全文获取类型
收费全文 | 72677篇 |
免费 | 10164篇 |
国内免费 | 13299篇 |
专业分类
测绘学 | 12347篇 |
大气科学 | 9986篇 |
地球物理 | 11083篇 |
地质学 | 28530篇 |
海洋学 | 7133篇 |
天文学 | 9652篇 |
综合类 | 5201篇 |
自然地理 | 12208篇 |
出版年
2024年 | 274篇 |
2023年 | 801篇 |
2022年 | 2369篇 |
2021年 | 2893篇 |
2020年 | 2784篇 |
2019年 | 3393篇 |
2018年 | 2437篇 |
2017年 | 3162篇 |
2016年 | 3064篇 |
2015年 | 3315篇 |
2014年 | 4171篇 |
2013年 | 4682篇 |
2012年 | 4545篇 |
2011年 | 4647篇 |
2010年 | 3974篇 |
2009年 | 4885篇 |
2008年 | 4816篇 |
2007年 | 5034篇 |
2006年 | 4836篇 |
2005年 | 4311篇 |
2004年 | 3839篇 |
2003年 | 3439篇 |
2002年 | 2817篇 |
2001年 | 2499篇 |
2000年 | 2357篇 |
1999年 | 2049篇 |
1998年 | 1715篇 |
1997年 | 1207篇 |
1996年 | 1077篇 |
1995年 | 895篇 |
1994年 | 857篇 |
1993年 | 725篇 |
1992年 | 500篇 |
1991年 | 428篇 |
1990年 | 302篇 |
1989年 | 242篇 |
1988年 | 201篇 |
1987年 | 117篇 |
1986年 | 88篇 |
1985年 | 83篇 |
1984年 | 54篇 |
1983年 | 37篇 |
1982年 | 37篇 |
1981年 | 23篇 |
1980年 | 31篇 |
1979年 | 10篇 |
1978年 | 22篇 |
1977年 | 29篇 |
1971年 | 9篇 |
1954年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
121.
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
122.
123.
124.
Groundbased radio observations indicate that Jupiter's ammonia is globally depleted from 0.6 bars to at least 4-6 bars relative to the deep abundance of ∼3 times solar, a fact that has so far defied explanation. The observations also indicate that (i) the depletion is greater in belts than zones, and (ii) the greatest depletion occurs within Jupiter's local 5-μm hot spots, which have recently been detected at radio wavelengths. Here, we first show that both the global depletion and its belt-zone variation can be explained by a simple model for the interaction of moist convection with Jupiter's cloud-layer circulation. If the global depletion is dynamical in origin, then important endmember models for the belt-zone circulation can be ruled out. Next, we show that the radio observations of Jupiter's 5-μm hot spots imply that the equatorial wave inferred to cause hot spots induces vertical parcel oscillation of a factor of ∼2 in pressure near the 2-bar level, which places important constraints on hot-spot dynamics. Finally, using spatially resolved radio maps, we demonstrate that low-latitude features exceeding ∼4000 km diameter, such as the equatorial plumes and large vortices, are also depleted in ammonia from 0.6 bars to at least 2 bars relative to the deep abundance of 3 times solar. If any low-latitude features exist that contain 3-times-solar ammonia up to the 0.6-bar ammonia condensation level, they must have diameters less than ∼4000 km. 相似文献
125.
本文使用红外天文卫星(IRAS)巡天数据的最新版本IRAS天空巡天图(ISSA),经过进一步处理,得到了S140S141和S142SharplessHII区-分子云复合体的红外发射强度、温度及其光深的分布.在此基础上对各HII区的一些物理参量进行了统计分析,得到了分子云复合体的红外发射总光度以及复合体中尘埃的分布情况,对小尺度尘埃(VSG)的丰度进行了分析.并对各恒星形成区中的致密团块进行了研究,揭示出其中一些可能的恒星形成区域.同时,对S140区中的有关红外点源作出了能谱分析,并对S141区的激发星进行了讨论. 相似文献
126.
DEVELOPMENTSTRATEGIESOFWATERANDLANDRESOURCESINTHEHEXIREGION,CHINA肖洪浪,高前兆,李福兴DEVELOPMENTSTRATEGIESOFWATERANDLANDRESOURCESINTHE... 相似文献
127.
Brian Warner Patrick A. Woudt 《Monthly notices of the Royal Astronomical Society》2006,367(4):1562-1576
We present new observations of the rapid oscillations in the dwarf nova VW Hyi, made late in outburst. These dwarf nova oscillations (DNOs) increase in period until they reach 33 s, when a transition to a strong 1st harmonic and weak fundamental takes place. After further period increase, the 2nd harmonic appears; often all three components are present simultaneously. This 1:2:3 frequency suite is similar to what has been seen in some neutron star and black hole X-ray binaries, but has not previously been seen in a cataclysmic variable. When studied in detail, the fundamental and 2nd harmonic vary similarly in phase, but the 1st harmonic behaves independently, though keeping close to twice the frequency of the fundamental. The fundamental period of the DNOs, as directly observed or inferred from the harmonics, increases to ∼100 s before the oscillation disappears as the star reaches quiescence. Its maximum period is close to that of the 'longer-period' DNOs observed in VW Hyi. The quasi-periodic oscillations (QPOs), which have fundamental periods 400–1000 s, behave in the same way, showing 1st and 2nd harmonics at approximately the same times as the DNOs. We explore some possible models. One in which the existence of the 1st harmonic is due to the transition from viewing a single accretion region to viewing two regions, and the rate of accretion on to the primary is modulated at the frequency of the 1st harmonic, as in the 'beat frequency model', can generate the suite of DNO frequencies observed. But the behaviour of the QPOs is not yet understood. 相似文献
128.
J. I. Read M. I. Wilkinson N. W. Evans G. Gilmore Jan T. Kleyna 《Monthly notices of the Royal Astronomical Society》2006,366(2):429-437
We present an improved analytic calculation for the tidal radius of satellites and test our results against N -body simulations.
The tidal radius in general depends upon four factors: the potential of the host galaxy, the potential of the satellite, the orbit of the satellite and the orbit of the star within the satellite . We demonstrate that this last point is critical and suggest using three tidal radii to cover the range of orbits of stars within the satellite. In this way we show explicitly that prograde star orbits will be more easily stripped than radial orbits; while radial orbits are more easily stripped than retrograde ones. This result has previously been established by several authors numerically, but can now be understood analytically. For point mass, power-law (which includes the isothermal sphere), and a restricted class of split power-law potentials our solution is fully analytic. For more general potentials, we provide an equation which may be rapidly solved numerically.
Over short times (≲1–2 Gyr ∼1 satellite orbit), we find excellent agreement between our analytic and numerical models. Over longer times, star orbits within the satellite are transformed by the tidal field of the host galaxy. In a Hubble time, this causes a convergence of the three limiting tidal radii towards the prograde stripping radius. Beyond the prograde stripping radius, the velocity dispersion will be tangentially anisotropic. 相似文献
The tidal radius in general depends upon four factors: the potential of the host galaxy, the potential of the satellite, the orbit of the satellite and the orbit of the star within the satellite . We demonstrate that this last point is critical and suggest using three tidal radii to cover the range of orbits of stars within the satellite. In this way we show explicitly that prograde star orbits will be more easily stripped than radial orbits; while radial orbits are more easily stripped than retrograde ones. This result has previously been established by several authors numerically, but can now be understood analytically. For point mass, power-law (which includes the isothermal sphere), and a restricted class of split power-law potentials our solution is fully analytic. For more general potentials, we provide an equation which may be rapidly solved numerically.
Over short times (≲1–2 Gyr ∼1 satellite orbit), we find excellent agreement between our analytic and numerical models. Over longer times, star orbits within the satellite are transformed by the tidal field of the host galaxy. In a Hubble time, this causes a convergence of the three limiting tidal radii towards the prograde stripping radius. Beyond the prograde stripping radius, the velocity dispersion will be tangentially anisotropic. 相似文献
129.
130.