全文获取类型
收费全文 | 62405篇 |
免费 | 793篇 |
国内免费 | 351篇 |
专业分类
测绘学 | 1486篇 |
大气科学 | 4245篇 |
地球物理 | 11515篇 |
地质学 | 25276篇 |
海洋学 | 4947篇 |
天文学 | 12766篇 |
综合类 | 239篇 |
自然地理 | 3075篇 |
出版年
2022年 | 281篇 |
2021年 | 463篇 |
2020年 | 522篇 |
2019年 | 608篇 |
2018年 | 3786篇 |
2017年 | 3563篇 |
2016年 | 2621篇 |
2015年 | 754篇 |
2014年 | 1198篇 |
2013年 | 2038篇 |
2012年 | 2305篇 |
2011年 | 4220篇 |
2010年 | 3848篇 |
2009年 | 4325篇 |
2008年 | 3573篇 |
2007年 | 4171篇 |
2006年 | 1753篇 |
2005年 | 1607篇 |
2004年 | 1525篇 |
2003年 | 1612篇 |
2002年 | 1301篇 |
2001年 | 956篇 |
2000年 | 896篇 |
1999年 | 788篇 |
1998年 | 778篇 |
1997年 | 798篇 |
1996年 | 649篇 |
1995年 | 614篇 |
1994年 | 526篇 |
1993年 | 498篇 |
1992年 | 437篇 |
1991年 | 454篇 |
1990年 | 460篇 |
1989年 | 419篇 |
1988年 | 398篇 |
1987年 | 432篇 |
1986年 | 463篇 |
1985年 | 546篇 |
1984年 | 587篇 |
1983年 | 580篇 |
1982年 | 534篇 |
1981年 | 496篇 |
1980年 | 476篇 |
1979年 | 440篇 |
1978年 | 389篇 |
1977年 | 402篇 |
1976年 | 366篇 |
1975年 | 374篇 |
1974年 | 359篇 |
1973年 | 391篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
We have studied the optical historical light curves of two Active Galactic Nuclei (AGNs): OJ287 and 3C345. We find a linear
dependence between the magnitude and the logarithm of the length of the time the objects stay below this magnitude. The relation
may originate from a fractal structure of the light curve.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
102.
J.C. Vega Beltrán W.W. Zeilinger A. Pizzella E.M. Corsini F. Bertola J.G. Funes J.E. Beckman 《Astrophysics and Space Science》2001,276(2-4):1201-1210
In this paper we present the kinematics of the gas and/or the stars of a sample of 20 disc galaxies. We investigate whether there is any relation between the kinematics of the gas and stars and the classical morphological type of the galaxies in the sample. We deduce that, in most of the late-type spirals we have studied, the stars and the ionized gas are moving with virtually circular velocity, except when the spectroscopic slit crosses a bar region. On the other hand, we found in the central parts of early-type disc galaxies a wider variety of different behaviour of stars and gas. We find many possible factors that complicate the classification of the kinematical properties of the galaxies by their morphological type: the presence of counter-rotations (star vs. stars or star vs. gas), misalignment between the different kinematic components present in the galaxy, the presence of a bar structure and its orientation with respect to the line of nodes of the galaxy, and interactions and mergers or external accretion processes are some of the problems we find in the study of the kinematics of a galaxy. 相似文献
103.
104.
Polarimetric observations of the light scattered by dust have been carried out at Pic-du-Midi Observatory with the 2 m telescope
in June and September–October 1996, and at Haute-Provence Observatory with the 0.80 m telescope in April 1997. They cover
a total number of 11 nights and a large (6.9°–47.7°) phase angle range. The spatial resolution allows to underline structures
in the coma, as well in the brightness images as in the polarization maps, with a correlation between the regions of bright
structures and the regions of higher polarization. A clear difference appears between the sunward and antisunward side, with
higher polarization on the antisunward side. The phase angle coverage allows us to obtain a polarimetric phase curve for the
whole coma and to compare it with other cometary phase curves. The degree of polarization is higher for Hale-Bopp than for
the comets previously observed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
105.
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
106.
Theoretical studies have shown the possibility of high-temperature ('high enthalpy') geothermal reservoirs in the pre-Tertiary rocks at 4–5 km depth range within the Pannonian Basin. This expectation was proven by the hotwater/steam blowout of Fábiánsebestyén-4 borehole (16.12.85–31.1.86). Exploration efforts carried out during 1987–88 in the broad vicinity of the borehole proved that reservoirs of this type can be found with the combination of seismic reflection, silica-thermometry and magnetotelluric sounding methods. Deliberate prospection should be continued in all suitable areas within the basin, since high enthalpy reservoirs promise profitable operation of geothermal power stations. 相似文献
107.
Anastassios E. Petropoulos James M. Longuski 《Celestial Mechanics and Dynamical Astronomy》1994,60(4):431-454
An exact analysis of the coverage obtained by spacecraft using cross-track scanning and nadir-centered conical imaging, under imposed viewing obliqueness and resolution requirements, is presented. In addition to exact expressions for the area acquired and the area acquisition rate, envelope theory is introduced to obtain the boundary of the imaged area. These expressions are relatively compact, allowing rapid machine computation. The effects of the sun phase angle, and of imaging system limitations are also examined. The Galileo mission encounter with Callisto is used as a numerical example, from which certain general conclusions are drawn regarding optimal imaging trajectories. 相似文献
108.
In this paper, two factors — the redistribution of the density and the variation in the angular velocity of the Earth rotation, that affect the adopted value of the flattening for equidensity surface within the Earth, are discussed. The computational results show that the contribution of the redistribution of the density in the Earth interior (especially in the core) on the change of the flattening at the core-mantle boundary (CMB) is marginal, and that the calculated value of the flattening at the CMB can be in good agreement with the VLBI observed value so long as the fact that the angular velocity of the Earth rotation has undergone the tidal evolution is taken into account. As a result, this paper presents a set of recommended values of the dynamical parameters of the Earth (see Table III) for computing Earth's forced nutation series. 相似文献
109.
M. Bellazzini F.R. Ferraro E. Pancino † 《Monthly notices of the Royal Astronomical Society》2001,327(1):L15-L20
We present wide-field multiband ( BVI ) CCD photometry (down to of the very low surface brightness dwarf spheroidal galaxy Sextans. In the derived colour–magnitude diagrams we find evidence suggesting the presence of multiple stellar populations in this dwarf spheroidal. In particular, we discover (i) a blue horizontal branch tail that appears to lie on a brighter sequence with respect to the prominent red horizontal branch and the RR Lyrae stars, very similar to what was found by Majewski et al. for the Sculptor dwarf spheroidal, (ii) hints of a bimodal distribution in colour of the red giant branch stars, (iii) a double red giant branch bump. All of these features suggest that (at least) two components are present in the old stellar population of this galaxy: the main one with and a minor component around . The similarity to the Sculptor case may indicate that multiple star formation episodes are also common in the most nearby dwarf spheroidals that ceased their star formation activity at very early epochs. 相似文献
110.