全文获取类型
收费全文 | 116175篇 |
免费 | 1840篇 |
国内免费 | 937篇 |
专业分类
测绘学 | 2936篇 |
大气科学 | 8049篇 |
地球物理 | 22730篇 |
地质学 | 43210篇 |
海洋学 | 9710篇 |
天文学 | 25733篇 |
综合类 | 385篇 |
自然地理 | 6199篇 |
出版年
2022年 | 551篇 |
2021年 | 957篇 |
2020年 | 1064篇 |
2019年 | 1122篇 |
2018年 | 4406篇 |
2017年 | 4152篇 |
2016年 | 3939篇 |
2015年 | 1841篇 |
2014年 | 2946篇 |
2013年 | 5312篇 |
2012年 | 3821篇 |
2011年 | 6052篇 |
2010年 | 5280篇 |
2009年 | 6584篇 |
2008年 | 5677篇 |
2007年 | 5949篇 |
2006年 | 4021篇 |
2005年 | 3441篇 |
2004年 | 3320篇 |
2003年 | 3153篇 |
2002年 | 2848篇 |
2001年 | 2485篇 |
2000年 | 2369篇 |
1999年 | 1934篇 |
1998年 | 2028篇 |
1997年 | 1940篇 |
1996年 | 1591篇 |
1995年 | 1630篇 |
1994年 | 1387篇 |
1993年 | 1276篇 |
1992年 | 1233篇 |
1991年 | 1151篇 |
1990年 | 1298篇 |
1989年 | 1102篇 |
1988年 | 1016篇 |
1987年 | 1269篇 |
1986年 | 1038篇 |
1985年 | 1330篇 |
1984年 | 1453篇 |
1983年 | 1406篇 |
1982年 | 1272篇 |
1981年 | 1209篇 |
1980年 | 1069篇 |
1979年 | 986篇 |
1978年 | 1005篇 |
1977年 | 904篇 |
1976年 | 889篇 |
1975年 | 835篇 |
1974年 | 822篇 |
1973年 | 834篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
41.
B. A. Ivanov 《Solar System Research》2005,39(5):381-409
Multi-ring impact basins have been found on the surfaces of almost all planetary bodies in the Solar system with solid crusts. The details of their formation mechanism are still unclear. We present results of our numerical modeling of the formation of the largest known terrestrial impact craters. The geological and geophysical data on these structures accumulated over many decades are used to place constraints on the parameters of available numerical models with a dual purpose: (i) to choose parameters in available mechanical models for the crustal response of planetary bodies to a large impact and (ii) to use numerical modeling to refine the possible range of original diameters and the morphology of partially eroded terrestrial craters. We present numerical modeling results for the Vredefort, Sudbury, Chicxulub, and Popigai impact craters and compare these results with available geological and geophysical information. 相似文献
42.
The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray 相似文献
43.
Daniela Lazzaro Marcos A. Florczak Alberto Betzler Othon C. Winter Silvia M. Giuliatti-Winter Claudia A. Angeli Dietmar W. Foryta 《Planetary and Space Science》1996,44(12):1547-1550
The results of photometric observations of comet/asteroid 2060 Chiron at the Observatório do Pico dos Dias (Brazil-OPD) and the Observatoire de Haute-Provence (France-OHP) during 1994 and 1995 are presented. The analysis of the data shows a decrease of 2060 Chiron brightness from its peak values of 1988–1991. The absolute magnitude, Hv, varies from a maximum of 6.6 in February 1994 up to a minimum of 6.8 in June 1995. Therefore 2060 Chiron is back to a minimum of activity close to that of 1983–1985. The slope parameter G is found to be G = 0.71 ± 0.15. It is suggested that the H-G magnitude system, generally adopted to present 2060 Chiron brightness, is not the most appropriate due to the cometary activity of this object. 相似文献
44.
Matthew J. GENGE Monica M. GRADY Robert HUTCHISON 《Meteoritics & planetary science》1996,31(5):627-632
Abstract— Cosmic dust accreted by the Earth can be extensively reprocessed during atmospheric encounters. The textures and compositions of reprocessed material provide important constraints by which the processes affecting extraterrestrial matter in the Earth's atmosphere can be better understood. Here we report results on an unusual Antarctic glassy cosmic spherule that demonstrates strong textural evidence for at least two grazing incidence encounters with the Earth's atmosphere prior to final reentry. The particle consists of a central glassy core with four peripheral glass lobes that transect a silicate particle rim. The texture of the particle confirms previous theoretical speculations that some high velocity, low incidence angle interplanetary particles experience numerous encounters with the Earth's atmosphere and also indicates that micrometeorites demonstrating multiple melting episodes should be interpreted with caution. 相似文献
45.
46.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques. 相似文献
47.
The orbits of (69230) Hermes and 2002 SY50 are similar and the Earth approaches both of them twice: at the end of October the local orbital minimum distances are smaller
than 0.007 AU, and at the end of April the distances are smaller than 0.04 AU. This gives us opportunities to observe the
meteors associated with these asteroids. Using the geocentric parameters of the orbital close encounters (the theoretical
radiants) and our D
N
distance function (Valsecchi et al. Mon. Not. R. Astron. Soc. 304 (1999) 743), we searched for meteoroids originated by Hermes and 2002 SY50. A search among 1830 good quality photographic meteors gave negative results: we found no meteor dynamically similar to Hermes
or 2002 SY50. In a second search, done in a set of 62150 radio meteors, we applied two methods (M1, M2) and in both cases we found two
streams; the streams found with the M1 method had 43 and 30 members, those found with the M2 method had 39 and 14 members.
However, these results do not look convincing, due to the small number of common members in the corresponding streams. We
therefore conclude that amongst the IAU meteors used in our search there are no compact streams associated with Hermes and
2002 SY50. 相似文献
48.
49.
A highly automated moving object detection package 总被引:1,自引:0,他引:1
50.
The UBV photometric observations of RT Per, from Sanwal and Chaubey (1981), were analyzed by the Wilson and Devinney code (1971). The light curves include reflection effects that for the first time has been suggested by Dugan (1911). RT Per has a semi-detached configuration where the lower-mass component is in contact with its respective Roche surface. The higher-mass component very nearly fills its Roche lobe. It has the characteristic of an Algol type system. The absolute dimensions for the primary and secondary of this system were calculated from its spectral types and by combining the photometric solution with inferred component radial velocities (Lu, 1990). 相似文献