全文获取类型
收费全文 | 52590篇 |
免费 | 506篇 |
国内免费 | 375篇 |
专业分类
测绘学 | 1038篇 |
大气科学 | 2739篇 |
地球物理 | 10050篇 |
地质学 | 20177篇 |
海洋学 | 4972篇 |
天文学 | 12435篇 |
综合类 | 193篇 |
自然地理 | 1867篇 |
出版年
2022年 | 608篇 |
2021年 | 899篇 |
2020年 | 940篇 |
2019年 | 1041篇 |
2018年 | 2189篇 |
2017年 | 1999篇 |
2016年 | 2151篇 |
2015年 | 905篇 |
2014年 | 1938篇 |
2013年 | 2905篇 |
2012年 | 2204篇 |
2011年 | 2545篇 |
2010年 | 2441篇 |
2009年 | 2765篇 |
2008年 | 2428篇 |
2007年 | 2704篇 |
2006年 | 2345篇 |
2005年 | 1359篇 |
2004年 | 1239篇 |
2003年 | 1209篇 |
2002年 | 1170篇 |
2001年 | 1142篇 |
2000年 | 944篇 |
1999年 | 700篇 |
1998年 | 713篇 |
1997年 | 752篇 |
1996年 | 557篇 |
1995年 | 576篇 |
1994年 | 537篇 |
1993年 | 434篇 |
1992年 | 452篇 |
1991年 | 431篇 |
1990年 | 478篇 |
1989年 | 422篇 |
1988年 | 390篇 |
1987年 | 412篇 |
1986年 | 339篇 |
1985年 | 465篇 |
1984年 | 473篇 |
1983年 | 453篇 |
1982年 | 433篇 |
1981年 | 387篇 |
1980年 | 397篇 |
1979年 | 348篇 |
1978年 | 378篇 |
1977年 | 320篇 |
1976年 | 288篇 |
1975年 | 293篇 |
1974年 | 274篇 |
1973年 | 344篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
The observed properties of the long-period comet system, and its periodic disturbance by galactic forces manifesting as terrestrial impact episodes, may be indicative of a comet capture/escape cycle as the Solar System orbits the Galaxy. A mean number density of comets in molecular clouds of ~10?1±1 AU?3 is implied. This is sufficient to deplete metals from the gaseous component of the interstellar medium, as observed, but leads to the problem of how stars are formed nevertheless with solar metal abundances. Formation of comets prior to stars in dense systems of near-zero energy may be indicated, and isotope signatures in cometary particles may be diagnostic of conditions in young spiral arm material. 相似文献
52.
Andrea PATZER Jochen SCHLÜTER Ludolf SCHULTZ Dolores H. HILL William V. BOYNTON 《Meteoritics & planetary science》2005,40(6):869-879
Abstract— Mineralogical and chemical studies of Dar al Gani 983 show that this meteorite is a eucrite. Its texture is that of an impact breccia. It contains cumulate pyroxene and feldspar megacrysts, a variety of recrystallized melt clasts, clasts of subophitic basalt, and mesostasis. These components are embedded in a matrix of fragmental pyroxene and plagioclase. In addition, the entire rock is penetrated by glassy melt veins and patches, and displays features of strong shock. The mineralogical and chemical evidence obtained for DaG 983 indicates that this meteorite experienced a complex evolutionary history. The presence of cumulate silicate crystals implies substantial, large scale cratering events on the HED asteroid. As a result of these impacts, rocks from different intrusive bodies to extrusive surface layers were laterally and vertically transported to form a thoroughly mixed megaregolith. DaG 983 represents a sample of this megabreccia. 相似文献
53.
54.
55.
56.
Using the basic Boussinesq's equation, the expression for the vertical stress distribution (σz) underneath any point on the ground surface due to a general triangular loaded region in a preferred orientation with a linearly varied loading has been successfully derived. When the triangle is not in a preferred orientation, a simple axis transformation is required and the expression will be equally applicable. Based on this expression, σz due to an arbitrarily shaped loaded foundation can simply be determined by first triangulating the loaded area and summing up the contributions from each generated triangular region. The procedures for triangulating and calculating the stress distribution can be simply automated through computer programs. 相似文献
57.
I.A. Bond F. Abe R.J. Dodd J.B. Hearnshaw M. Honda J. Jugaku P.M. Kilmartin A. Marles K. Masuda Y. Matsubara Y. Muraki T. Nakamura G. Nankivell S. Noda C. Noguchi K. Ohnishi N.J. Rattenbury M. Reid To. Saito H. Sato M. Sekiguchi J. Skuljan D.J. Sullivan T. Sumi M. Takeuti Y. Watase S. Wilkinson R. Yamada T. Yanagisawa P.C.M. Yock 《Monthly notices of the Royal Astronomical Society》2001,327(3):868-880
We describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extrasolar planets and faint stars. Approximately 17 deg2 were monitored at a sampling rate of up to six times per night. The images were analysed in real time using a difference imaging technique. 20 microlensing candidates were detected, of which eight were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (≳10), at least one had very high magnification (≳50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid. 相似文献
58.
Summary
Silica-undersaturated phlogopite schists from the Cackleberry Metamorphics, Arunta Inlier, central Australia, preserve relatively
low-temperature sapphirine-bearing parageneses that developed during low-pressure upper amphibolite facies metamorphism. Peak
metamorphic phlogopite–cordierite–sapphirine assemblages are interpreted to have formed during the same event recorded in
nearby metapelites, at c.3 kbar and 650–700 °C. Initial cooling of the terrain resulted in the breakdown of sapphirine to corundum–chlorite–phlogopite
and corundum–spinel–chlorite assemblages. Further retrogression at greenschist facies conditions resulted in the replacement
of sapphirine by diaspore–chlorite intergrowths. The reaction textures are consistent with a near-isobaric heating-cooling
path at low-pressure, and provide evidence for the stability of sapphirine at c.700 °C at low pressures in rocks of an appropriate Mg- and Fe3+-rich bulk composition.
Received August 15, 2001 accepted December 27, 2001 相似文献
59.
We develop a general formalism for analysing parameter information from non-Gaussian cosmic fields. The method can be adapted to include the non-linear effects in galaxy redshift surveys, weak lensing surveys and cosmic velocity field surveys as part of parameter estimation. It can also be used as a test of non-Gaussianity of the cosmic microwave background. Generalizing maximum-likelihood analysis to second order, we calculate the non-linear Fisher information matrix and likelihood surfaces in parameter space. To this order we find that the information content is always increased by including non-linearity. Our methods are applied to a realistic model of a galaxy redshift survey, including non-linear evolution, galaxy bias, shot-noise and redshift-space distortions to second order. We find that including non-linearities allows all of the degeneracies between parameters to be lifted. Marginalized parameter uncertainties of a few per cent will then be obtainable using forthcoming galaxy redshift surveys. 相似文献
60.
A very long series of photographic observations of the comet Hale-Bopp has been made during January–April 1997 at the double
astrograph (400/2000) of the Main Astronomical Observatory (Kyiv, Ukraine). Some of the cometary photos were obtained with
two wide-band filter combinations. One of these combinations isolates C2 emission, another — the nearby dust continuum. The images were digitized by means of AMDPH-XY machine and then calibrated
following the standard procedure. After subtraction of the dust continuum the distribution of surface brightness in the C2 emission coma of comet Hale-Bopp was studied. We found an asymmetric brightness distribution both pre- and post-perihelion.
On 21.77 April 1997 a secondary brightness peak is found at the distance of 1.03 × 105 km from the nucleus. It is possible that this peak is related to the extended source of the C2 molecules.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献