全文获取类型
收费全文 | 75125篇 |
免费 | 1293篇 |
国内免费 | 747篇 |
专业分类
测绘学 | 1865篇 |
大气科学 | 5465篇 |
地球物理 | 15312篇 |
地质学 | 26898篇 |
海洋学 | 6333篇 |
天文学 | 16756篇 |
综合类 | 288篇 |
自然地理 | 4248篇 |
出版年
2022年 | 438篇 |
2021年 | 777篇 |
2020年 | 838篇 |
2019年 | 865篇 |
2018年 | 2034篇 |
2017年 | 1895篇 |
2016年 | 2437篇 |
2015年 | 1453篇 |
2014年 | 2370篇 |
2013年 | 4021篇 |
2012年 | 2491篇 |
2011年 | 3293篇 |
2010年 | 2812篇 |
2009年 | 3713篇 |
2008年 | 3408篇 |
2007年 | 3167篇 |
2006年 | 2985篇 |
2005年 | 2454篇 |
2004年 | 2329篇 |
2003年 | 2166篇 |
2002年 | 1993篇 |
2001年 | 1863篇 |
2000年 | 1761篇 |
1999年 | 1426篇 |
1998年 | 1529篇 |
1997年 | 1430篇 |
1996年 | 1154篇 |
1995年 | 1189篇 |
1994年 | 1008篇 |
1993年 | 910篇 |
1992年 | 879篇 |
1991年 | 780篇 |
1990年 | 884篇 |
1989年 | 751篇 |
1988年 | 675篇 |
1987年 | 849篇 |
1986年 | 689篇 |
1985年 | 885篇 |
1984年 | 956篇 |
1983年 | 910篇 |
1982年 | 865篇 |
1981年 | 737篇 |
1980年 | 687篇 |
1979年 | 639篇 |
1978年 | 635篇 |
1977年 | 584篇 |
1976年 | 555篇 |
1975年 | 522篇 |
1974年 | 528篇 |
1973年 | 494篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
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
42.
M. A. Vashkov’yak 《Astronomy Letters》2001,27(7):464-469
We consider the structural peculiarities of Uranus’s satellite system associated with its separation into two groups: inner equatorial satellites moving in nearly circular orbits and distant irregular satellites with retrograde motion in highly elliptical orbits. The intermediate region is free from satellites in a wide range of semimajor axes. By analyzing the evolution of satellite orbits under the combined effect of solar attraction and Uranus’s oblateness, we offer a celestial-mechanical explanation for the absence of equatorial satellites in this region. M.L. Lidov’s studies during 1961–1963 have served as a basis for our analysis. 相似文献
43.
44.
F. M. F. El-Sabaa 《Celestial Mechanics and Dynamical Astronomy》1985,37(1):71-79
The equation of motion of a rigid body in Kovaleveskaya case is reduced to a plane motion. By using the method of small parameters introduced by Poincaré, the existence of a periodic solution is established. 相似文献
45.
Progress in the introduction of coulometry for the analysis of total carbon dioxide (TCO2) in marine waters is described. An extractor—stripper removes CO2 that is measured coulometrically by the quantity of electricity (coulombs) used to electrogenerate OH? ions for the titration of the acid formed by the reaction of CO2 and ethanolamine. The equivalence point is detected photometrically with thymolphthalein as the indicator, and Faraday's Law relates coulombs to equivalents of titrant generated and CO2 determined so that there are no standard curves needed or titrants to standardize or store. Accuracy was determined by adding gelatin capsules containing 100–1500 μg C of pure CaCO3 into the stripper, and accuracies of better than ± 1 μg C were achieved. The best precision for natural seawater (± 1 standard error) of ± 0.5 μmol l?1 was found for 17 samples of Bermuda coastal waters having a mean TCO2 of 2007.2 μmol l?1 (0.05% CV). Sources of error and precautions are discussed. This method, which has been used successfully at sea, can be used to study a variety of marine phenomena involving TCO2. 相似文献
46.
Alan M Bond Julie R Bradbury Henry A Hudson John S Garnham Peter J Hanna Stanley Strother 《Marine Chemistry》1985,16(1):1-9
Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent. 相似文献
47.
48.
J. CAMPSIE M. H. RASMUSSEN L. C. KOVACS F. DITTMER J. C. BAILEY N. O. HANSEN J. LAURSEN L. JOHNSON 《Polar research》1990,8(2):237-243
New aeromagnetic data, K-Ar age determinations of dredged marine igneous rocks, as well as other geophysical evidence have shed light on the chronology, nature and evolution of the northern Iceland Plateau. Correspondence between seismic refraction profiles taken on the Jan Mayen Ridge and westward through Jan Mayen Island, suppressed aeromagnetic anomalies, earthquake surface wave studies, and ages of dredged igneous rocks suggest these strata may form an extended region of thickened crust, possibly of Caledonian age, extending westward toward the Kolbeinsey Ridge and northwest to the south wall of the Jan Mayen Fracture Zone. 相似文献
49.
Structural crust is a thin layer formed on the soil surface after a rainstorm. The crust is the result of a physical segregation and rearrangement of soil particles in a way that affects some of the soil properties, such as infiltration, runoff and soil erosion. In practice, there is no rapid, in situ method for monitoring, assessing and mapping crust intensity and quality. In this study, a controlled spectral investigation of the structural crust across the NIR–SWIR spectral region was conducted on three selected Israeli soils, to study the potential of reflectance radiation to detect structural crust in soils. Two major factors served as the driving forces for this study: (1) there is no valid method for in situ assessment of the crust's characteristics in the agriculture field, and (2) the crust might bias thematic remote sensing of soils, because the thin layer of crust blocks photon–matter interaction, which represents the relevant soil body. Through the use of a laboratory rainfall simulator and a sensitive spectrometer, it was revealed that for three selected soils, significant spectral differences occurred between the crust and its bulk soil. The spectral information was found to be related to changes in particle size distribution and texture at the surface of the soil. This conclusion was based on indications of absorption of OH in clay lattice, OH in adsorbed water and CO3 in carbonates. It was concluded that the structural crust is a phenomenon that should not be ignored by remote-sensing users. In fact, in the field of agriculture, the spectral properties of crust can be used as tools for estimating the crust's intensity. 相似文献
50.