全文获取类型
收费全文 | 71046篇 |
免费 | 859篇 |
国内免费 | 445篇 |
专业分类
测绘学 | 1793篇 |
大气科学 | 4935篇 |
地球物理 | 13145篇 |
地质学 | 27910篇 |
海洋学 | 6044篇 |
天文学 | 14776篇 |
综合类 | 257篇 |
自然地理 | 3490篇 |
出版年
2022年 | 294篇 |
2021年 | 574篇 |
2020年 | 567篇 |
2019年 | 648篇 |
2018年 | 3933篇 |
2017年 | 3594篇 |
2016年 | 2870篇 |
2015年 | 896篇 |
2014年 | 1400篇 |
2013年 | 2595篇 |
2012年 | 2461篇 |
2011年 | 4449篇 |
2010年 | 4113篇 |
2009年 | 4758篇 |
2008年 | 3888篇 |
2007年 | 4502篇 |
2006年 | 2047篇 |
2005年 | 1999篇 |
2004年 | 1850篇 |
2003年 | 1889篇 |
2002年 | 1659篇 |
2001年 | 1279篇 |
2000年 | 1233篇 |
1999年 | 954篇 |
1998年 | 979篇 |
1997年 | 974篇 |
1996年 | 763篇 |
1995年 | 764篇 |
1994年 | 720篇 |
1993年 | 653篇 |
1992年 | 591篇 |
1991年 | 581篇 |
1990年 | 592篇 |
1989年 | 567篇 |
1988年 | 544篇 |
1987年 | 647篇 |
1986年 | 556篇 |
1985年 | 697篇 |
1984年 | 735篇 |
1983年 | 686篇 |
1982年 | 608篇 |
1981年 | 661篇 |
1980年 | 574篇 |
1979年 | 543篇 |
1978年 | 510篇 |
1977年 | 504篇 |
1976年 | 442篇 |
1975年 | 437篇 |
1974年 | 440篇 |
1973年 | 445篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
33.
M.S. Hanner E. Tedesco A.T. Tokunaga G.J. Veeder D.F. Lester F.C. Witteborn J.D. Bregman J. Gradie L. Lebofsky 《Icarus》1985,64(1):11-19
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ~2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ~0.04 at 1.25 μm and ~0.05 at 2.2 μm. 相似文献
34.
35.
36.
37.
In the first paper of this series, we presented EBAS – Eclipsing Binary Automated Solver, a new fully automated algorithm to analyse the light curves of eclipsing binaries, based on the ebop code. Here, we apply the new algorithm to the whole sample of 2580 binaries found in the Optical Gravitational Lensing Experiment (OGLE) Large Magellanic Cloud (LMC) photometric survey and derive the orbital elements for 1931 systems. To obtain the statistical properties of the short-period binaries of the LMC, we construct a well-defined subsample of 938 eclipsing binaries with main-sequence B-type primaries. Correcting for observational selection effects, we derive the distributions of the fractional radii of the two components and their sum, the brightness ratios and the periods of the short-period binaries. Somewhat surprisingly, the results are consistent with a flat distribution in log P between 2 and 10 d. We also estimate the total number of binaries in the LMC with the same characteristics, and not only the eclipsing binaries, to be about 5000. This figure leads us to suggest that (0.7 ± 0.4) per cent of the main-sequence B-type stars in the LMC are found in binaries with periods shorter than 10 d. This frequency is substantially smaller than the fraction of binaries found by small Galactic radial-velocity surveys of B stars. On the other hand, the binary frequency found by Hubble Space Telescope ( HST ) photometric searches within the late main-sequence stars of 47 Tuc is only slightly higher and still consistent with the frequency we deduced for the B stars in the LMC. 相似文献
38.
We have investigated the central regions of the galaxies in the NGC 3169/NGC 3166/NGC 3156 group with the multipupil fiber spectrograph of the 6-m telescope; the first (central) galaxy in the group is a spiral (Sa) one and the other two galaxies are lenticular ones. The group is known to have an extended HI cloud with a size of more than 100 kpc that is associated in its position, orientation, and rotation with the central galaxy NGC 3169. The mean age of the stellar populations in the centers of all three galaxies has been found to be approximately the same, ~1 Gyr. Since the galaxies are early-type ones and since NGC 3166 and NGC 3156 show no global star formation, we are dealing here with a synchronous star formation burst in the centers of all three galaxies. 相似文献
39.
D. Montes M. C. Gálvez M. J. Fernández-Figueroa I. Crespo-Chacón 《Astrophysics and Space Science》2006,304(1-4):367-369
High resolution echelle spectroscopic observations taken with the FEROS spectrograph at the 2.2 m telescope ESO confirm the binary nature of the flare M3.5V star LU Vel (GJ 375, RE J0958-462) previously reported by Christian and Mathioudakis (2002). Emission of similar intensity from both components is detected in the Balmer, Na i D1&D2, He i D3, Ca ii H&K, and Ca ii IRT lines. We have determined precise radial velocities by cross correlation with radial velocity standard stars, which have allowed us to obtain for the first time the orbital solution of the system. The binary consists of two near-equal M3.5V components with an orbital period shorter than 2 days. We have analyzed the behaviour of the chromospheric activity indicators (variability and possible flares). In addition, we have determined its rotational velocity and kinematics. 相似文献
40.
Measurements of 18O concentrations in precipitation, soil solution, spring and runoff are used to determine water transit time in the small granitic Strengbach catchment (0·8 km2; 883–1146 m above sea level) located in the Vosges Mountains of northeastern France. Water transit times were calculated by applying the exponential, exponential piston and dispersion models of the FlowPC program to isotopic input (rainfall) and output (spring and stream water) data sets during the period 1989–95. The input function of the model was modified compared with the former version of the model and estimated by a deterministic approach based on a simplified hydrological balance. The fit between observed and calculated output data showed marked improvements compared with results obtained using the initial version of the model. An exponential piston version of the model applied to spring water indicates a 38·5 month mean transit time, which suggests that the volume in the aquifer, expressed in water depth, is 2·4 m. A considerable thickness (>45 m) of fractured bedrock may be involved for such a volume of water to be stored in the aquifer. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献