全文获取类型
收费全文 | 72142篇 |
免费 | 11585篇 |
国内免费 | 15011篇 |
专业分类
测绘学 | 7533篇 |
大气科学 | 9961篇 |
地球物理 | 13886篇 |
地质学 | 35242篇 |
海洋学 | 9039篇 |
天文学 | 4619篇 |
综合类 | 5074篇 |
自然地理 | 13384篇 |
出版年
2024年 | 280篇 |
2023年 | 812篇 |
2022年 | 2222篇 |
2021年 | 2694篇 |
2020年 | 2709篇 |
2019年 | 3427篇 |
2018年 | 2483篇 |
2017年 | 2982篇 |
2016年 | 3052篇 |
2015年 | 3308篇 |
2014年 | 4203篇 |
2013年 | 4422篇 |
2012年 | 4393篇 |
2011年 | 4665篇 |
2010年 | 3860篇 |
2009年 | 4857篇 |
2008年 | 4718篇 |
2007年 | 5065篇 |
2006年 | 4966篇 |
2005年 | 4432篇 |
2004年 | 3964篇 |
2003年 | 3658篇 |
2002年 | 3078篇 |
2001年 | 2637篇 |
2000年 | 2501篇 |
1999年 | 2257篇 |
1998年 | 2012篇 |
1997年 | 1595篇 |
1996年 | 1345篇 |
1995年 | 1173篇 |
1994年 | 1024篇 |
1993年 | 917篇 |
1992年 | 682篇 |
1991年 | 556篇 |
1990年 | 389篇 |
1989年 | 384篇 |
1988年 | 260篇 |
1987年 | 156篇 |
1986年 | 113篇 |
1985年 | 95篇 |
1984年 | 52篇 |
1983年 | 35篇 |
1982年 | 39篇 |
1981年 | 29篇 |
1980年 | 30篇 |
1979年 | 25篇 |
1978年 | 32篇 |
1977年 | 29篇 |
1976年 | 30篇 |
1973年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
152.
Jacques Laskar 《Celestial Mechanics and Dynamical Astronomy》1993,56(1-2):191-196
Frequency analysis is a new method for analyzing the stability of orbits in a conservative dynamical system. It was first devised in order to study the stability of the solar system (Laskar, Icarus, 88, 1990). It is a powerful method for analyzing weakly chaotic motion in hamiltonian systems or symplectic maps. For regular motions, it yields an analytical representation of the solutions. In cases of 2 degrees of freedom system with monotonous torsion, precise numerical criterions for the destruction of KAM tori can be found. For a 4D symplectic map, plotting the frequency map in the frequency plane provides a clear representation of the global dynamics and describes the actual Arnold web of the system. 相似文献
153.
154.
155.
L. I. Matveenko 《Astronomy Letters》2001,27(11):686-690
The core (injector) and the jet (relativistic plasma outflow) of AGN objects are surrounded by an ionized medium, an H II region observed in emission lines. The synchrotron radiation from the core and the jet is observed through a thin screen that cocoons the structure under consideration. The screen transparency depends on wavelength and distance from the injector. We consider the objects 3C 345 and 1803+784 whose core emission at decimeter wavelengths is absorption by more than 25 dB. The visible bright compact component is the bright nearby portion of the jet that extends outside the dense part of the screen. We explore the possibility of measuring the screen transparency from absorption in Hα recombination lines with different quantum numbers at centimeter wavelengths. 相似文献
156.
157.
G. A. Krasinsky 《Celestial Mechanics and Dynamical Astronomy》2002,84(1):27-55
Differential equations describing the tidal evolution of the earth's rotation and of the lunar orbital motion are presented in a simple close form. The equations differ in form for orbits fixed to the terrestrial equator and for orbits with the nodes precessing along the ecliptic due to solar perturbations. Analytical considerations show that if the contemporary lunar orbit were equatorial the evolution would develop from an unstable geosynchronous orbit of the period about 4.42 h (in the past) to a stable geosynchronous orbit of the period about 44.8 days (in the future). It is also demonstrated that at the contemporary epoch the orbital plane of the fictitious equatorial moon would be unstable in the Liapunov's sense, being asymptotically stable at early stages of the evolution. Evolution of the currently near-ecliptical lunar orbit and of the terrestrial rotation is traced backward in time by numerical integration of the evolutional equations. It is confirmed that about 1.8 billion years ago a critical phase of the evolution took place when the equatorial inclination of the moon reached small values and the moon was in a near vicinity of the earth. Before the critical epoch t
cr
two types of the evolution are possible, which at present cannot be unambiguously distinguished with the help of the purely dynamical considerations. In the scenario that seems to be the most realistic from the physical point of view, the evolution also has started from a geosynchronous equatorial lunar orbit of the period 4.19 h. At t < t
cr
the lunar orbit has been fixed to the precessing terrestrial equator by strong perturbations from the earth's flattening and by tidal effects; at the critical epoch the solar perturbations begin to dominate and transfer the moon to its contemporary near-ecliptical orbit which evolves now to the stable geosynchronous state. Probably this scenario is in favour of the Darwin's hypothesis about originating the moon by its separation from the earth. Too much short time scale of the evolution in this model might be enlarged if the dissipative Q factor had somewhat larger values in the past than in the present epoch. Values of the length of day and the length of month, estimated from paleontological data, are confronted with the results of the developed model. 相似文献
158.
159.
张赤军 《大地测量与地球动力学》2002,22(4):31-34
介绍当今确定天文大地垂偏差的新仪器-CCD自动天体测量仪和确定两点间高程异常差的新方法,该方法用单极坐标代替繁琐的双极坐标进行计算,讨论了天文重力水准的误差及垂线偏差非线性影响等问题,估计在不久的将来,用这一手段施测山区似大地水准面的精度可望达到厘米级。 相似文献
160.
案例推理的地学应用背景和方法 总被引:3,自引:1,他引:2
案例推理 (CBR)研究 ,是人类认知的一种合理性模型 ,接近于人类认识、解决问题最原始的思维方式 ,具有在无法获取机理模型、确定规则或统计模型时 ,采用简单的历史相似性推理实现问题的定量求解和预测 ,成为人工智能领域近年来的研究热点。 相似文献