全文获取类型
收费全文 | 24512篇 |
免费 | 4550篇 |
国内免费 | 8967篇 |
专业分类
测绘学 | 1323篇 |
大气科学 | 11349篇 |
地球物理 | 3247篇 |
地质学 | 9012篇 |
海洋学 | 3840篇 |
天文学 | 5497篇 |
综合类 | 1416篇 |
自然地理 | 2345篇 |
出版年
2024年 | 139篇 |
2023年 | 345篇 |
2022年 | 777篇 |
2021年 | 1001篇 |
2020年 | 1017篇 |
2019年 | 1253篇 |
2018年 | 1005篇 |
2017年 | 1000篇 |
2016年 | 1037篇 |
2015年 | 1220篇 |
2014年 | 1673篇 |
2013年 | 1821篇 |
2012年 | 1863篇 |
2011年 | 1889篇 |
2010年 | 1731篇 |
2009年 | 2166篇 |
2008年 | 2069篇 |
2007年 | 2126篇 |
2006年 | 1920篇 |
2005年 | 1795篇 |
2004年 | 1483篇 |
2003年 | 1268篇 |
2002年 | 1028篇 |
2001年 | 978篇 |
2000年 | 912篇 |
1999年 | 808篇 |
1998年 | 730篇 |
1997年 | 491篇 |
1996年 | 447篇 |
1995年 | 407篇 |
1994年 | 378篇 |
1993年 | 306篇 |
1992年 | 188篇 |
1991年 | 172篇 |
1990年 | 93篇 |
1989年 | 87篇 |
1988年 | 87篇 |
1987年 | 37篇 |
1986年 | 36篇 |
1985年 | 54篇 |
1984年 | 44篇 |
1983年 | 31篇 |
1982年 | 26篇 |
1981年 | 26篇 |
1980年 | 22篇 |
1979年 | 2篇 |
1978年 | 10篇 |
1977年 | 21篇 |
1954年 | 3篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
对青藏东北缘现今块体划分、运动及变形的初步研究 总被引:19,自引:9,他引:10
利用2维非连续变形分析方法(DDA),以位移代替围压作为边界约束力,研究青藏东北缘现今块体划分及其运动变形。根据该地区地质构造及地震活动,以GPS点测量位移作为模拟结果约束点,得出了较合理的块体划分模型和随时间演化的主应变分布图,并把应变高值区与近几年来发生的5级以上地震作对比,得出了研究区内地震危险性可能较大的区域。另外,对模拟的甘青块体与阿拉善块体的边缘带断裂左旋运动做了大概计算。 相似文献
13.
14.
C. Beaugé 《Celestial Mechanics and Dynamical Astronomy》1996,64(4):313-350
Starting with a simple Taylor-based expansion of the inverse of the distance between two bodies, we are able to obtain a series expansion of the disturbing function of the three-body problem (planar elliptic case) which is valid for all points of the phase space outside the immediate vicinity of the collision points. In particular, the expansion is valid for very high values of the eccentricity of the perturbed body. Furthermore, in the case of an interior mean-motion resonant configuration, the above-mentioned expression is easily averaged with respect to the synodic period, yielding once again a global expansion of (R) valid for very high eccentricities.Comparisons between these results and the numerically computed exact function are presented for various resonances and values of the eccentricity. Maximum errors are determined in each case and their origin is established. Lastly, we discuss the applicability of the present expansion to practical problems. 相似文献
15.
16.
Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (~0.1 ms?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators. 相似文献
17.
Optimal deflection of NEOs en route of collision with the Earth 总被引:1,自引:0,他引:1
Recently, a method for the n-body analysis of the velocity change required to deflect a hazardous near-Earth object (NEO) was presented by Carusi et al. [Carusi, A., Valsecchi, G.B., D'Abramo, G., Boattini A., 2002. Icarus 159, 417-422]. We extent this method in order to optimize the velocity change vector instead of its along-track magnitude. From an application of both methods to a fictitious NEO we find Carusi's parallel approach to be reasonable for phases of unperturbed two-body motion. But, for orbit phases inhering third-body perturbations, i.e., for planetary close approaches or prior to a collision, the results obtained from the new method show the radial component of deflection impulse to play a major role. We show that a fivefold greater efficiency can be achieved by a deflection impulse being non-parallel to orbital velocity. The new method is applied to two possible 99942 Apophis impact trajectories in order to provide constraints for future Apophis deflection mission analysis. 相似文献
18.
19.
20.
We present results from a new simulation code that accounts for the evolution of the reservoirs of carbon dioxide on Mars, from its early years to the present. We establish a baseline model parameter set that produces results compatible with the present (i.e., Patm?6.5 mbar with permanent CO2 ice cap) for a wide range of initial inventories. We find that the initial inventory of CO2 broadly determines the evolutionary course of the reservoirs of CO2. The reservoirs include the atmosphere, ice cap, adsorbed CO2 in the regolith, and carbonate rocks. We track the evolution of the free inventory: the atmosphere, ice cap and regolith. Simulations begin at 4.53 Gyr before present with a rapid loss of free inventory to space in the early Noachian. Models that assume a relatively small initial inventory (?5 bar) have pronounced minima in the free inventory of CO2 toward the end of the Noachian. Under baseline parameters, initial inventories below ∼4.5 bar result in a catastrophic loss of the free inventory to space. The current free inventory would be then determined by the balance between outgassing, sputtering losses and chemical weathering following the end of the late bombardment. We call these “thin” models. They generically predict small current free inventories in line with expectations of a small present CO2 ice cap. For “thick” models, with initial inventories ?5 bar, a surplus of 300-700 mbar of free CO2 remains during the late-Noachian. The histories of free inventory in time for thick models tend to converge within the last 3.5 Gyr toward a present with an ice cap plus atmospheric inventory of about 100 mbar. For thick models, the convergence is largely due to the effects of chemical weathering, which draws down higher free inventories more rapidly than the low. Thus, thick models have ?450 mbar carbonate reservoirs, while thin models have ?200 mbar. Though both thick and thin scenarios can reproduce the current atmospheric pressure, the thick models imply a relatively large current CO2 ice cap and thin models, little or none. While the sublimation of a massive cap at a high obliquity would create a climate swing of greenhouse warming for thick models, under the thin model, mean temperatures and pressures would be essentially unaffected by increases in obliquity. 相似文献