全文获取类型
收费全文 | 131722篇 |
免费 | 2380篇 |
国内免费 | 931篇 |
专业分类
测绘学 | 3042篇 |
大气科学 | 9224篇 |
地球物理 | 26245篇 |
地质学 | 46430篇 |
海洋学 | 11909篇 |
天文学 | 29578篇 |
综合类 | 327篇 |
自然地理 | 8278篇 |
出版年
2021年 | 1274篇 |
2020年 | 1468篇 |
2019年 | 1606篇 |
2018年 | 3191篇 |
2017年 | 2993篇 |
2016年 | 3668篇 |
2015年 | 2075篇 |
2014年 | 3569篇 |
2013年 | 6815篇 |
2012年 | 3838篇 |
2011年 | 5206篇 |
2010年 | 4683篇 |
2009年 | 6152篇 |
2008年 | 5424篇 |
2007年 | 5329篇 |
2006年 | 5132篇 |
2005年 | 3929篇 |
2004年 | 3900篇 |
2003年 | 3671篇 |
2002年 | 3577篇 |
2001年 | 3136篇 |
2000年 | 3084篇 |
1999年 | 2564篇 |
1998年 | 2588篇 |
1997年 | 2436篇 |
1996年 | 2154篇 |
1995年 | 2100篇 |
1994年 | 1839篇 |
1993年 | 1739篇 |
1992年 | 1629篇 |
1991年 | 1558篇 |
1990年 | 1678篇 |
1989年 | 1482篇 |
1988年 | 1328篇 |
1987年 | 1602篇 |
1986年 | 1413篇 |
1985年 | 1728篇 |
1984年 | 2002篇 |
1983年 | 1905篇 |
1982年 | 1743篇 |
1981年 | 1648篇 |
1980年 | 1438篇 |
1979年 | 1400篇 |
1978年 | 1409篇 |
1977年 | 1263篇 |
1976年 | 1215篇 |
1975年 | 1184篇 |
1974年 | 1166篇 |
1973年 | 1217篇 |
1972年 | 747篇 |
排序方式: 共有10000条查询结果,搜索用时 976 毫秒
831.
J. Mikolajewska 《Astrophysics and Space Science》1987,131(1-2):713-723
Periodic variations of emission line intensities and radial velocities in three S-type symbiotic stars: BF Cyg, CI Cyg and AX Per are presented and discussed. The behavior of emission lines is different in these objects and suggests that significant differences in physical conditions and geometry may occur in these seemingly similar systems.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986. 相似文献
832.
Two-charged bodiesM 1 andM 2 revolve round their centre of mass in circular orbits under Newton's inverse-square law and the so similar Coulomb's law. A third-charged-bodyM, without mass and charge (i.e., such that it is attracted or repulsed byM 1 andM 2, but does not influence their motion), moves in a field with a force function, namely $$U = {\text{ }}\frac{{q - \mu }}{{r_1 }}{\text{ }} + {\text{ }}\frac{{\mu - q}}{{r_2 }}$$ , which is created byM 1 andM 2. In what follows, the existence and location of the collinear and equilateral Lagrangian points or solutions with be discussed and the interpretation of them will be given. This work is a generalization of the classical restricted circular three-body problem. 相似文献
833.
J. Schubart 《Celestial Mechanics and Dynamical Astronomy》1987,43(1-4):309-317
Selected orbits of asteroids close to the 3/2 and 4/3 resonances in the outer belt are studied by extended numerical integrations of the four-body problem Sun-Jupiter-Saturn-asteroid. Jupiter's variable eccentricity causes strong or dominant effects in the asteroidal eccentricity, rate of perihelion, and critical argument. However, a suitable transformation removes most of these effects by the transition to respective new quantities. Cases of circulation can change to permanent libration in the new critical argument. This happens in the case of (1256) Normannia, one of the two objects found to be circulating near the 3/2 resonance in former work. The other object, (334) Chicago, remains a case of circulation and shows significant deviations from quasi-periodic motion, contrary to all the other studied Hildas (3/2 cases) and to (279) Thule (4/3). Temporary libration with respect to a resonance of high order is visible in the long-period evolution of Chicago's orbit. There are cases of some analogy further inward in the belt: (903) Nealley changes to permanent libration at the 2/1 resonance by the use of the new critical argument, and (522) Helga's orbit shows a non-quasi-periodic behaviour. 相似文献
834.
835.
Rapid variations of the radial velocities of absorption components of Ti II lines in CH Cyg are presented. The periods of these variations are determined to 1.89 and 41.07 days in 1982. The variations are interpreted through oscilliations in the mass transfer from the M component onto the accretion disk of the companion during periastron passage.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986. 相似文献
836.
Theoretical rotational excitation rates were computed for C3H2 in collisions with He atoms at temperatures from 30 to 120 K. The intermolecular forces were obtained from accurate self-consistent field and perturbation theory calculations, and collision dynamics were treated within the infinite-order sudden approximation. The accuracy of the latter was examined by comparing with the more exact coupled states approximation. 相似文献
837.
A fully relativistic electron maser is proposed for the explanation of certain non-thermal solar and stellar radio bursts. This mechanism (maser synchrotron) is based on a gyroresonant interaction between waves and electrons of high energies and uses the free energy contained in an electronic distribution function that peaks for energies around 1 MeV.By a calculation of the growth rates of the three electromagnetic modes, we show that the X-mode prevails for values of
p/
cup to 2 or 3. This result is very different from the standard cyclotron maser case where such values of
p/
clead to quench the X-mode growth. Hence, the synchrotron maser instability appears to be a direct and efficient amplification process for considerably larger physical conditions than the cyclotron maser. In addition, the radiation, emitted over the second gyroharmonic, freely propagates without a strong reabsorbtion at the 2
clayer. All these points can constitute major advantages of this mechanism in an astrophysical context.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986. 相似文献
838.
A. A. Karaballi 《Celestial Mechanics and Dynamical Astronomy》1987,41(1-4):323-332
In this paper we consider the problem concerning the reduction of the two-body motion to that of a single particle in a central field. As a force function we takeU(r)=r
–, where is some positive real number. Making use of the variational equations we study the ejection solutions of the differential equations of motion.
Resumé Nous considérons dans cet article le problème concernant la réduction du mouvement de deux corps à celui d'une particule dans un champ de forces central. Comme fonction de forces nous prenonsU(r)=r –; où est un réel positif. Nous étudions à l'aide des équations aux variations les solutions d'éjection des équations du mouvement.相似文献
839.
A. H. Jupp 《Celestial Mechanics and Dynamical Astronomy》1987,43(1-4):127-138
The critical inclination problem in artificial satellite theory, first discovered 30 years ago, has aroused great interest and provoked much discussion and controversy in the intervening years. It was this problem which essentially provided the seed corn for the development of the theory of the Ideal Resonance Problem (IRP). The latter theory provides good first-approximation solutions to a number of important resonance problems in celestial mechanics. It is not applicable, however, to certain other interesting resonant systems within the solar system. For these resonances a new fundamental mathematical model of resonance, in the spirit of the IRP, has recently been formulated and successfully applied.This paper reviews the history of the critical inclination problem and highlights the controversies it has generated over the years. The Problem's strong connection with the IRP is outlined with both thenormal andabnormal forms featuring. Finally, with reference to the critical inclination problem, the essential properties of the newer fundamental model are described and compared with the IRP. A strong correspondence is established between recent independent investigations of a variety of resonance problems and earlier work of Andoyer. 相似文献
840.