全文获取类型
收费全文 | 1285篇 |
免费 | 60篇 |
国内免费 | 57篇 |
专业分类
测绘学 | 124篇 |
大气科学 | 70篇 |
地球物理 | 100篇 |
地质学 | 94篇 |
海洋学 | 63篇 |
天文学 | 830篇 |
综合类 | 38篇 |
自然地理 | 83篇 |
出版年
2023年 | 10篇 |
2022年 | 11篇 |
2021年 | 20篇 |
2020年 | 20篇 |
2019年 | 24篇 |
2018年 | 9篇 |
2017年 | 23篇 |
2016年 | 23篇 |
2015年 | 19篇 |
2014年 | 29篇 |
2013年 | 35篇 |
2012年 | 25篇 |
2011年 | 35篇 |
2010年 | 21篇 |
2009年 | 96篇 |
2008年 | 76篇 |
2007年 | 95篇 |
2006年 | 97篇 |
2005年 | 97篇 |
2004年 | 107篇 |
2003年 | 93篇 |
2002年 | 44篇 |
2001年 | 70篇 |
2000年 | 58篇 |
1999年 | 58篇 |
1998年 | 71篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 22篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 4篇 |
1989年 | 14篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1980年 | 6篇 |
1954年 | 1篇 |
排序方式: 共有1402条查询结果,搜索用时 15 毫秒
101.
J. C. Pickering A. P. Thorne & J. K. Webb 《Monthly notices of the Royal Astronomical Society》1998,300(1):131-134
Strong ultraviolet resonance transitions are observed routinely both in the Galactic interstellar medium and in quasar absorption systems. The quality of the astronomical spectroscopic data now available demands more precise laboratory rest wavelengths. Of particular interest is the accuracy with which one can constrain space–time variations in fundamental constants using quasar spectra. A recent analysis by Webb et al. of 25 quasar spectra using Mg and Fe transitions tentatively suggests that the fine-structure constant was smaller at earlier epochs. To permit a check on this result, and to allow further more extensive investigations, we have carried out a new determination of the laboratory wavelengths of Mg i 2853 Å, Mg II 2796 Å and Mg II 2803 Å by high-resolution Fourier transform spectroscopy. Our results for Mg II 2796 Å are consistent with the value measured independently by two other groups. To our knowledge, no previous measurements of comparable precision exist for Mg I 2853 Å and Mg II 2803 Å. 相似文献
102.
The Upper Himenoura Subgroup exposed in the island of Amakusa-Shimojima, Kyushu, Japan shows an example of the terminal Cretaceous stratigraphic record in the circum Pacific region. This sequence is a part of the Upper Cretaceous intra-arc basins of southwest Japan. Four cycles of upward coarse-graded facies are recognized. Each cycle consists of a basinal mud facies in the lower part and a tide-dominated shallow marine to brackish coarse clastic facies in the upper part. Biostratigraphic correlation chiefly based on ammonites, inocerami and trigoniids indicates that this sequence is Campanian to Maastrichtian in age. The occurrence of the above three fossils decreases upward and is terminated at the top of the sequence, being replaced by a molluscan assemblage similar to the Danian. This suggests that the sedimentation may have continued to the very end of the Cretaceous period and possibly to the beginning of the Tertiary. 相似文献
103.
D. P. Naidin I. G. Sasonova Z. N. Pojarkova M. R. Djalilov G. N. Papulov Yu. Senkovsky V. N. Benjamovsky L. F. Kopaevich 《Cretaceous Research》1980,1(4):375-387
This paper is a brief explanation of the diagrams of the Cretaceous transgressions and regressions on the Russian Platform, in the Crimea (Figures 1–3) and some regions of central Asia—the western flanks of the Tien Shan mountains, the Fergana basin, the Zeravshan-Gissar and Alaj mountains, and the Northern Pamirs (Figures 5–7).Internationally recognized stages are employed. They are interpreted by Sasonova (1967) for the Lower Cretaceous (K1) of the Russian Platform, by Naidin (1977) for the Upper Cretaceous (K2) of the Platform and the Crimea, by Djalilov (1971) and Pojarkova (1976) for the Upper Cretaceous of central Asia. General data on the stratigraphy of the Cretaceous of central Asia may be found in Anon 1977. 相似文献
104.
我们利用南京大学太阳塔中的多波段光谱仪,在H_α、H_β和CaⅡ H、K三个波段同时拍到了1982年12月2日日面S15W11处的一个SB级耀斑的光谱。本文给出其中七个时刻的谱线轮廓及有关参数的序列。在非局部热动平衡条件下计算了耀斑随时间变化的半经验模型,结果显示了色球耀斑的演化过程。利用模型得到了一些色球物质蒸发参数,结果同根据SMM的X射线观测所作的估计相一致。 相似文献
105.
K. R. Anantharamaiah 《Journal of Astrophysics and Astronomy》1986,7(3):131-139
Radio recombination lines are known to be observable at positions along the galactic ridge which are free of discrete continuum
sources. Based on the results of a recent survey of H272α lines it is shown that most of the observed galactic ridge recombination
lines can be explained as emission from outer low-density envelopes of normal Hn regions. The distribution of low-density
ionized gas and discrete HII regions as a function of the distance from the galactic centre is also derived. 相似文献
106.
107.
J. F. Palacián 《Celestial Mechanics and Dynamical Astronomy》2007,98(4):219-249
We study the dynamics of a satellite (artificial or natural) orbiting an Earth-like planet at low altitude from an analytical
point of view. The perturbation considered takes into account the gravity attraction of the planet and in particular it is
caused by its inhomogeneous potential. We begin by truncating the equations of motion at second order, that is, incorporating
the zonal and the tesseral harmonics up to order two. The system is formulated as an autonomous Hamiltonian and has three
degrees of freedom. After three successive Lie transformations, the system is normalised with respect to two angular co-ordinates
up to order five in a suitable small parameter given by the quotient between the angular velocity of the planet and the mean
motion of the satellite. Our treatment is free of power expansions of the eccentricity and of truncated Fourier series in
the anomalies. Once these transformations are performed, the truncated Hamiltonian defines a system of one degree of freedom
which is rewritten as a function of two variables which generate a phase space which takes into account all of the symmetries
of the problem. Next an analysis of the system is achieved obtaining up to six relative equilibria and three types of bifurcations.
The connection with the original system is established concluding the existence of various families of invariant 3-tori of
it, as well as quasiperiodic and periodic trajectories. This is achieved by using KAM theory techniques. 相似文献
108.
109.
利用现场可编程门阵列(FPGA)的内部资源,在FPGA中实现了高精度时间间隔测量系统的设计。 相似文献
110.