全文获取类型
收费全文 | 45814篇 |
免费 | 3918篇 |
国内免费 | 5856篇 |
专业分类
测绘学 | 3264篇 |
大气科学 | 4769篇 |
地球物理 | 8943篇 |
地质学 | 20647篇 |
海洋学 | 4976篇 |
天文学 | 7652篇 |
综合类 | 1170篇 |
自然地理 | 4167篇 |
出版年
2023年 | 369篇 |
2022年 | 1005篇 |
2021年 | 1189篇 |
2020年 | 1163篇 |
2019年 | 1356篇 |
2018年 | 1602篇 |
2017年 | 1453篇 |
2016年 | 1604篇 |
2015年 | 1594篇 |
2014年 | 1750篇 |
2013年 | 2721篇 |
2012年 | 2189篇 |
2011年 | 2606篇 |
2010年 | 2316篇 |
2009年 | 2769篇 |
2008年 | 2498篇 |
2007年 | 2424篇 |
2006年 | 2393篇 |
2005年 | 1965篇 |
2004年 | 1692篇 |
2003年 | 1526篇 |
2002年 | 1489篇 |
2001年 | 1404篇 |
2000年 | 1192篇 |
1999年 | 907篇 |
1998年 | 762篇 |
1997年 | 747篇 |
1996年 | 593篇 |
1995年 | 550篇 |
1994年 | 499篇 |
1993年 | 429篇 |
1992年 | 397篇 |
1991年 | 369篇 |
1990年 | 395篇 |
1989年 | 360篇 |
1988年 | 293篇 |
1987年 | 413篇 |
1986年 | 320篇 |
1985年 | 415篇 |
1984年 | 452篇 |
1983年 | 445篇 |
1982年 | 404篇 |
1981年 | 374篇 |
1980年 | 358篇 |
1979年 | 326篇 |
1978年 | 335篇 |
1977年 | 298篇 |
1976年 | 302篇 |
1975年 | 297篇 |
1973年 | 277篇 |
排序方式: 共有10000条查询结果,搜索用时 859 毫秒
21.
22.
The UBV photometric observations of RT Per, from Sanwal and Chaubey (1981), were analyzed by the Wilson and Devinney code (1971). The light curves include reflection effects that for the first time has been suggested by Dugan (1911). RT Per has a semi-detached configuration where the lower-mass component is in contact with its respective Roche surface. The higher-mass component very nearly fills its Roche lobe. It has the characteristic of an Algol type system. The absolute dimensions for the primary and secondary of this system were calculated from its spectral types and by combining the photometric solution with inferred component radial velocities (Lu, 1990). 相似文献
23.
24.
为了便于大型软件系统的后续开发和用户软件系统的升级,介绍了软件系统在Internet/Intranet上更新功能或数据的原理和方法,并用C#.Net进行了编程实现。 相似文献
25.
W. H. Jefferys G. F. Benedict P. D. Hemenway P. J. Shelus R. L. Duncombe 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):299-305
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed. 相似文献
26.
27.
Electron impact excitation rates for transitions in the S v ion, calculated with theR-matrix code, are used to derive the electron temperature sensitive emission line ratiosR
1 =I(854.8 Å)/I(786.9 Å),R
2 =I(852.2 Å)/I(786.9 Å),R
3 =I(849.2 Å)/I(786.9 Å), andR
4 =I(1199.1 Å)/I(786.9 Å), which are found to be significantly different from previous estimates. A comparison of the present results with observational data for a sunspot obtained with the Harvard S-055 spectrometer on boardSkylab reveals generally good agreement between theory and experiment, except in the case ofR
1, which is probably due to blending in the 854.8 Å feature. The possible effects of Lyman continuum absorption on the observed line ratios is briefly discussed. 相似文献
28.
M.S. Hanner E. Tedesco A.T. Tokunaga G.J. Veeder D.F. Lester F.C. Witteborn J.D. Bregman J. Gradie L. Lebofsky 《Icarus》1985,64(1):11-19
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ~2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ~0.04 at 1.25 μm and ~0.05 at 2.2 μm. 相似文献
29.
介绍了MATLAB语言特点和系统建模方法的基本理论.根据南海气象数据的实际建模处理过程,给出了建模的详细步骤及其MATLAB实现过程以及MATLABTM的主要实现程序.试验讨论和结果表明利用MATLAB语言可以方便地对南海气象数据用系统建模方法进行建模和处理,MATLAB在运用系统建模法处理南海气象数据方面具有明显的优越性. 相似文献
30.
Y.-B. WU S. GAO H.-J. GONG H. XIANG W.-F. JIAO S.-H. YANG Y.-S. LIU H.-L. YUAN 《Journal of Metamorphic Geology》2009,27(6):461-477
U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and 176 Lu/177 Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean 207 Pb/206 Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ LuN ≥2213; 14.7 ≤ LuN /SmN ≤ 354) and 176 Lu/177 Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean 207 Pb/206 Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T DM2 ) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust. 相似文献