全文获取类型
收费全文 | 43236篇 |
免费 | 3755篇 |
国内免费 | 4446篇 |
专业分类
测绘学 | 1630篇 |
大气科学 | 5607篇 |
地球物理 | 10517篇 |
地质学 | 17488篇 |
海洋学 | 4581篇 |
天文学 | 6104篇 |
综合类 | 1775篇 |
自然地理 | 3735篇 |
出版年
2023年 | 383篇 |
2022年 | 836篇 |
2021年 | 1034篇 |
2020年 | 881篇 |
2019年 | 921篇 |
2018年 | 1201篇 |
2017年 | 1134篇 |
2016年 | 1401篇 |
2015年 | 1113篇 |
2014年 | 1417篇 |
2013年 | 2249篇 |
2012年 | 1443篇 |
2011年 | 1768篇 |
2010年 | 1695篇 |
2009年 | 1869篇 |
2008年 | 1647篇 |
2007年 | 1664篇 |
2006年 | 1465篇 |
2005年 | 1338篇 |
2004年 | 1199篇 |
2003年 | 1221篇 |
2002年 | 1207篇 |
2001年 | 1073篇 |
2000年 | 1208篇 |
1999年 | 1474篇 |
1998年 | 1296篇 |
1997年 | 1318篇 |
1996年 | 1185篇 |
1995年 | 1101篇 |
1994年 | 1007篇 |
1993年 | 896篇 |
1992年 | 787篇 |
1991年 | 614篇 |
1990年 | 609篇 |
1989年 | 484篇 |
1988年 | 518篇 |
1987年 | 477篇 |
1986年 | 436篇 |
1985年 | 548篇 |
1984年 | 581篇 |
1983年 | 585篇 |
1982年 | 476篇 |
1981年 | 458篇 |
1980年 | 482篇 |
1979年 | 416篇 |
1978年 | 410篇 |
1977年 | 355篇 |
1976年 | 382篇 |
1974年 | 389篇 |
1973年 | 365篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Expressions for the mass quadrupole moment tensor in the model for a wide variety of astrophysical objects are shown to be identical in form. This makes it possible to obtain analytical expressions for the gravitational radiation emitted by the sources, as well as the angular distribution, polarization dependence, and the wave forms of the radiation. 相似文献
142.
The reflectance coefficient of the regolith layer of celestial bodies has been studied in relation to the physical properties of regolith particles (size, refractive index, and packing density) on the basis of an accurate numerical radiative-transfer algorithm for a semi-infinite flat layer. Using the geometric-optics approximation, we have found that a shape mixture of randomly oriented spheroids can successfully model the single-scattering phase function of independent soil grains. In order to take into account the effect of packing density in a regolith layer, the concept of the so-called static structure factor was used. The main effect of increasing packing density is to suppress the forward-scattering peak of the phase function and to increase the albedo of the reflecting surface. We also investigated the influence of fine dust on the reflected light. An addition of small particles not only increases the surface albedo, but also changes the brightness profile and enhances the backscattering. Although the problem of unique solution, which is inherent in the retrieval of the properties of a medium from the measurements of the intensity of light scattered by this media, cannot be removed in the proposed model, the procedure used here, in contrast to widely used approximations, allows us to fit observational data with a set of real characteristics of the regolith. Semiempirical approaches are able to fit the measurements well with a small number of free parameters, but they do not explicitly contain crucial physical characteristics of the regolith such as grain sizes or the refractive index. We compared the numerical solution of the radiative-transfer equation with the Hapke approximation, which is most often used by investigators. The errors introduced by the Hapke model are small only for near-isotropic scattering by isolated particles. However, independent regolith grains are known to scatter light mainly in the forward direction. 相似文献
143.
Da-Ming Chen 《中国天文和天体物理学报》2001,1(4):287-290
1 INTRODUCTIONObservations of the most famous starburst galaxy M82 with the High-Resolution Cameraon board the Chandra X-Ray Observatory, showed that there are nine sources in the celltral11 x 11 region, but no son-roe was detected at the galactic center (Matsumoto et al. 2001).Comparing the observations on 1999 October 28 and those on 2000 January 20, the authorsfound an extremely large time variability of the source CXO M82 J095550.2 694047, which islocated gll away from the galact… 相似文献
144.
低频扩频中伪随机序列产生方法的研究 总被引:2,自引:0,他引:2
伪随机序列的产生是实现BPC低频时码发播台扩展频谱技术升级的一个重要环节。针对BPC低频时码发播台的现有系统,提出了伪随机序列的产生方案,并对其性能作了分析。 相似文献
145.
CHRIS W. LEE 《Geology Today》1991,7(4):133-136
Each year many hundreds, if not thousands, of Earth-science students visit the coastline at Ogmore-by-Sea, mid-Glamorgan. They study the excellent exposures of Lower Carboniferous Limestone, with their well-preserved coral and other shelly faunas, together with the presumed late Triassic 'breccias'* that rest unconformably on top. A study of the cements within the breccias may give an insight into syndepositional mineralization and the formation of sedimentary basins linked to the opening of the Atlantic in early Mesozoic times. 相似文献
146.
147.
Jeremy Allington-Smith Graham Murray Robert Content George Dodsworth Bryan W. Miller James Turner Inger Jorgensen Isobel Hook 《Experimental Astronomy》2002,13(1):1-37
We describe the integral field unit (IFU) which converts the Gemini Multiobject Spectrograph (GMOS) installed on the Gemini-North telescope to an integral field spectrograph,which produces spectra over a contiguous field of view of 7 × 5 arcsec with spatial sampling of 0.2 arcsecover the wavelength range 0.4-1.0 μm.GMOS is converted to this mode by the remote insertion of the IFU into thebeam in place of the masks used for the multiobject mode. A separate fieldof half the area of the main field, but otherwise identical, is alsoprovided to improve background subtraction. The IFU contains 1500lenslet-coupled fibres and was the first facility of any type for integralfield spectroscopy employed on an 8/10 m telescope.We describe the design, construction and testing of the GMOS IFU and present measurements of the throughput both in the laboratory and at the telescope. We compare these with a theoretical prediction made before construction started. All are in good agreement with each other, with the on-telescope throughput exceeding 60% (averaged over wavelength). Finallywe show an example of data obtained during commissioning to illustrate the power of the device. 相似文献
148.
W.J. Boulton 《Planetary and Space Science》1985,33(6):721-734
Orbital parameters of the satellite NIMBUS 1 ROCKET (1964-52B) have been determined at 285 epochs using about 14,500 optical and radar observations during the last 2233 days of its life. 1964-52B was in an orbit inclined at about 98.7° to the Equator and it decayed on 13 August 1974. The orbital elements were found using the RAE orbit refinement program PROP 6 giving average orbital accuracies in perigee height and inclination at 110 m and 0.002° respectively. 相似文献
149.
150.
A new updated version of the MSK macroseismic intensity scale has been prepared by a Working Group of the European Seismological Commission and has been published in April 1993 (European Macroseismic Scale 1992: updated MSK scale, 1993, ed. by G. Grünthal, Cahiers du Centre Européen de Geodynamique et de Séismologie, no. 7). 相似文献