全文获取类型
收费全文 | 68404篇 |
免费 | 1369篇 |
国内免费 | 1243篇 |
专业分类
测绘学 | 1593篇 |
大气科学 | 4358篇 |
地球物理 | 13691篇 |
地质学 | 25885篇 |
海洋学 | 6308篇 |
天文学 | 15513篇 |
综合类 | 333篇 |
自然地理 | 3335篇 |
出版年
2022年 | 708篇 |
2021年 | 1068篇 |
2020年 | 1099篇 |
2019年 | 1179篇 |
2018年 | 2326篇 |
2017年 | 2164篇 |
2016年 | 2427篇 |
2015年 | 1248篇 |
2014年 | 2242篇 |
2013年 | 3563篇 |
2012年 | 2683篇 |
2011年 | 3352篇 |
2010年 | 3036篇 |
2009年 | 3637篇 |
2008年 | 3180篇 |
2007年 | 3378篇 |
2006年 | 3127篇 |
2005年 | 1975篇 |
2004年 | 1809篇 |
2003年 | 1725篇 |
2002年 | 1632篇 |
2001年 | 1506篇 |
2000年 | 1357篇 |
1999年 | 1028篇 |
1998年 | 1084篇 |
1997年 | 1095篇 |
1996年 | 869篇 |
1995年 | 863篇 |
1994年 | 775篇 |
1993年 | 674篇 |
1992年 | 660篇 |
1991年 | 630篇 |
1990年 | 730篇 |
1989年 | 622篇 |
1988年 | 581篇 |
1987年 | 666篇 |
1986年 | 519篇 |
1985年 | 686篇 |
1984年 | 789篇 |
1983年 | 706篇 |
1982年 | 689篇 |
1981年 | 618篇 |
1980年 | 630篇 |
1979年 | 527篇 |
1978年 | 567篇 |
1977年 | 517篇 |
1976年 | 464篇 |
1975年 | 457篇 |
1974年 | 457篇 |
1973年 | 526篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
11.
12.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars. 相似文献
13.
14.
In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In
order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic
showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data
from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying
and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the
temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray
and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation
level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events
for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like
the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect
the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the
cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density
fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the
signal to noise ratio.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
16.
17.
18.
The surface digital photometry for NGC 5253 and NGC 2915 is given. The luminosity profiles and colour gradients are discussed and compared. It is concluded that NGC 2915 differs from NGC 5253, both in the extension and intensity of the star burst event because of the kinematical behaviour of the captured gas.CONICET, Buenos Aires, Visiting Astronomer, Cerro Tololo Inter-American Observatory, La Serena, Chile, operated by NSF 相似文献
19.
Randy L. Korotev 《Meteoritics & planetary science》1996,31(3):403-412
Abstract Two types of texturally and compositionally similar breccias that consist largely of fragmental debris from meteorite impacts occur at the Apollo 16 lunar site: Feldspathic fragmental breccias (FFBs) and ancient regolith breccias (ARBs). Both types of breccia are composed of a suite of mostly feldspathic components derived from the early crust of the Moon and mafic impact-melt breccias produced during the time of basin formation. The ARBs also contain components, such as agglutinates and glass spherules, indicating that the material of which they are composed occurred at the surface of the Moon as fine-grained regolith prior to lithification of the breccias. These components are absent from the FFBs, suggesting that the FFBs might be the protolith of the ARBs. However, several compositional differences exist between the two types of breccia, making any simple genetic relationship implausible. First, clasts of mafic impact-melt breccia occurring in the FFBs are of a different composition than those in the ARBs. Also the feldspathic “prebasin” components of the FFBs have a lower average Mg/Fe ratio than the corresponding components of the ARBs; the average composition of the plagioclase in the FFBs is more sodic than that of the ARBs; and there are differences in relative abundances of rare earth elements. The two breccia types also have different provenances: the FFBs occur primarily in ejecta from North Ray crater and presumably derive from the Descartes Formation, while the ARBs are restricted to the Cayley plains. Together these observations suggest that although some type of fragmental breccia may have been a precursor to the ARBs, the FFBs of North Ray crater are not a significant component of the ARBs and, by inference, the Cayley plains. The average compositions of the prebasin components of the two types of fragmental breccia are generally similar to the composition of the feldspathic lunar meteorites. With 30–31% Al2O3, however, they are slightly richer in plagioclase than the most feldspathic lunar meteorites (~29% Al2O3), implying that the crust of the early central nearside of the Moon contained a higher abundance of highly feldspathic anorthosite than typical lunar highlands, as inferred from the lunar meteorites. The ancient regolith breccias, as well as the current surface regolith of the Cayley plains, are more mafic than (1) prebasin regoliths in the Central Highlands and (2) regions of highlands presently distant from nearside basins because they contain a high abundance (~30%) of mafic impact-melt breccias produced during the time of basin formation that is absent from other regoliths. 相似文献
20.
Summary. Four box cores collected from the Ontong—Java plateau during the Eurydice expedition have been used to make relative geomagnetic palaeo-intensity measurements. Rock magnetic measurements on the sediments show that they are characterized by a uniform magnetic mineralogy, and that they are suitable for relative intensity estimates. These are obtained by normalizing the NRM by an ARM imparted in a low DC bias field. the palaeoceanographic event known as the preservation spike is used to establish a crude time-scale for the record so that it may be compared with other data from the same region, and also with global palaeointensity estimates. the marine sediment data are quite similar to Australian intensity data from lake sediments and archaeomagnetic sources, but as might be expected exhibit some obvious differences from the global record. 相似文献