全文获取类型
收费全文 | 104518篇 |
免费 | 1425篇 |
国内免费 | 711篇 |
专业分类
测绘学 | 2329篇 |
大气科学 | 7065篇 |
地球物理 | 20227篇 |
地质学 | 37665篇 |
海洋学 | 9445篇 |
天文学 | 23663篇 |
综合类 | 258篇 |
自然地理 | 6002篇 |
出版年
2022年 | 686篇 |
2021年 | 1189篇 |
2020年 | 1267篇 |
2019年 | 1389篇 |
2018年 | 2916篇 |
2017年 | 2738篇 |
2016年 | 3202篇 |
2015年 | 1644篇 |
2014年 | 3110篇 |
2013年 | 5487篇 |
2012年 | 3384篇 |
2011年 | 4412篇 |
2010年 | 3982篇 |
2009年 | 5038篇 |
2008年 | 4391篇 |
2007年 | 4491篇 |
2006年 | 4211篇 |
2005年 | 3042篇 |
2004年 | 2968篇 |
2003年 | 2765篇 |
2002年 | 2748篇 |
2001年 | 2446篇 |
2000年 | 2374篇 |
1999年 | 1913篇 |
1998年 | 1944篇 |
1997年 | 1862篇 |
1996年 | 1578篇 |
1995年 | 1570篇 |
1994年 | 1367篇 |
1993年 | 1262篇 |
1992年 | 1211篇 |
1991年 | 1218篇 |
1990年 | 1220篇 |
1989年 | 1068篇 |
1988年 | 1019篇 |
1987年 | 1161篇 |
1986年 | 1013篇 |
1985年 | 1264篇 |
1984年 | 1378篇 |
1983年 | 1369篇 |
1982年 | 1270篇 |
1981年 | 1168篇 |
1980年 | 1098篇 |
1979年 | 1015篇 |
1978年 | 964篇 |
1977年 | 844篇 |
1976年 | 841篇 |
1975年 | 816篇 |
1974年 | 804篇 |
1973年 | 853篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
811.
812.
The phenomenon of rows—straight features in the spiral patterns of galaxies, which was discovered by Vorontsov-Velyaminov, is investigated. The rows are not artifacts; in several cases, they outline regular spiral arms almost over their entire lengths. The galaxies M 101, M 51, and a number of more distant spirals are used as examples to demonstrate major geometrical and physical properties of these structures. It is shown that the row lengths increase nearly linearly with distance from the disk center, and that the angle between adjacent rows is almost always close to 2π/3. The galaxies with rows generally belong to moderate-luminosity Sbc-Sc systems with low rotational velocities, regular spiral patterns (Grand Design), and an H I content normal for these types of galaxies. Two types of rows are shown to exist, which differ in thickness and appear to be evolutionarily related. The formation mechanism of the rows should probably be sought in the peculiar behavior of the gas-compression wave in spiral density waves. 相似文献
813.
Results of lithostratigraphic and mineral magnetic analysis of two surficial sediment cores (21 cm and 45 cm in length) collected from the Southern basin of Lake Baikal at a water depth of 1390 m, are presented. The sediments have been measured for a wide range of mineral magnetic parameters in order to assess their value in the identification of turbidite layers. Particle size and geochemical data are also presented and these explain some of the down core variations in magnetic mineralogy. It is suggested that changes in the particle size frequency distributions down core may be related to fossil diatom shells. One of the cores has been dated using 210Pb. The sediment cores were cross-correlated using low frequency magnetic susceptibility (f) and these cores can also be correlated with a nearby core collected earlier in 1992. Changes in the magnetic parameters of lf, IRMs and HIRM210 suggest that there are significant changes in the concentration of ferrimagnetic minerals in the sediment cores, indicating changing sediment sources and/or increasing concentrations of spheroidal carbonaceous particles and the dissolution of minerals through reduction below the oxidised layer within the sediment core. 相似文献
814.
An upper limit has been calculated on the effective aperture separation or detector thickness of ion drift meters of two fundamental types. The limit applies to meters which compare currents collected by detectors with different view directions at the same retarding potential and to meters which measure the entire thermal ion distribution function. For both types, a single scanned detector may be employed instead of multiple detectors, in which case the limit applies to twice the radius of curvature of the path followed by the detector aperture during a scan, (i.e. the diameter of a spinning payload). The limit was found to be important in two cases. First, in the F region on spacecraft with stringent electrostatic cleanliness requirements, the 10% error limit was found to be 40 cm. Second, in the E region, the limit was found to be 1 cm.Originally submitted to the journalSpace Science Instrumentation.Deceased. 相似文献
815.
Radio stars are identified optically with bright stars located in the direction of the cluster A3487 (RA(J) = 11h31m58s, DEC(J) = −31°11’34".8) with an accuracy determined by the refraction of radio waves in the interstellar medium in this direction
of the sky. Within an area of 1 sq. degree, 60% of the strong radio sources with P > 0.004 Jy are identified with stars brighter
than 10 m. 相似文献
816.
817.
Introduction to the solar space telescope 总被引:2,自引:0,他引:2
The design of the space solar telescope (SST) (phase B) has been completed. The manufacturing is under development. At the
end of 2000, it will be assembled. The basic aspect will be introduced in this paper. 相似文献
818.
Parallaxes with an accuracy better than 10% and proper motions from the Gaia DR1 TGAS catalogue, radial velocities from the Pulkovo Compilation of Radial Velocities (PCRV), accurate Tycho-2 photometry, theoretical PARSEC, MIST, YaPSI, BaSTI isochrones, and the most accurate reddening and interstellar extinction estimates have been used to analyze the kinematics of 9543 thin-disk B-F stars as a function of their dereddened color. The stars under consideration are located on the Hertzsprung–Russell diagram relative to the isochrones with an accuracy of a few hundredths of a magnitude, i.e., at the level of uncertainty in the parallax, photometry, reddening, extinction, and the isochrones themselves. This has allowed us to choose the most plausible reddening and extinction estimates and to conclude that the reddening and extinction were significantly underestimated in some kinematic studies of other authors. Owing to the higher accuracy of TGAS parallaxes than that of Hipparcos ones, the median accuracy of the velocity components U, V, W in this study has improved to 1.7 km s?1, although outside the range ?0.1 m < (B T ? V T )0 < 0.5 m the kinematic characteristics are noticeably biased due to the incompleteness of the sample. We have confirmed the variations in the mean velocity of stars relative to the Sun and the stellar velocity dispersion as a function of their dereddened color known from the Hipparcos data. Given the age estimates for the stars under consideration from the TRILEGAL model and the Geneva–Copenhagen survey, these variations may be considered as variations as a function of the stellar age. A comparison of our results with the results of other studies of the stellar kinematics near the Sun has shown that selection and reddening underestimation explain almost completely the discrepancies between the results. The dispersions and mean velocities from the results of reliable studies fit into a ±2 km s?1 corridor, while the ratios σ V /σ U and σ W /σ U fit into ±0.05. Based on all reliable studies in the range ?0.1 m < (B T ? V T )0 < 0.5m, i.e., for an age from 0.23 to 2.4 Gyr, we have found: W⊙ = 7.15 km s?1, \({\sigma _U} = 16.0{e^{1.29({B_T} - {V_T})o}}\), \({\sigma _V} = 10.9{e^{1.11({B_T} - {V_T})o}}\), \({\sigma _W} = 6.8{e^{1.46({B_T} - {V_T})o}}\), the stellar velocity dispersions in km s?1 are proportional to the age in Gyr raised to the power β U = 0.33, β V = 0.285, and β W = 0.37. 相似文献
819.
M. G. Revnivtsev A. Kniazev D. I. Karasev L. Berdnikov S. Barway 《Astronomy Letters》2013,39(8):523-531
Increasing the identification completeness of sources from new X-ray sky surveys is a necessary condition for further works on analyzing the formation and long-term evolution of star systems in our Galaxy. Infrared observations of several sources selected from Galactic plane surveys as candidates for low-mass X-ray binaries with the IRSF telescope at the South African Astronomical Observatory are presented. The infrared fluxes have been reliably measured from five of the eight sources (4U 1556-60, 4U 1708-40, AX J165901-4208, IGR J16287-5021, IGR J17350-2045, AX J171922-3703, SAX J1712.6-3739, 4U 1705-32). One of the objects (AX J165901-4208) may be a candidate for symbiotic X-ray binaries, i.e., binaries in which the companion of a relativistic object is a giant star. The distances have been estimated for three sources and the orbital periods have been estimated for two. 相似文献
820.