全文获取类型
收费全文 | 20544篇 |
免费 | 532篇 |
国内免费 | 200篇 |
专业分类
测绘学 | 446篇 |
大气科学 | 202篇 |
地球物理 | 536篇 |
地质学 | 1857篇 |
海洋学 | 256篇 |
天文学 | 17363篇 |
综合类 | 95篇 |
自然地理 | 521篇 |
出版年
2024年 | 44篇 |
2023年 | 76篇 |
2022年 | 157篇 |
2021年 | 96篇 |
2020年 | 95篇 |
2019年 | 163篇 |
2018年 | 58篇 |
2017年 | 44篇 |
2016年 | 74篇 |
2015年 | 175篇 |
2014年 | 161篇 |
2013年 | 156篇 |
2012年 | 242篇 |
2011年 | 235篇 |
2010年 | 299篇 |
2009年 | 1614篇 |
2008年 | 1560篇 |
2007年 | 1842篇 |
2006年 | 1840篇 |
2005年 | 1690篇 |
2004年 | 1787篇 |
2003年 | 1539篇 |
2002年 | 1344篇 |
2001年 | 1162篇 |
2000年 | 947篇 |
1999年 | 891篇 |
1998年 | 1059篇 |
1997年 | 240篇 |
1996年 | 108篇 |
1995年 | 295篇 |
1994年 | 306篇 |
1993年 | 129篇 |
1992年 | 77篇 |
1991年 | 93篇 |
1990年 | 91篇 |
1989年 | 146篇 |
1988年 | 97篇 |
1987年 | 96篇 |
1986年 | 79篇 |
1985年 | 45篇 |
1984年 | 30篇 |
1983年 | 28篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1977年 | 6篇 |
1905年 | 3篇 |
1900年 | 3篇 |
1897年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
461.
462.
The integrated optical spectrum of the Crab nebula and evidence for its fading synchrotron continuum
Nathan Smith † 《Monthly notices of the Royal Astronomical Society》2003,346(3):885-889
A flux-calibrated optical spectrum integrated over the entire Crab nebula was obtained by making drift scans with a long-slit spectrograph. Compared to observations obtained over the past 40 years, these new data confirm an earlier controversial result that the [O iii ] λλ4959, 5007 equivalent width is increasing with time, although the rate of ∼0.9 per cent yr−1 is somewhat slower than that measured previously. Additionally, the Hβ equivalent width is increasing at a comparable rate, but the measured fluxes of both Hβ and [O iii ] have changed less than their respective equivalent widths. The different rates of change in the measured fluxes and equivalent widths of these lines suggest that the optical synchrotron continuum from the Crab nebula is indeed fading rapidly. The apparent decline is consistent with a rate around −0.5 (±0.2) per cent yr−1 at wavelengths near 5000 Å inferred independently from measurements of the optical continuum flux during the same time period. 相似文献
463.
464.
465.
466.
467.
468.
469.
We present predictions for the numbers of ultracool dwarfs in the Galactic disc population that could be detected by the WFCAM/UKIDSS Large Area Survey and Ultra Deep Survey. Simulated samples of objects are created with masses and ages drawn from different mass functions and birthrates. Each object is then given absolute magnitudes in different passbands based on empirically derived bolometric correction versus effective temperature relationships (or model predictions for Y dwarfs). These are then combined with simulated space positions, velocities and photometric errors to yield observables such as apparent magnitudes and proper motions. Such observables are then passed through the survey selection mechanism to yield histograms in colour. This technique also produces predictions for the proper motion histograms for ultracool dwarfs and estimated numbers for the as yet undetected Y dwarfs. Finally, it is shown that these techniques could be used to constrain the ultra-low-mass mass function and birthrate of the Galactic disc population. 相似文献
470.
A flare of OH maser emission was discovered in W75N in 2000. Its location was determined with the Very Long Baseline Array (VLBA) to be within 110 au from one of the ultracompact H ii regions, Very Large Array 2 (VLA2). The flare consisted of several maser spots. Four of the spots were found to form Zeeman pairs, all of them with a magnetic field strength of about 40 mG. This is the highest ever magnetic field strength found in OH masers, an order of magnitude higher than in typical OH masers. Three possible sources for the enhanced magnetic field are discussed: (i) the magnetic field of the exciting star dragged out by the stellar wind; (ii) the general interstellar field in the gas compressed by the magnetohydrodynamic shock; and (iii) the magnetic field of planets which orbit the exciting star and produce maser emission in gaseous envelopes. 相似文献