ejecta deposits at the time of their emplacement. Finite-element studies of the stress magnitudes, distributions, and directions in hypothetical Martian rampart ejecta blankets reveal that the material most likely failed when the shear stresses were less than 500 kPa and the angle of internal friction was between 26 and 36°. These figures imply that the ejecta has a water content between 16 and 72%. Whether the upper limit or the lower limit is more appropriate depends on the mode of failure which one presumes; namely, viscous flow or plastic deformation. 相似文献
705.
Having analyzed the light curve for the Herbig Ae star BF Ori, we justify the hypothesis of a giant protocomet, GPC I BF Ori, with a period of 6.3 years and semimajor axis a = 10 ± 3 AU. Passing through periastron, such a giant protocomet partially breaks up. During each passage through periastron, the protocomet and the fragments that follow it supply dust to circumstellar space for a certain period of time. This hypothesis can account for the entire complex of observable phenomena of cyclic Algol-like activity in Herbig Ae/Be and T Tauri stars. Conditions in a protoplanetary disk after cocoon breakup are discussed. We adduce arguments for the absence of a dust disk and for the weak effect of objects other than comets on cyclic large-scale variability. 相似文献
706.
707.
708.
Ivan I. Shevchenko Andrej G. Sokolsky 《Celestial Mechanics and Dynamical Astronomy》1995,62(4):289-304
The Norma specialized program package, intended for normalization of autonomous Hamiltonian systems by means of computer algebra, is used in studies of small-amplitude periodic motions in the neighbourhood of regular precessions of a dynamically symmetric satellite on a circular orbit. The case of hyperboloidal precession is considered. Analytical expressions for normal forms and generating functions depending on frequencies of the system as on parameters are derived. Possible resonances are considered in particular. The 6th order of normalization is achieved. Though the intermediate analytical expressions occupy megabytes of computer's main memory, final ones are quite compact. Obtained analytical expressions are applied to the analysis of stability of small-amplitude periodic motions in the neighbourhood of hyperboloidal precession. 相似文献
709.
The statistical behaviour of intermittent trajectories at the 3/1resonance is investigated. The elliptic planar restricted three-body problemis used as a model. Distribution functions for time intervals D betweeneccentricity bursts are obtained and theoretically interpreted. For smallervalues of D the distribution is found to be of Poisson type, while in itstail it is described by a power law D. This change in thedistribution occurs for values of D in the range105106Jupiter periods. The power-lawindexfor the integral distributions lies in the range (-2,-1) and istrajectory dependent. The algebraic decay in the tails of the distributionsis explained by the phenomenon of sticking of orbits to the chaos borderduring long intervals between bursts. 相似文献
710.