全文获取类型
收费全文 | 73472篇 |
免费 | 1356篇 |
国内免费 | 564篇 |
专业分类
测绘学 | 1936篇 |
大气科学 | 5152篇 |
地球物理 | 14912篇 |
地质学 | 25525篇 |
海洋学 | 6481篇 |
天文学 | 16970篇 |
综合类 | 191篇 |
自然地理 | 4225篇 |
出版年
2021年 | 668篇 |
2020年 | 769篇 |
2019年 | 917篇 |
2018年 | 1763篇 |
2017年 | 1714篇 |
2016年 | 2040篇 |
2015年 | 1138篇 |
2014年 | 1934篇 |
2013年 | 3526篇 |
2012年 | 2181篇 |
2011年 | 2911篇 |
2010年 | 2632篇 |
2009年 | 3443篇 |
2008年 | 3052篇 |
2007年 | 3150篇 |
2006年 | 2904篇 |
2005年 | 2141篇 |
2004年 | 2160篇 |
2003年 | 2081篇 |
2002年 | 2032篇 |
2001年 | 1775篇 |
2000年 | 1650篇 |
1999年 | 1468篇 |
1998年 | 1462篇 |
1997年 | 1438篇 |
1996年 | 1201篇 |
1995年 | 1150篇 |
1994年 | 1035篇 |
1993年 | 929篇 |
1992年 | 822篇 |
1991年 | 849篇 |
1990年 | 855篇 |
1989年 | 820篇 |
1988年 | 751篇 |
1987年 | 852篇 |
1986年 | 806篇 |
1985年 | 969篇 |
1984年 | 1129篇 |
1983年 | 1041篇 |
1982年 | 979篇 |
1981年 | 916篇 |
1980年 | 824篇 |
1979年 | 787篇 |
1978年 | 772篇 |
1977年 | 736篇 |
1976年 | 653篇 |
1975年 | 660篇 |
1974年 | 627篇 |
1973年 | 697篇 |
1972年 | 470篇 |
排序方式: 共有10000条查询结果,搜索用时 593 毫秒
131.
D. H. Lumb A. Finoguenov R. Saxton B. Aschenbach P. Gondoin M. Kirsch I. M. Stewart 《Experimental Astronomy》2003,15(2):89-111
We describe measurements of the mirror vignetting in the XMM-Newton Observatory made in-orbit, using observations of SNR G21.5-09
and SNR 3C58 with the EPIC imaging cameras. The instrument features that complicate these measurements are briefly described.
We show the spatial and energy dependences of measured vignetting, outlining assumptions made in deriving the eventual agreement
between simulation and measurement. Alternate methods to confirm these are described, including an assessment of source elongation
with off-axis angle, the surface brightness distribution of the diffuse X-ray background, and the consistency of Coma cluster
emission at different position angles. A synthesis of these measurements leads to a change in the XMM calibration data base,
for the optical axis of two of the three telescopes, by in excess of 1 arcmin. This has a small but measureable effect on
the assumed spectral responses of the cameras for on-axis targets.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
132.
133.
A major difficulty in remote sensing is handling the many data from sensors aboard aircraft and satellites. In this paper we identify an optimal procedure for sampling remotely sensed data before their storage or on their retrieval. The procedure depends on spatial correlation in the scene and uses kriging to estimate values that have been lost. An example in which data from an airborne multispectral scanner could be diminished to only about one tenth without serious loss of precision illustrates the method. 相似文献
134.
We present two spectra of the supernova SN1988A in M58 (NGC4579) over the wavelength range 4000–9700 Å, as recorded by the Faint Object Spectrograph on the 4.2 m William Herschel Telescope at the Observatorio del Roque de los Muchachos. We conclude that SN1988A was a type II supernova. 相似文献
135.
G. A. GRAHAM P. G. GRANT R. J. CHATER A. J. WESTPHAL A. T. KEARSLEY C. SNEAD G. DOMÍNGUEZ A. L. BUTTERWORTH D. S. McPHAIL G. BENCH J. P. BRADLEY 《Meteoritics & planetary science》2004,39(9):1461-1473
Abstract— In 2006, the Stardust spacecraft will return to Earth with cometary and perhaps interstellar dust particles embedded in silica aerogel collectors for analysis in terrestrial laboratories. These particles will be the first sample return from a solid planetary body since the Apollo missions. In preparation for the return, analogue particles were implanted into a keystone of silica aerogel that had been extracted from bulk silica aerogel using the optical technique described in Westphal et al. (2004). These particles were subsequently analyzed using analytical techniques associated with the use of a nuclear microprobe. The particles have been analyzed using: a) scanning transmission ion microscopy (STIM) that enables quantitative density imaging; b) proton elastic scattering analysis (PESA) and proton backscattering (PBS) for the detection of light elements including hydrogen; and c) proton‐induced X‐ray emission (PIXE) for elements with Z > 11. These analytical techniques have enabled us to quantify the composition of the encapsulated particles. A significant observation from the study is the variable column density of the silica aerogel. We also observed organic contamination within the silica aerogel. The implanted particles were then subjected to focused ion beam (FIB) milling using a 30 keV gallium ion beam to ablate silica aerogel in site‐specific areas to expose embedded particles. An ion polished flat surface of one of the particles was also prepared using the FIB. Here, we show that ion beam techniques have great potential in assisting with the analysis and exposure of Stardust particles. 相似文献
136.
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
137.
138.
The occultation of Io by Ganymede as observed on 10 June 1985 is reported. The middle of the occultation minimum was found to occur at 14h 14m 5.7s UT. In the plane perpendicular to the line of sight the centres of the two satellite disks passed to within ~ 2530 km of each other, at a relative velocity, in this plane, of about 11 kms–1. The values of these last two quantities, however, depend on what assumptions are made about the light distribution over io's disk. 相似文献
139.
A non-static exact solution of the Einstein equations corresponding to a conformally invariant scalar field with trace-free energy momentum tensor is obtained for the Robertson-Walker type metric. Some physical properties of the solution are discussed. 相似文献
140.