全文获取类型
收费全文 | 24299篇 |
免费 | 4570篇 |
国内免费 | 5689篇 |
专业分类
测绘学 | 2763篇 |
大气科学 | 2545篇 |
地球物理 | 6648篇 |
地质学 | 13060篇 |
海洋学 | 3352篇 |
天文学 | 1968篇 |
综合类 | 1643篇 |
自然地理 | 2579篇 |
出版年
2024年 | 48篇 |
2023年 | 256篇 |
2022年 | 703篇 |
2021年 | 871篇 |
2020年 | 909篇 |
2019年 | 1090篇 |
2018年 | 889篇 |
2017年 | 1049篇 |
2016年 | 1084篇 |
2015年 | 1254篇 |
2014年 | 1577篇 |
2013年 | 1546篇 |
2012年 | 1631篇 |
2011年 | 1692篇 |
2010年 | 1400篇 |
2009年 | 1666篇 |
2008年 | 1648篇 |
2007年 | 1787篇 |
2006年 | 1758篇 |
2005年 | 1503篇 |
2004年 | 1446篇 |
2003年 | 1219篇 |
2002年 | 1008篇 |
2001年 | 899篇 |
2000年 | 889篇 |
1999年 | 814篇 |
1998年 | 714篇 |
1997年 | 572篇 |
1996年 | 451篇 |
1995年 | 401篇 |
1994年 | 379篇 |
1993年 | 313篇 |
1992年 | 259篇 |
1991年 | 173篇 |
1990年 | 139篇 |
1989年 | 159篇 |
1988年 | 117篇 |
1987年 | 68篇 |
1986年 | 48篇 |
1985年 | 38篇 |
1984年 | 17篇 |
1983年 | 10篇 |
1982年 | 15篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1976年 | 1篇 |
1972年 | 1篇 |
1954年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
介绍了自动高精度测距系统。该系统能以5×10-7的测距精度测定多个观测点的形变,可作为跨断层动态连续观测的新仪器。 相似文献
82.
M. V. Sazhin V. E. Zharov T. A. Kalinina 《Monthly notices of the Royal Astronomical Society》2001,323(4):952-964
Parallax measurements allow distances to celestial objects to be determined. Together with measurements of their position on the celestial sphere, they give a full three-dimensional picture of the location of the objects relative to the observer. The distortion of the parallax value of a distant source affected by weak microlensing is considered. This means that the weak microlensing leads to distortion of the distance scale. The gravitational deflection causes a retrograde apparent motion of the image, which is revealed as a negative parallax. It is shown that the distortions may significantly change the parallax values when they amount to several microseconds of arc. In particular, at this level many measured values of parallaxes should be negative. 相似文献
83.
成都及附近地区旅游气候资源研究 总被引:9,自引:0,他引:9
通过对成都及附近等7个地区多年气候资源及人体舒适指数进行分级比较,得出了7个地区的人体舒适指数的时间分布特征及对当地旅游的影响。 相似文献
84.
Numerical methods are usually used for the computation of ephemerides with perturbations for the precise orbital determination of an artificial satellite. But their numerical stability will be encountered in a long arc. In this case the use the improved Encke special perturbation methods has been suggested. The results of this paper show that Encke's method does indeed have a certain effectiveness, but cannot yet completely resolve the numerical stability, and the more efficient method is to use the energy integral or its variational relation to control the growth of the along-track error in general numerical calculations so that the aim of stabilization can be achieved. 相似文献
85.
R.G. Sharp R.G. McMahon M.J. Irwin S.T. Hodgkin 《Monthly notices of the Royal Astronomical Society》2001,326(4):L45-L49
We report the first results of an observational programme designed to determine the luminosity density of high-redshift quasars quasars) using deep multicolour CCD data. We report the discovery and spectra of three high-redshift quasars, including one with . At , this is the fourth highest redshift quasar currently published. Using these preliminary results we derive an estimate of the quasar space density in the redshift range of . When completed, the survey will provide a firm constraint on the contribution to the ionizing UV background in the redshift range from quasars by determining the faint-end slope of the quasar luminosity function. The survey uses imaging data taken with the 2.5-m Isaac Newton Telescope as part of the Public Isaac Newton Group Wide Field Survey (WFS). This initial sample of objects is taken from two fields of effective area ∼12.5 deg2 from the final ∼100 deg2 . 相似文献
86.
87.
在太阳活动区的物理研究中,特别是在二维动力学光谱分析中,迫切需要相应活动区的磁图资料。本文介绍了在太阳光谱仪的入射狭缝后安装一种新型偏振器进行活动区二维磁场观测的新方法。这种方法不仅能获得日面上任一点的磁场强度,且可快速获得活动区的纵向场磁图。除此之外,还可利用多条谱线的同时观测,获得有关磁力线管结构等方面的资料。 相似文献
88.
89.
This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given. 相似文献
90.
C. Tadhunter K. Wills R. Morganti T. Oosterloo R. Dickson 《Monthly notices of the Royal Astronomical Society》2001,327(1):227-232
We present new spectroscopic observations of the southern radio galaxy . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O iii ] λλ 5007, 4959, are unusually broad for a narrow-line radio galaxy , and are blueshifted by 600 km s−1 relative to the low-ionization lines such as [O ii ] λλ 3726,3729. The [O ii ] lines are also considerably narrower than the [O iii ] lines, and have a redshift consistent with that of the recently detected H i 21-cm absorption-line system. Whereas the kinematics of the [O iii ] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O ii ] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H i absorption-line system. The unusually broad [O iii ] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that is a radio source in an early stage of evolution. 相似文献