全文获取类型
收费全文 | 65119篇 |
免费 | 817篇 |
国内免费 | 621篇 |
专业分类
测绘学 | 1774篇 |
大气科学 | 4525篇 |
地球物理 | 12130篇 |
地质学 | 25766篇 |
海洋学 | 5112篇 |
天文学 | 14043篇 |
综合类 | 347篇 |
自然地理 | 2860篇 |
出版年
2021年 | 451篇 |
2020年 | 480篇 |
2019年 | 486篇 |
2018年 | 3822篇 |
2017年 | 3567篇 |
2016年 | 2644篇 |
2015年 | 816篇 |
2014年 | 1219篇 |
2013年 | 2223篇 |
2012年 | 2307篇 |
2011年 | 4213篇 |
2010年 | 3801篇 |
2009年 | 4472篇 |
2008年 | 3625篇 |
2007年 | 4198篇 |
2006年 | 1767篇 |
2005年 | 1710篇 |
2004年 | 1624篇 |
2003年 | 1615篇 |
2002年 | 1431篇 |
2001年 | 1119篇 |
2000年 | 1036篇 |
1999年 | 880篇 |
1998年 | 881篇 |
1997年 | 848篇 |
1996年 | 725篇 |
1995年 | 736篇 |
1994年 | 659篇 |
1993年 | 557篇 |
1992年 | 519篇 |
1991年 | 522篇 |
1990年 | 599篇 |
1989年 | 507篇 |
1988年 | 465篇 |
1987年 | 594篇 |
1986年 | 484篇 |
1985年 | 614篇 |
1984年 | 691篇 |
1983年 | 658篇 |
1982年 | 574篇 |
1981年 | 615篇 |
1980年 | 515篇 |
1979年 | 472篇 |
1978年 | 476篇 |
1977年 | 434篇 |
1976年 | 424篇 |
1975年 | 417篇 |
1974年 | 396篇 |
1973年 | 426篇 |
1971年 | 260篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
Abstract— We have analyzed several types of data associated with the well‐documented fall of the Neuschwanstein meteorites on April 6, 2002 (a total of three meteorites have been recovered). This includes ground‐based photographic and radiometer data as well as infrasound and seismic data from this very significant bolide event (Spurný et al. 2002, 2003). We have also used these data to model the entry of Neuschwanstein, including the expected dynamics, energetics, panchromatic luminosity, and associated fragmentation effects. In addition, we have calculated the differential efficiency of acoustical waves for Neuschwanstein and used these values to compare against the efficiency calculated using available ground‐based infrasound data. This new numerical technique has allowed the source height to be determined independent of ray tracing solutions. We have also carried out theoretical ray tracing for a moving point source (not strictly a cylindrical line emission) and for an infinite speed line source. In addition, we have determined the ray turning heights as a function of the source height for both initially upward and downward propagating rays, independent of the explicit ray tracing (detailed propagation path) programs. These results all agree on the origins of the acoustic emission and explicit source heights for Neuschwanstein for the strongest infrasonic signals. Calculated source energies using more than four different independent approaches agree that Neuschwanstein was certainly <500 kg in initial mass, given the initial velocity of 20.95 km/s, resulting in an initial source energy ≤0.0157‐0.0276 kt TNT equivalent (4.185 times 1012 J). Local source energies at the calculated infrasonic/seismic source altitudes are up to two orders of magnitude smaller than this initial source energy. 相似文献
22.
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
23.
24.
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
25.
26.
27.
28.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during
1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal
that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles.
The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease
of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles
of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed. 相似文献
29.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem. 相似文献
30.
W. H. Jefferys G. F. Benedict P. D. Hemenway P. J. Shelus R. L. Duncombe 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):299-305
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed. 相似文献