全文获取类型
收费全文 | 114891篇 |
免费 | 1685篇 |
国内免费 | 759篇 |
专业分类
测绘学 | 2678篇 |
大气科学 | 7862篇 |
地球物理 | 21775篇 |
地质学 | 43062篇 |
海洋学 | 9817篇 |
天文学 | 25456篇 |
综合类 | 379篇 |
自然地理 | 6306篇 |
出版年
2022年 | 679篇 |
2021年 | 1119篇 |
2020年 | 1234篇 |
2019年 | 1379篇 |
2018年 | 4475篇 |
2017年 | 4047篇 |
2016年 | 3904篇 |
2015年 | 1698篇 |
2014年 | 2970篇 |
2013年 | 5285篇 |
2012年 | 3877篇 |
2011年 | 5876篇 |
2010年 | 5233篇 |
2009年 | 6451篇 |
2008年 | 5489篇 |
2007年 | 5942篇 |
2006年 | 4156篇 |
2005年 | 3273篇 |
2004年 | 3178篇 |
2003年 | 3080篇 |
2002年 | 2908篇 |
2001年 | 2428篇 |
2000年 | 2387篇 |
1999年 | 1930篇 |
1998年 | 1974篇 |
1997年 | 1814篇 |
1996年 | 1544篇 |
1995年 | 1533篇 |
1994年 | 1314篇 |
1993年 | 1251篇 |
1992年 | 1155篇 |
1991年 | 1178篇 |
1990年 | 1156篇 |
1989年 | 1025篇 |
1988年 | 931篇 |
1987年 | 1115篇 |
1986年 | 962篇 |
1985年 | 1197篇 |
1984年 | 1365篇 |
1983年 | 1326篇 |
1982年 | 1214篇 |
1981年 | 1147篇 |
1980年 | 1046篇 |
1979年 | 968篇 |
1978年 | 965篇 |
1977年 | 829篇 |
1976年 | 831篇 |
1975年 | 828篇 |
1974年 | 774篇 |
1973年 | 836篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
We consider the effects of projection, internal absorption, and gas-or stellar-velocity dispersion on the measured rotation curves of galaxies with edge-on disks. Axisymmetric disk models clearly show that the rotational velocity in the inner galaxy is highly underestimated. As a result, an extended portion that imitates nearly rigid rotation appears. At galactocentric distances where the absorption is low (i.e., it does not exceed 0.3–0.5m kpc?1), the line profiles can have two peaks, and a rotation curve with minimum distortions can be obtained by estimating the position of the peak that corresponds to a higher rotational velocity. However, the high-velocity peak disappears in high-absorption regions and the actual shape of the rotation curve cannot be reproduced from line-of-sight velocity estimates. In general, the optical rotation curves for edge-on galaxies are of little use in reconstructing the mass distribution in the inner regions, particularly for galaxies with a steep velocity gradient in the central region. In this case, estimating the rotation velocities for outer (transparent) disk regions yields correct results. 相似文献
172.
Of the impact craters on Earth larger than 20 km in diameter, 10-15% (3 out of 28) are doublets, having been formed by the simultaneous impact of two well-separated projectiles. The most likely scenario for their formation is the impact of well-separated binary asteroids. If a population of binary asteroids is capable of striking the Earth, it should also be able to hit the other terrestrial planets as well. Venus is a promising planet to search for doublet craters because its surface is young, erosion is nearly nonexistent, and its crater population is significantly larger than the Earth's. After a detailed investigation of single craters separated by less than 150 km and “multiple” craters having diameters greater than 10 km, we found that the proportion of doublet craters on Venus is at most 2.2%, significantly smaller than Earth's, although several nearly incontrovertible doublets were recognized. We believe this apparent deficit relative to the Earth's doublet population is a consequence of atmospheric screening of small projectiles on Venus rather than a real difference in the population of impacting bodies. We also examined “splotches,” circular radar reflectance features in the Magellan data. Projectiles that are too small to form craters probably formed these features. After a careful study of these patterns, we believe that the proportion of doublet splotches on Venus (14%) is comparable to the proportion of doublet craters found on Earth (10-15%). Thus, given the uncertainties of interpretation and the statistics of small numbers, it appears that the doublet crater population on Venus is consistent with that of the Earth. 相似文献
173.
174.
175.
Andrew Mennim Richard A. Battye Carsten van de Bruck 《Astrophysics and Space Science》2003,283(4):633-638
We discuss the problems faced in trying to deduce the evolution of cosmological perturbations in brane-world models. There
are two natural ways to formulate the problem: one which makes the equations of motion simple and the other which makes the
boundary condition simple. Unfortunately the problem is difficult to solve, even numerically, in either formalism. We present
a more phenomenological approach which, while it does not solve the problem for any given model, illustrates some generic
features one might expect to see in the tensor part of the cosmic microwave background power spectrum. We find that the observed
scale invariance of the cosmic microwave background provides bounds on brane world models.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
176.
Piero F. Spinnato Michael Fellhauer Simon F. Portegies Zwart 《Monthly notices of the Royal Astronomical Society》2003,344(1):22-32
We study the efficiency at which a black hole or dense star cluster spirals in to the Galactic Centre. This process takes place on a dynamical friction time-scale, which depends on the value of the Coulomb logarithm (ln Λ). We determine the accurate value of this parameter using the direct N -body method, a tree algorithm and a particle-mesh technique with up to two million plus one particles. The three different techniques are in excellent agreement. Our measurement for the Coulomb logarithm appears to be independent of the number of particles. We conclude that ln Λ= 6.6 ± 0.6 for a massive point particle in the inner few parsec of the Galactic bulge. For an extended object, such as a dense star cluster, ln Λ is smaller, with a value of the logarithm argument Λ inversely proportional to the object size. 相似文献
177.
178.
D. H. Lumb A. Finoguenov R. Saxton B. Aschenbach P. Gondoin M. Kirsch I. M. Stewart 《Experimental Astronomy》2003,15(2):89-111
We describe measurements of the mirror vignetting in the XMM-Newton Observatory made in-orbit, using observations of SNR G21.5-09
and SNR 3C58 with the EPIC imaging cameras. The instrument features that complicate these measurements are briefly described.
We show the spatial and energy dependences of measured vignetting, outlining assumptions made in deriving the eventual agreement
between simulation and measurement. Alternate methods to confirm these are described, including an assessment of source elongation
with off-axis angle, the surface brightness distribution of the diffuse X-ray background, and the consistency of Coma cluster
emission at different position angles. A synthesis of these measurements leads to a change in the XMM calibration data base,
for the optical axis of two of the three telescopes, by in excess of 1 arcmin. This has a small but measureable effect on
the assumed spectral responses of the cameras for on-axis targets.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
179.
J. A. WOOD 《Meteoritics & planetary science》1996,31(5):641-645
Abstract— A widely held view of nebular evolution is that during the ~0.5 Ma while interstellar material was collapsing onto the disk, the latter grew in mass to the point of gravitational instability. It responded to this by losing axial symmetry, growing spiral arms that had the capacity to tidally redistribute disk mass (inward) and angular momentum (outward) and prevent further increase in the disk/protosun mass ratio. The spiral arms (density waves) rotated differently than the substance of the nebula, and in some parts of the disk, nebular material may have encountered the arms at supersonic velocities. The disk gas, and solid particles entrained in it, would have been heated to some degree when they passed through shock fronts at the leading edges of the spiral arms. The present paper proposes this was the energetic nebular setting or environment that has long been sought, in which the material now in the planets and chondritic meteorites was thermally processed. 相似文献
180.