全文获取类型
收费全文 | 21322篇 |
免费 | 3726篇 |
国内免费 | 5303篇 |
专业分类
测绘学 | 436篇 |
大气科学 | 888篇 |
地球物理 | 4473篇 |
地质学 | 14652篇 |
海洋学 | 2176篇 |
天文学 | 5094篇 |
综合类 | 1128篇 |
自然地理 | 1504篇 |
出版年
2024年 | 56篇 |
2023年 | 219篇 |
2022年 | 560篇 |
2021年 | 661篇 |
2020年 | 721篇 |
2019年 | 874篇 |
2018年 | 786篇 |
2017年 | 695篇 |
2016年 | 860篇 |
2015年 | 894篇 |
2014年 | 1263篇 |
2013年 | 1318篇 |
2012年 | 1366篇 |
2011年 | 1427篇 |
2010年 | 1410篇 |
2009年 | 1809篇 |
2008年 | 1707篇 |
2007年 | 1666篇 |
2006年 | 1596篇 |
2005年 | 1361篇 |
2004年 | 1255篇 |
2003年 | 1094篇 |
2002年 | 900篇 |
2001年 | 836篇 |
2000年 | 825篇 |
1999年 | 716篇 |
1998年 | 602篇 |
1997年 | 444篇 |
1996年 | 399篇 |
1995年 | 385篇 |
1994年 | 311篇 |
1993年 | 317篇 |
1992年 | 202篇 |
1991年 | 165篇 |
1990年 | 144篇 |
1989年 | 126篇 |
1988年 | 90篇 |
1987年 | 68篇 |
1986年 | 47篇 |
1985年 | 35篇 |
1984年 | 34篇 |
1983年 | 20篇 |
1982年 | 16篇 |
1981年 | 7篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 7篇 |
1977年 | 17篇 |
1954年 | 7篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
121.
122.
123.
124.
125.
126.
127.
Ignacio Ferreras Alessandro Melchiorri Domenico Tocchini-Valentini 《Monthly notices of the Royal Astronomical Society》2003,344(1):257-261
The age of the Universe has been increasingly constrained by different techniques, such as the observations of type Ia supernovae (SNIa) at high redshift or dating the stellar populations of globular clusters. In this paper, we present a complementary approach using the colours of the brightest elliptical galaxies in clusters over a wide redshift range ( z ≲ 1) . We put new and independent bounds on the dark energy equation of state parametrized by a constant pressure-to-density ratio w Q and by a parameter (ξ) which determines the scaling between the matter and dark energy densities. We find that accurate estimates of the metallicities of the stellar populations in moderate and high-redshift cluster galaxies can pose stringent constraints on the parameters that describe dark energy. Our results are in good agreement with the analysis of dark energy models using SNIa data as a constraint. Accurate estimates of the metallicities of stellar populations in cluster galaxies at z ≲ 2 will make this approach a powerful complement to studies of cosmological parameters using high-redshift SNIa. 相似文献
128.
Short-lived asteroids in the 7/3 Kirkwood gap and their relationship to the Koronis and Eos families
A population of 23 asteroids is currently observed in a very unstable region of the main belt, the 7/3 Kirkwood gap. The small size of these bodies—with the notable exception of (677) Aaltje (∼30 km)—as well as the computation of their dynamical lifetimes (3<TD<172 Myr) shows that they cannot be on their primordial orbits, but were recently injected in the resonance. The distribution of inclinations appears to be bimodal, the two peaks being close to 2° and 10°. We argue that the resonant population is constantly being replenished by the slow leakage of asteroids from both the Koronis (I∼2°) and Eos (I∼10°) families, due to the drift of their semi-major axes, caused by the Yarkovsky effect. Assuming previously reported values for the Yarkovsky mean drift rate, we calculate the flux of family members needed to sustain the currently observed population in steady state. The number densities with respect to semi-major axis of the observed members of both families are in very good agreement with our calculations. The fact that (677) Aaltje is currently observed in the resonance is most likely an exceptional event. This asteroid should not be genetically related to any of the above families. Its size and the eccentricity of its orbit suggest that the Yarkovsky effect should have been less efficient in transporting this body to the resonance than close encounters with Ceres. 相似文献
129.
Starting with analysis on the evolving course of oasis and the characteristics and evolution of transitional zone between oasis and desert, in consideration of ecological elements including plant stomata resistance, area covered by vegetation, and physical elements including albedo of vegetation and bare soil, atmosphere temperature, and humidity, under the condition of the balance among net radiation flux, latent heat flux, and sensible heat flux, the following are calculated: temperatures of vegetation and bare soil in different conditions, as well as the evapotranspiration rate of ecosystem. Analysis on evapotranspiration rate indicates that it depends on both the climate of environment and the physiological and ecological conditions of plants. On certain conditions, the evapotranspiration rate of transitional zone between oasis and desert (i.e. area covered by vegetation less than 20%), in some parameter domains, appears in bifurcation or multiequilibrium state. Meanwhile, in such area, ecosystem is extremely unstable. Any minor change to the balance will cause either increase or reduction of area covered by vegetation in ecosystem, on the basis of discussion on the emergency of these phenomena. This paper is attempting to propose an effective way of destruction and rebuilt ecosystem in transitional zone. The way is to control the evaporation of plant through selecting anti-drought country plant with big stomata resistance, and modify the roughness of the underlying surface in ecosystem by establishing rational interspace structure of plant community, so as to put the degenerative ecosystem into the natural succession track. This primary theory is being verified through observation and analysis on historical data. 相似文献
130.
Frédéric Zagury 《Monthly notices of the Royal Astronomical Society》2006,370(4):1763-1770
The hypothesis of an extended red emission (ERE) in diffuse Galactic light (DGL) has been put forward in 1998 by Gordon, Witt & Friedmann who found that scattered starlight was not enough to explain the amount of DGL in the R band, in some high Galactic latitude directions. This paper re-investigates, for high Galactic latitudes, the brightnesses and colours of DGL, integrated star and galaxy light (ISGL), and of the total extrasolar light (ISGL+DGL) measured by Pioneer. Under the traditional assumption that DGL is forward scattering of background starlight by interstellar dust on the line of sight, ISGL and Pioneer have very close colours, as it is found by Gordon, Witt & Friedmann. Pioneer observations at high |b| thus accept an alternative and simple interpretation, with no involvement of ERE in DGL. 相似文献