全文获取类型
收费全文 | 67756篇 |
免费 | 946篇 |
国内免费 | 650篇 |
专业分类
测绘学 | 2010篇 |
大气科学 | 4922篇 |
地球物理 | 13313篇 |
地质学 | 23678篇 |
海洋学 | 5841篇 |
天文学 | 15635篇 |
综合类 | 204篇 |
自然地理 | 3749篇 |
出版年
2021年 | 632篇 |
2020年 | 623篇 |
2019年 | 726篇 |
2018年 | 1601篇 |
2017年 | 1474篇 |
2016年 | 1950篇 |
2015年 | 1103篇 |
2014年 | 1798篇 |
2013年 | 3466篇 |
2012年 | 1975篇 |
2011年 | 2662篇 |
2010年 | 2338篇 |
2009年 | 3058篇 |
2008年 | 2601篇 |
2007年 | 2632篇 |
2006年 | 2529篇 |
2005年 | 2065篇 |
2004年 | 1970篇 |
2003年 | 1949篇 |
2002年 | 1860篇 |
2001年 | 1649篇 |
2000年 | 1590篇 |
1999年 | 1364篇 |
1998年 | 1331篇 |
1997年 | 1328篇 |
1996年 | 1144篇 |
1995年 | 1098篇 |
1994年 | 1036篇 |
1993年 | 936篇 |
1992年 | 849篇 |
1991年 | 844篇 |
1990年 | 850篇 |
1989年 | 820篇 |
1988年 | 770篇 |
1987年 | 904篇 |
1986年 | 799篇 |
1985年 | 986篇 |
1984年 | 1045篇 |
1983年 | 987篇 |
1982年 | 907篇 |
1981年 | 882篇 |
1980年 | 785篇 |
1979年 | 723篇 |
1978年 | 692篇 |
1977年 | 693篇 |
1976年 | 612篇 |
1975年 | 593篇 |
1974年 | 621篇 |
1973年 | 657篇 |
1972年 | 384篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
861.
The photometric elements of the Algol type binary TT Hydrae derived by the authors from theirUBV observations during 1973–77 have been combined with the spectroscopic elements given by Sanford (1937) and Sahade and Cesco
(1946) to obtain the absolute dimensions of the system. It is found that the spectroscopic orbital elements given by Sanford
represent the evolutionary status of the secondary component better than those of Sahade and Cesco. The primary appears to
be an Al v main sequence star of mass
and radius ∼2.3R
⊙. The secondary fills its Roche lobe; it can be represented by a K0iii star of mass
and radius ∼6.0R
⊙. Better spectroscopic data are needed for confirmation of these results. 相似文献
862.
A spectrum scanning Stokes polarimeter 总被引:1,自引:0,他引:1
863.
Oliver K. Manuel 《Icarus》1980,41(2):312-315
Isotopically anomalous xenon in chondrites is closely associated with low-Z noble gases, but there is no helium (or neon) in the noble gas component with normal xenon. The correlation of elemental and isotopic heterogeneities in meteoritic noble gases places stringent limits on the origin of isotopically anomalous elements in meteorites and on the formation of the solar system. 相似文献
864.
Lowell's value for the mass of Planet X was about seven times that of the Earth. Postdiscovery determinations of the mass of Pluto from analysis of the observed motions of Uranus and Neptune reduced this value to about one Earth mass. More extended analyses in the past 10 years have lowered this value to about one-tenth of an Earth mass. The mass so derived, however, fails to agree by a factor of 50 with that determined from the motion of the newly discovered satellite Charon. The discrepancy may arise from unmodeled effects in the motions of the outer planets. 相似文献
865.
KAr and/or 40Ar39Ar plateau ages of Allende samples—whole rock, matrix, chondrules, white inclusions–range from 3.8 AE for matrix of ?5 AE for some white inclusions, but cluster strongly near 4.53 AE. This age marks the dominant KAr resetting of Allende materials. Age spectra show disturbances due to 39Ar recoil or some other argon redistribution processes. Possible explanations for the apparent presolar ages (>4.6 AE) include: ?20% loss of 39Ar; ?40% loss of 40K ~3.8 AE ago with no loss of 40Arl trapped argon of unique 40Ar/36Ar isotopic composition; admixture of “very old” presolar grains. 相似文献
866.
We have conducted a targeted high-sensitivity search for narrow-band signals near γ18 cm using the 91-m radiotelescope of the National Radio Astronomy Observatory. The search included 201 nearby solar-type stars and achieved a frequency resolution of 5.5 Hz over a 1.4-MHz bandwidth. This high spectral Mark I VLBI recording terminal in conjunction with the CDC 7600 computational facility at the NASA-Ames Research Center. This is the first high-resolution search for narrow-band signals in this wavelength regime. To date it is the most sensitive search per unit observing time of any search strategy which does not postulate a unique magic frequency. Our data show no evidence for narrow-band signals due to extraterrestrial intelligence at a 12-σ upper limit on signal strength of 1.1 × 10?23 W m?2. 相似文献
867.
Saikia D. J. Shastri P. Cornwell T. J. Junor W. Muxlow T. W. B. 《Journal of Astrophysics and Astronomy》1989,10(2):203-235
As part of our study to understand the nature of extragalactic radio sources which are very asymmetric in the surface brightness
of the two lobes, often with radio emission on only one side of the nucleus, we have observed a large number of them with
high angular resolution and good surface brightness sensitivity at radio frequencies. In this paper we present VLA and MERLIN
observations of 15 such sources. We discuss their observed structures and spectra, and possible explanations for their morphologies.
We report evidence of a possible correlation between the hot-spot brightness ratio and the degree of core prominence, used
as a Statistical measure of source orientation, suggesting that relativistic beaming of the hot-spot emission does play a
significant role in the observed brightness asymmetry. To explain the apparently one-sided sources within the relativistic
beaming framework, the velocities required are in the range of 0.2 to 0.8c. We discuss the possibility that the lobe which is seen to the south of the jet in 3C273 is the counter-lobe seen in projection.
We also draw-attention to a number of one-sided sources with very weak cores, and discuss their possible nature. 相似文献
868.
Results are presented for several theoretical line ratios in Nev involving transitions between multiplets in the 2s
22p
2 and 2s2p
3 configurations. A comparison of these with solar data from the S082A and S-055 instruments on board Skylab reveals generally good agreement between theory and experiment, especially in the case of the high-resolution (S082A) observations. However the 2s
22p
2
1
D – 2s2p
3
1
P (365.6 Å) and 2s
22p
2
3P – 2s2p
3
3
S (359 Å) lines appear to be blended, possibly with transitions in Fex and Fexi/Fexiii, respectively. We note that the intensity ratio I(365.6 Å)/I(416.2 Å) should be a valuable calibration check for a high-resolution extreme ultraviolet instrument in the spectral range 360–420 Å. 相似文献
869.
We present results of modeling rubble piles as collections of polyhedra. The use of polyhedra allows more realistic (irregular) shapes and interactions (e.g. collisions), particularly for objects of different sizes. Rotational degrees of freedom are included in the modeling, which may be important components of the motion. We solved the equations of rigid-body dynamics, including frictional/inelastic collisions, for collections of up to several hundred elements. As a demonstration of the methods and to compare with previous work by other researchers, we simulated low-speed collisions between km-scale bodies with the same general parameters as those simulated by Leinhardt et al. [Leinhardt, Z.M., Richardson, D.C., Quinn, T., 2000. Icarus 146, 133-151]. High-speed collisions appropriate to present-day asteroid encounters require additional treatment of shock effects and fragmentation and are the subject of future work; here we study regimes appropriate to planetesimal accretion and re-accretion in the aftermath of catastrophic events. Collisions between equal-mass objects at low speeds () were simulated for both head-on and off-center collisions between rubble piles made of a power-law mass spectrum of sub-elements. Very low-speed head-on collisions produce single objects from the coalescence of the impactors. For slightly higher speeds, extensive disruption occurs, but re-accretion produces a single object with most of the total mass. For increasingly higher speeds, the re-accreted object has smaller mass, finally resulting in complete catastrophic disruption with all sub-elements on escape trajectories and only small amounts of mass in re-accreted bodies. Off-center collisions at moderately low speeds produce two re-accreted objects of approximately equal mass, separating at greater than escape speed. At high speed, complete disruption occurs as with the high-speed head-on collisions. Head-on collisions at low to moderate speeds result in objects of mostly oblate shape, while higher speed collisions produce mostly prolate objects, as do off-center collisions at moderate and high speeds. Collisions carried out with the same dissipative coefficients (coefficient of restitution ?n=0.8, zero friction) as used by Leinhardt et al. [Leinhardt, Z.M., Richardson, D.C., Quinn, T., 2000. Icarus 146, 133-151] result in a value for specific energy for disruption , somewhat lower than the value of 2 J/kg found by them, while collisions with a lower coefficient of restitution and friction [?n=0.5, ?t=0, μ=0.5, similar to those used by Michel, et al. [Michel, P., Benz, W., Richardson, D.C., 2004. Planet. Space Sci. 52, 1109-1117] for SPH + N-body calculations] yield . 相似文献
870.
ICE, the electric field experiment on DEMETER 总被引:9,自引:0,他引:9