全文获取类型
收费全文 | 51041篇 |
免费 | 9311篇 |
国内免费 | 11611篇 |
专业分类
测绘学 | 4167篇 |
大气科学 | 5943篇 |
地球物理 | 11822篇 |
地质学 | 28778篇 |
海洋学 | 7521篇 |
天文学 | 2404篇 |
综合类 | 3513篇 |
自然地理 | 7815篇 |
出版年
2024年 | 161篇 |
2023年 | 481篇 |
2022年 | 1279篇 |
2021年 | 1523篇 |
2020年 | 1614篇 |
2019年 | 2055篇 |
2018年 | 1586篇 |
2017年 | 1783篇 |
2016年 | 1916篇 |
2015年 | 2167篇 |
2014年 | 2620篇 |
2013年 | 2643篇 |
2012年 | 2971篇 |
2011年 | 3249篇 |
2010年 | 2736篇 |
2009年 | 3315篇 |
2008年 | 3281篇 |
2007年 | 3747篇 |
2006年 | 3603篇 |
2005年 | 3165篇 |
2004年 | 2997篇 |
2003年 | 2854篇 |
2002年 | 2518篇 |
2001年 | 2238篇 |
2000年 | 2078篇 |
1999年 | 1942篇 |
1998年 | 1696篇 |
1997年 | 1523篇 |
1996年 | 1444篇 |
1995年 | 1221篇 |
1994年 | 1228篇 |
1993年 | 1033篇 |
1992年 | 807篇 |
1991年 | 583篇 |
1990年 | 498篇 |
1989年 | 396篇 |
1988年 | 309篇 |
1987年 | 190篇 |
1986年 | 134篇 |
1985年 | 92篇 |
1984年 | 37篇 |
1983年 | 34篇 |
1982年 | 32篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 30篇 |
1978年 | 36篇 |
1977年 | 25篇 |
1975年 | 4篇 |
1954年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
81.
冰雹云雷达回波自动识别系统 总被引:2,自引:0,他引:2
利用CAPPI资料对立体风暴进行识别,计算并提供出实用的风暴结构参数,采用矩心踊跃法和矩不变量法相结合对单体风暴和混合性风暴回波进行跟踪;最小二第线性外推预报;根据WSD-88D的冰雹算法,在风暴结构基础上本文建立了Windows98操作平台上冰雹识别系统,经单站1年11次强对流天气过程的资料检验,结果表明:雹云识别精度达82%。系统建立了大量人机对话框以方便用户,增加实用性及推广性。 相似文献
82.
Barney Rickett 《Astrophysics and Space Science》2001,278(1-2):5-10
A review is given of the interplay between studies of compact radio sources and the scattering and scintillations that occur
as the signals travel through the irregular refractive index of the interstellar and interplanetary plasmas.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
83.
Polarimetric observations of the light scattered by dust have been carried out at Pic-du-Midi Observatory with the 2 m telescope
in June and September–October 1996, and at Haute-Provence Observatory with the 0.80 m telescope in April 1997. They cover
a total number of 11 nights and a large (6.9°–47.7°) phase angle range. The spatial resolution allows to underline structures
in the coma, as well in the brightness images as in the polarization maps, with a correlation between the regions of bright
structures and the regions of higher polarization. A clear difference appears between the sunward and antisunward side, with
higher polarization on the antisunward side. The phase angle coverage allows us to obtain a polarimetric phase curve for the
whole coma and to compare it with other cometary phase curves. The degree of polarization is higher for Hale-Bopp than for
the comets previously observed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
84.
The first World Atlas of the artificial night sky brightness 总被引:5,自引:0,他引:5
P. Cinzano F. Falchi C.D. Elvidge 《Monthly notices of the Royal Astronomical Society》2001,328(3):689-707
We present the first World Atlas of the zenith artificial night sky brightness at sea level. Based on radiance-calibrated high-resolution DMSP satellite data and on accurate modelling of light propagation in the atmosphere, it provides a nearly global picture of how mankind is proceeding to envelop itself in a luminous fog. Comparing the Atlas with the United States Department of Energy (DOE) population density data base, we determined the fraction of population who are living under a sky of given brightness. About two-thirds of the World population and 99 per cent of the population in the United States (excluding Alaska and Hawaii) and European Union live in areas where the night sky is above the threshold set for polluted status. Assuming average eye functionality, about one-fifth of the World population, more than two-thirds of the United States population and more than one half of the European Union population have already lost naked eye visibility of the Milky Way. Finally, about one-tenth of the World population, more than 40 per cent of the United States population and one sixth of the European Union population no longer view the heavens with the eye adapted to night vision, because of the sky brightness. 相似文献
85.
The dust shell around the evolved star HD 179821 has been detected in scattered light in near-IR imaging polarimetry observations. Here, we subtract the contribution of the unpolarized stellar light to obtain an intrinsic linear polarization of between 30 and 40 per cent in the shell that seems to increase with radial offset from the star. The J - and K -band data are modelled using a scattering code to determine the shell parameters and dust properties. We find that the observations are well described by a spherically-symmetric distribution of dust with an r −2 density law, indicating that when mass-loss was occurring, the mass-loss rate was constant. The models predict that the detached nature of a spherically-symmetric, optically-thin dust shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optically-thin circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness we derive an optical depth to the star that is consistent with V -band observations and that, assuming a distance of 6 kpc, gives an inner-shell radius of , a dust number density of at r in and a dust mass of . We have explored axisymmetric shell models but conclude that any deviations from spherical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to fit simultaneously the high linear polarizations and the small colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 either are highly elongated or consist of aggregates of smaller particles. 相似文献
86.
关于非球形粒子光散射的T-矩阵数值计算方法的研究 总被引:1,自引:0,他引:1
首先介绍了无规取向轴对称非球形粒子光散射的T-矩阵数值方法。然后,进一步研究了T-矩阵计算的收敛问题,提出了一种新的关于无规取向无吸收非线形散射物体的物理收敛方法,我们研究了椭球粒子的收敛问题,结果表明我们的数值方法和T-矩阵计算程序是有效的, 收敛精度与粒子的尺度和形状有很强的依赖性,在一定的条件下,我们的物理收敛速度优于NASA Mishchenko的数学收敛速序。 相似文献
87.
矿床谱系是对成矿多样性的理论概括,而成矿多样性又是由不同级别、不同性质的致矿地质异常决定的.本文论述了山东省内生金矿矿床谱系及其致矿地质异常. 相似文献
88.
本文分析了国外用于测定度盘分划改正的几种方法,并分析了固定角距法的优点和不足之处;详细叙述了组合固定角距法测定对径改正的原理和计算公式;并且讨论了两对显微镜之间夹角变化以及显微镜比例尺变化对对径改正的影响,给出了相应的修正方法。用固定角距法所测的对径改正精度可达±0″.009。 相似文献
89.
90.