全文获取类型
收费全文 | 4891篇 |
免费 | 211篇 |
国内免费 | 52篇 |
专业分类
测绘学 | 122篇 |
大气科学 | 403篇 |
地球物理 | 1124篇 |
地质学 | 1816篇 |
海洋学 | 357篇 |
天文学 | 870篇 |
综合类 | 17篇 |
自然地理 | 445篇 |
出版年
2022年 | 28篇 |
2021年 | 64篇 |
2020年 | 67篇 |
2019年 | 93篇 |
2018年 | 145篇 |
2017年 | 149篇 |
2016年 | 153篇 |
2015年 | 133篇 |
2014年 | 185篇 |
2013年 | 259篇 |
2012年 | 206篇 |
2011年 | 267篇 |
2010年 | 209篇 |
2009年 | 265篇 |
2008年 | 214篇 |
2007年 | 191篇 |
2006年 | 176篇 |
2005年 | 174篇 |
2004年 | 184篇 |
2003年 | 161篇 |
2002年 | 162篇 |
2001年 | 81篇 |
2000年 | 89篇 |
1999年 | 73篇 |
1998年 | 90篇 |
1997年 | 69篇 |
1996年 | 65篇 |
1995年 | 61篇 |
1994年 | 55篇 |
1993年 | 63篇 |
1992年 | 46篇 |
1991年 | 39篇 |
1990年 | 40篇 |
1989年 | 41篇 |
1988年 | 47篇 |
1987年 | 50篇 |
1986年 | 47篇 |
1985年 | 57篇 |
1984年 | 67篇 |
1983年 | 51篇 |
1982年 | 57篇 |
1981年 | 47篇 |
1980年 | 47篇 |
1979年 | 46篇 |
1978年 | 43篇 |
1977年 | 37篇 |
1976年 | 31篇 |
1975年 | 32篇 |
1974年 | 33篇 |
1973年 | 31篇 |
排序方式: 共有5154条查询结果,搜索用时 15 毫秒
991.
A total of 82 images of Hyperion was returned by the Voyager spacecraft; the most detailed views have a nominal resolution of 8.7 km/line pair. Hyperion had a rotation period of about 13 days and a spin vector lying close to its orbital plane at the time of the Voyager 2 encounter in 1981. The satellite's shape is very irregular, and cannot be approximated suitably by an ellipsoid. The largest cross section (A × C) is about 370 × 225 km; the B × C cross section is approximately 280 × 225 km. Most prominent among the surface features is a 120-km-diameter crater with an estimated depth of 10 km, and a series of arcuate scarps 300 km long that have relief in excess of 5 km. The density of large craters of Hyperion is smaller than that on other small Saturnian satellites and suggests the possibility that the last significant fragmentation of Hyperion occurred near the end of or after initial heavy bombardment. Voyager photometry yields an average normal reflectance of the surface material of 0.21 in the clear filter (0.47 μm) and evidence of slight albedo mottling over the surface. The disk-integrated phase coefficient between phase angles of 22° and 82° is 0.018 mag/de; there is little indication of a strong opposition effect in Voyager data down to phase angles of 3°. Hyperion's average color is definitely redder than that of Phobe, but matches that of the dark material on the leading hemisphere of Iapetus quite well. The satellite's albedo and color are consistent with those of contaminated water ice but since no mass determinations of Hyperion exist we do not know whether the bulk composition is icy or rocky. 相似文献
992.
Thomas H. Burbine Michael J. Gaffey Jeffrey F. Bell 《Meteoritics & planetary science》1992,27(4):424-434
Abstract— Asteroids 387 Aquitania and 980 Anacostia are anomalous members of the S-class. Their reflectance spectra exhibit a strong broad absorption feature longwards of 1.5 μm and no significant feature near 1 μm. Their spectra indicate the presence of spinel, an aluminum-magnesium oxide mineral commonly present in inclusions in CV3 and CO3 meteorites. Spinel probably makes up only a small percentage of the surface assemblages of these asteroids, but its spectral effect may be enhanced by its presence in fine-grained white inclusions in immature asteroid regoliths. It is speculated that Aquitania and Anacostia represent material formed in the same nebular zone as the CV3 and CO3 chondrites but either: A) at an earlier time in the nebula when such inclusions might have been a relatively larger fraction of the nebular grain population, or B) in local regions where nebular processes (e.g., settling to the midplane) had concentrated such inclusions. The close similarity of two orbital elements (a, i) suggests that Aquitania and Anacostia may be members of a partially dispersed asteroid family produced by the early disruption of a spinel-bearing parent body. 相似文献
993.
Frank Thomas Tveter 《Celestial Mechanics and Dynamical Astronomy》1994,60(4):409-419
This paper deals with the Hamilton equations of motion and non conservative forces. The paper will show how the Hamilton formalism may be expanded so that the auxiliary equations for any problem may be found in any set of canonical variables, regardless of the nature of the forces involved. Although the expansion does not bring us closer to an analytical solution of the problem, it's simplicity makes it worth noticing.The starting point is a conservative system (for instance a satellite orbiting an oblate planet) with a known Hamiltonian (K) and canonical variables {Q, P}. This system is placed under influence of a non-conservative force (for instance drag-force). The idea is then to use, as far as possible, the same definitions used in the conservative problem, in the process of finding the auxiliary equations for the perturbed system. 相似文献
994.
995.
996.
The applicability of metropolitan-derived explanations of gender differences in worktrip lengths is examined based on a sample of 435 university employees in the nonmetropolitan town of Blacksburg, VA. Women's worktrips are found to be longer than men's in contrast to metropolitan findings. This pattern results from the interaction of spatial structures of housing and employment with women's lower incomes, gender segregation of employment, and the tendency for married women to find jobs after their residence is established. Upward trends in female labor force participation rates and in nonmetropolitan growth require continued analysis of their joint impact on women's spatial behavior. 相似文献
997.
Evaluating yearly net migration for the Federal Republic of Germany establishes the importance of demographic restructuring and government policies as explanations for the counterurbanization phenomenon. Counterurbanization, the spatial demographic deconcentration of regional population, is measured as an inverse relationship between the net migration rate and population sizes of functional urban regions. A counterurbanization pattern filtered-down from older to younger age-groups of the population, and appears related to an increasing preference for small-sized regions with natural amenities. For foreigners, a counterurbanization direction of movement can be explained by changes in immigration laws. While counterurbanization may be slowing down during the 1980s in certain developed countries, such as the United States, in the Federal Republic the phenomenon intensifies throughout the 1970–84 time period. Based on broad national demographic changes in combination with agespecific patterns of movement, this strong counterurbanization trend should continue well into the future. 相似文献
998.
H. Mavromichalaki X. Moussas J. J. Quenby J. F. Valdes-Galicia E. J. Smith B. T. Thomas 《Solar physics》1988,116(2):377-390
Pioneer 11 and 10 observations of the wave structure seen in a corotating interaction region at 2.5 AU on day 284 of 1973 and 8 days later at 5 AU reveal large-amplitude Alfvénic structures with many detailed correlations seen between their features at the two radial distances. Hodogram analysis suggests the dominance of near plane polarized, transverse Alfvénic mode fluctuations with periods between 2 min and one hour or more. Some wave evolution close to the Corotating Interaction Region (CIR) shock is noticed, but waves towards the centre of the compression seem to propagate with little damping between the spacecraft observation positions. 相似文献
999.
Joint investigations of the Middle Pliocene climate I: PRISM paleoenvironmental reconstructions 总被引:1,自引:0,他引:1
Harry Dowsett Robert Thompson John Barron Thomas Cronin Farley Fleming Scott Ishman Richard Poore Debra Willard Thomas Holtz Jr. 《Global and Planetary Change》1994,9(3-4)
The Pliocene epoch represents an important transition from a climate regime with high-frequency, low-amplitude oscillations when the Northern Hemisphere lacked substantial ice sheets, to the typical high-frequency, high-amplitude Middle to Late Pleistocene regime characterized by glacial—interglacial cycles that involve waxing and waning of major Northern Hemisphere ice sheets. Analysis of middle Pliocene (3 Ma) marine and terrestrial records throughout the Northern Hemisphere forms the basis of an integrated synoptic Pliocene paleoclimate reconstruction of the last significantly warmer than present interval in Earth history. This reconstruction, developed primarily from paleontological data, includes middle Pliocene sea level, vegetation, land—ice distribution, sea—ice distribution, and sea-surface temperature (SST), all of which contribute to our conceptual understanding of this climate system. These data indicate middle Pliocene sea level was at least 25 m higher than present, presumably due in large part to a reduction in the size of the East Antarctic Ice Sheet. Sea surface temperatures were essentially equivalent to modern temperatures in tropical regions but were significantly warmer at higher latitudes. Due to increased heat flux to high latitudes, both the Arctic and Antarctic appear to have been seasonally ice free during the middle Pliocene with greatly reduced sea ice extent relative to today during winter. Vegetation changes, while more complex, are generally consistent with marine SST changes and show increased warmth and moisture at higher latitudes during the middle Pliocene. 相似文献
1000.
The applicability of the principle of material frame-indifference to turbulence and its consequences for turbulence modelling are considered. By introducing a distinction between arbitrary time-dependent orthogonal transformations of (i) the fluid motion, and (ii) the observer, we find that frame-invariant tensorial measures of the absolute angular velocity and acceleration can be defined. This follows from consideration of a four-dimensional formulation of classical mechanics which formally possesses the property of invariance under arbitrary changes of frame of the observer. These tensors allow the construction of constitutive equations which depend, for example, on the absolute angular velocity of the fluid, and which also satisfy the principle of material frame-indifference. The results of this analysis have application outside the field of turbulence modelling. The invariance properties of some previously published Reynolds stress transport turbulence models are examined and modifications are suggested. 相似文献