全文获取类型
收费全文 | 49693篇 |
免费 | 8841篇 |
国内免费 | 11351篇 |
专业分类
测绘学 | 4398篇 |
大气科学 | 6596篇 |
地球物理 | 11166篇 |
地质学 | 28241篇 |
海洋学 | 5630篇 |
天文学 | 2565篇 |
综合类 | 3560篇 |
自然地理 | 7729篇 |
出版年
2024年 | 164篇 |
2023年 | 471篇 |
2022年 | 1281篇 |
2021年 | 1533篇 |
2020年 | 1599篇 |
2019年 | 1955篇 |
2018年 | 1549篇 |
2017年 | 1765篇 |
2016年 | 1855篇 |
2015年 | 2106篇 |
2014年 | 2595篇 |
2013年 | 2582篇 |
2012年 | 2908篇 |
2011年 | 3151篇 |
2010年 | 2691篇 |
2009年 | 3231篇 |
2008年 | 3201篇 |
2007年 | 3588篇 |
2006年 | 3471篇 |
2005年 | 3096篇 |
2004年 | 2909篇 |
2003年 | 2806篇 |
2002年 | 2442篇 |
2001年 | 2163篇 |
2000年 | 2048篇 |
1999年 | 1880篇 |
1998年 | 1642篇 |
1997年 | 1468篇 |
1996年 | 1341篇 |
1995年 | 1144篇 |
1994年 | 1171篇 |
1993年 | 967篇 |
1992年 | 766篇 |
1991年 | 545篇 |
1990年 | 470篇 |
1989年 | 374篇 |
1988年 | 282篇 |
1987年 | 181篇 |
1986年 | 125篇 |
1985年 | 93篇 |
1984年 | 41篇 |
1983年 | 32篇 |
1982年 | 30篇 |
1981年 | 20篇 |
1980年 | 26篇 |
1979年 | 30篇 |
1978年 | 36篇 |
1977年 | 25篇 |
1975年 | 4篇 |
1954年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 67 毫秒
91.
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
92.
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
93.
J.M. Rodríguez-Ramos J.J. Fuensalida 《Monthly notices of the Royal Astronomical Society》2001,328(1):167-173
The point spread function of a segmented-mirror telescope is severely affected by segment misalignment, which can nullify the performance of adaptive optics systems. The piston and tilt of each segment must be precisely adjusted in relation to the other segments. Furthermore, the direct detection of the alignment error with natural stars would be desirable in order to monitor the errors during astronomical observation.
We have studied the lost information of the piston error caused by the presence of atmospheric turbulence in the measurements of curvature, and present a new algorithm for obtaining the local piston using the curvature sensor. A phase-wrapping effect is shown as responsible for the loss of curvature information and so the piston errors can no longer adequately be mapped; this happens not only in the presence of atmospheric turbulence, but also in its absence.
Good results are obtained using a new iterative method for obtaining the local piston error map. In the presence of atmospheric perturbation, the turbulent phase information obtained from a Shack–Hartmann sensor is introduced in our new iterative method. We propose a hybrid sensor composed of a curvature sensor and a Shack–Hartmann sensor, in order to complete all the information for the phasing. This design takes a short computation time and could be used in real time inside an adaptive optics system, where tilt and piston errors must be corrected. 相似文献
We have studied the lost information of the piston error caused by the presence of atmospheric turbulence in the measurements of curvature, and present a new algorithm for obtaining the local piston using the curvature sensor. A phase-wrapping effect is shown as responsible for the loss of curvature information and so the piston errors can no longer adequately be mapped; this happens not only in the presence of atmospheric turbulence, but also in its absence.
Good results are obtained using a new iterative method for obtaining the local piston error map. In the presence of atmospheric perturbation, the turbulent phase information obtained from a Shack–Hartmann sensor is introduced in our new iterative method. We propose a hybrid sensor composed of a curvature sensor and a Shack–Hartmann sensor, in order to complete all the information for the phasing. This design takes a short computation time and could be used in real time inside an adaptive optics system, where tilt and piston errors must be corrected. 相似文献
94.
95.
We have analysed short-exposure high-resolution images obtained on the William Herschel Telescope (WHT), Big Telescope Azimuthal (BTA) and CanadaFranceHawaii Telescope (CFHT) in order to investigate mechanical vibrations that are capable of reducing the high angular resolution allowed by speckle-interferometric methods. After filtering the photon noise we have computed power spectra of the image centres of gravity. In these spectra we have found vibration features that differ from one telescope to another and that vary in particular with the zenith angle. In the case of the WHT we discuss these results in terms of possible causes and in terms of degradation of the transfer function. We present some means to improve the data. 相似文献
96.
97.
G. M. Karataeva V. A. Yakovleva V. A. Hagen-Thorn O. V. Mikolaichuk 《Astronomy Letters》2001,27(2):74-80
We analyze our BVR c photometry for UGC 5600, a candidate polar ring galaxy, obtained with the 6-m telescope. We have confirmed the existence of an inner polar ring and show that the outer ring-shaped structure represents spiral arms; i.e., UGC 5600 belongs to the rare class of gas-rich spiral galaxies with inner polar rings. 相似文献
98.
Donald C. Gordon Peter J. Cranford Con Desplanque 《Estuarine, Coastal and Shelf Science》1985,20(2):205-227
The Cumberland Basin, a 118 km2 estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin. 相似文献
99.
100.
月日潮汐摩擦和地球惯量矩变化是日长长期变化的主要原因.在本文中,利用最新的地球物理和古生物钟数据,对过去15亿年以来的月日潮汐摩擦、地球惯量矩变化和日长长期变化等作了数值对比研究.由此得到二个重要结论:一是仅利用地球的自转形变不能解释J2的变化,这说明地球的重力分异现象至今仍存在着;其二是在几亿年前的潮汐摩擦比现在大得多,若取潮汐耗散与距离的立方成反比时,理论结果与由古生物钟得到的回归年日数和朔望月日数数据较为符合。 相似文献