全文获取类型
收费全文 | 51274篇 |
免费 | 513篇 |
国内免费 | 281篇 |
专业分类
测绘学 | 1372篇 |
大气科学 | 3809篇 |
地球物理 | 9167篇 |
地质学 | 21428篇 |
海洋学 | 3937篇 |
天文学 | 9785篇 |
综合类 | 209篇 |
自然地理 | 2361篇 |
出版年
2021年 | 269篇 |
2020年 | 297篇 |
2019年 | 297篇 |
2018年 | 3661篇 |
2017年 | 3425篇 |
2016年 | 2365篇 |
2015年 | 592篇 |
2014年 | 830篇 |
2013年 | 1560篇 |
2012年 | 1967篇 |
2011年 | 3871篇 |
2010年 | 3508篇 |
2009年 | 3858篇 |
2008年 | 3111篇 |
2007年 | 3751篇 |
2006年 | 1204篇 |
2005年 | 1311篇 |
2004年 | 1194篇 |
2003年 | 1201篇 |
2002年 | 1018篇 |
2001年 | 734篇 |
2000年 | 705篇 |
1999年 | 518篇 |
1998年 | 491篇 |
1997年 | 571篇 |
1996年 | 438篇 |
1995年 | 470篇 |
1994年 | 465篇 |
1993年 | 372篇 |
1992年 | 382篇 |
1991年 | 353篇 |
1990年 | 358篇 |
1989年 | 327篇 |
1988年 | 334篇 |
1987年 | 358篇 |
1986年 | 317篇 |
1985年 | 417篇 |
1984年 | 408篇 |
1983年 | 441篇 |
1982年 | 410篇 |
1981年 | 372篇 |
1980年 | 418篇 |
1979年 | 320篇 |
1978年 | 299篇 |
1977年 | 293篇 |
1976年 | 264篇 |
1975年 | 259篇 |
1974年 | 267篇 |
1973年 | 254篇 |
1971年 | 172篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
51.
Geometric Aspects and Testing of the Galactic Center Distance Determination from Spiral Arm Segments
We consider the problem of determining the geometric parameters of a Galactic spiral arm from its segment by including the distance to the spiral pole, i.e., the distance to the Galactic center (R0). The question about the number of points belonging to one turn of a logarithmic spiral and defining this spiral as a geometric figure has been investigated numerically and analytically by assuming the direction to the spiral pole (to the Galactic center) to be known. Based on the results obtained, in an effort to test the new approach, we have constructed a simplified method of solving the problem that consists in finding the median of the values for each parameter from all possible triplets of objects in the spiral arm segment satisfying the condition for the angular distance between objects. Applying the method to the data on the spatial distribution of masers in the Perseus and Scutum arms (the catalogue by Reid et al. (2014)) has led to an estimate of R0 = 8.8 ± 0.5 kpc. The parameters of five spiral arm segments have been determined from masers of the same catalogue. We have confirmed the difference between the spiral arms in pitch angle. The pitch angles of the arms revealed by masers are shown to generally correlate with R0 in the sense that an increase in R0 leads to a growth in the absolute values of the pitch angles. 相似文献
52.
The study of the chemical composition of stars and galaxies is a key topic for understanding their origin and evolution. In this study, we present the results of the calculation of solar abundances of the isotopes 1H, 4He, 12C, 14N, 15O, 16O, 17O, and 18O during the four phases of the solar life: hydrogen burning, onset of rapid growth and red giant, helium burning and helium exhaustion. The open source package “NucNet Tools” from the Webnucleo Group in Clemson University, SC, USA was used for this purpose. The results for all isotopes are listed in tables for future use. Abundances found, globally, agree fairly well with those predicted in the literature. Results obtained for the last two phases have no equivalents elsewhere. 相似文献
53.
Analytical study of ground motion caused by seismic wave propagation across faulted rock masses 下载免费PDF全文
Studying seismic wave propagation across rock masses and the induced ground motion is an important topic, which receives considerable attention in design and construction of underground cavern/tunnel constructions and mining activities. The current study investigates wave propagation across a rock mass with one fault and the induced ground motion using a recursive approach. The rocks beside the fault are assumed as viscoelastic media with seismic quality factors, Qp and Qs. Two kinds of interactions between stress waves and a discontinuity and between stress waves and a free surface are analyzed, respectively. As the result of the wave superposition, the mathematical expressions for induced ground vibration are deduced. The proposed approach is then compared with the existing analysis for special cases. Finally, parametric studies are carried out, which includes the influences of fault stiffness, incident angle, and frequency of incident waves on the peak particle velocities of the ground motions. 相似文献
54.
The ultrarelativistic generalized Lorentzian quasi-equilibrium thermodynamic energy distribution is tentatively applied to the energy spectrum of galactic cosmic ray fluxes. It is found that the inferred power law slopes contain a component which evolves with cosmic ray energy in steps of thirds, resembling the sequence of structure functions in fully developed Kolmogorov turbulence. Within the generalized thermodynamics the chemical potential is estimated from the deviation of the fluxes at decreasing energy, presumably throwing light on the cosmic ray acceleration mechanism. A relation between involved turbulent volumina and structure functions is obtained. The conclusions drawn hold only within the realm of applicability of thermodynamics to cosmic ray spectra. 相似文献
55.
N. C. Wickramasinghe Dayal T. Wickramasinghe Edward J. Steele 《Astrophysics and Space Science》2018,363(12):244
A growing body of evidence suggests the operation of life and life processes in comets as well in larger icy bodies in the solar system including Enceladus. Attempts to interpret such data without invoking active biology are beginning to look weak and flawed. The emerging new paradigm is that life is a cosmic phenomenon as proposed by Hoyle and Wickramasinghe (Lifecloud: the Origin of Life in the Galaxy, 1978) and first supported by astronomical spectroscopy (Wickramasinghe and Allen, Nature 287:518, 1980; Allen and Wickramasinghe, Nature 294:239, 1981; Wickramasinghe and Allen, Nature 323:44, 1986). Comets are the transporters and amplifiers of microbial life throughout the Universe and are also, according to this point of view, the carriers of viruses that contribute to the continued evolution of life. Comets brought life to Earth 4.2 billion years ago and they continue to do so. Space extrapolations of comets, Enceladus and possibly Pluto supports this point of view. Impacts of asteroids and comets on the Earth as well as on other planetary bodies leads to the ejection of life-bearing dust and rocks and a mixing of microbiota on a planetary scale and on an even wider galactic scale. It appears inevitable that the entire galaxy will be a single connected biosphere. 相似文献
56.
S. A. Naroenkov D. O. Glazachev A. P. Kartashova O. P. Popova I. S. Turuntaev 《Solar System Research》2018,52(6):534-546
Quick assessment of hazardous effects from impacts of large celestial bodies is achieved through the development of a new consequence calculator. A distinctive feature of this calculator is a new block, the Hazardous-Orbit Constructor, which simulates the conditions of entry of a celestial body into the Earth’s atmosphere and determines the orbital parameters of the body based on given atmospheric entry conditions. This block is used to simulate the atmospheric entry conditions of known asteroids and meteoroids and to determine the orbital parameters of known bolides leading to meteorite fall events. For the case of asteroid 2008 TC3 and the P?ibram meteorite, it is shown that within the potential impact area of the celestial body, the atmospheric entry angle may vary considerably. 相似文献
57.
O. M. Alifanov A. L. Medvedskiy V. V. Terentyev D. V. Lyskov 《Solar System Research》2018,52(7):590-596
The development project of a prototype demonstrator for a descent vehicle and the possibilities of descent from orbit using aero-elastic braking devices deployable in space and in the atmosphere is considered. The project was carried out jointly by the Lavochkin Scientific Production Association and the Moscow Aviation Institute teams in 2013–2015. The results are evaluated for both the project itself and the cooperation of the two organizations. 相似文献
58.
N. P. Danilkin S. V. Zhuravlev N. G. Kotonaeva V. B. Lapshin I. V. Romanov M. Y. Filippov E. N. Khotenko G. A. Zhbankov 《Solar System Research》2018,52(7):684-690
The satellite ionosondes in highly elliptical orbits are proposed to be used for the task of continuous monitoring of the Arctic ionosphere. The monitoring scheme with the ionosonde location aboard the Arktika-M satellites is presented. The calculations of the vertical topside and transionospheric sounding using the SIMP1 ionospheric model are performed, which show the feasibility of continuous monitoring. 相似文献
59.
Analytical solutions for generalizing the Ekman stationary flow of a viscous incompressible fluid in an infinite layer are obtained. The solution of an overdetermined system of the Oberbeck–Boussinesq equations is considered. It is suggested to use a class of exact solutions for this problem. It is shown that the structure of the solutions allows one to preserve the advective derivative in the heat-conductivity equation; this makes it possible to model the stratification of the temperature and pressure fields and describe the oceanic countercurrents. 相似文献
60.
A. A. Vetrov N. V. Lobus A. N. Drozdova N. A. Belyaev E. A. Romankevich 《Oceanology》2018,58(2):198-204
The methane content in water and bottom sediments was measured in vertical sections: Lena River Delta–continental slope (the Laptev Sea), Taimyr Peninsula–Voronin Trough, and along the Novaya Zemlya Archipelago. The methane concentrations varied from 2.5 to 70 nmol/L and from 590 to 2600 nmol/L in the water column and sediments, respectively. Most of the surface water samples showed oversaturation in methane (up to 19-fold, fourfold on average), which determined the water–atmosphere direction of methane fluxes, which amounted to 1–400 mol/km2 day (52 mol/km2 day on average). 相似文献