全文获取类型
收费全文 | 281805篇 |
免费 | 10862篇 |
国内免费 | 10969篇 |
专业分类
测绘学 | 9163篇 |
大气科学 | 25192篇 |
地球物理 | 60178篇 |
地质学 | 101790篇 |
海洋学 | 24598篇 |
天文学 | 56522篇 |
综合类 | 4488篇 |
自然地理 | 21705篇 |
出版年
2022年 | 2642篇 |
2021年 | 3874篇 |
2020年 | 3881篇 |
2019年 | 4415篇 |
2018年 | 4556篇 |
2017年 | 4135篇 |
2016年 | 7137篇 |
2015年 | 5742篇 |
2014年 | 8802篇 |
2013年 | 15923篇 |
2012年 | 7941篇 |
2011年 | 9044篇 |
2010年 | 8324篇 |
2009年 | 10735篇 |
2008年 | 9540篇 |
2007年 | 8774篇 |
2006年 | 10740篇 |
2005年 | 8636篇 |
2004年 | 8308篇 |
2003年 | 7776篇 |
2002年 | 7391篇 |
2001年 | 6771篇 |
2000年 | 6890篇 |
1999年 | 6619篇 |
1998年 | 6336篇 |
1997年 | 6074篇 |
1996年 | 5636篇 |
1995年 | 5269篇 |
1994年 | 4833篇 |
1993年 | 4491篇 |
1992年 | 4143篇 |
1991年 | 4007篇 |
1990年 | 4056篇 |
1989年 | 3797篇 |
1988年 | 3544篇 |
1987年 | 3969篇 |
1986年 | 3531篇 |
1985年 | 4299篇 |
1984年 | 4806篇 |
1983年 | 4452篇 |
1982年 | 4362篇 |
1981年 | 3959篇 |
1980年 | 3666篇 |
1979年 | 3534篇 |
1978年 | 3481篇 |
1977年 | 3282篇 |
1976年 | 3041篇 |
1975年 | 2954篇 |
1974年 | 2912篇 |
1973年 | 3071篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
We consider the evolution of certain low-mass binaries, incorporating models of (a) internal evolution, (b) tidal friction,
(c) dynamo activity driven by an elementary α,Ω dynamo, (d) stellar wind driven by the activity, and (e) magnetic braking
as a consequence of wind and poloidal dynamo-generated magnetic field. In some circumstances the stellar wind is found to
remove mass on a nuclear timescale, as is necessary to explain some observed systems.
We can hope that various uncertainties in the model may be clarified by a careful comparison of the models with such observed
quantities as rotation periods. These are modified by processes (a), (b) and (e). Assuming that stellar evolution is slow,
rotation rate should in some circumstances represent a balance between magnetic braking trying to slow the star down and tidal
friction trying to spin it up. Preliminary attempts are promising, but indicate that some fine tuning is necessary.
When there is a third body present, in an orbit which is inclined but not necessarily of short period, the eccentricity of
a close binary can be strongly modified by ‘Kozai cycles’. We show that this may complicate attempts to account for spin rates
of stars in close binaries. 相似文献
102.
Daniela Lazzaro Marcos A. Florczak Alberto Betzler Othon C. Winter Silvia M. Giuliatti-Winter Claudia A. Angeli Dietmar W. Foryta 《Planetary and Space Science》1996,44(12):1547-1550
The results of photometric observations of comet/asteroid 2060 Chiron at the Observatório do Pico dos Dias (Brazil-OPD) and the Observatoire de Haute-Provence (France-OHP) during 1994 and 1995 are presented. The analysis of the data shows a decrease of 2060 Chiron brightness from its peak values of 1988–1991. The absolute magnitude, Hv, varies from a maximum of 6.6 in February 1994 up to a minimum of 6.8 in June 1995. Therefore 2060 Chiron is back to a minimum of activity close to that of 1983–1985. The slope parameter G is found to be G = 0.71 ± 0.15. It is suggested that the H-G magnitude system, generally adopted to present 2060 Chiron brightness, is not the most appropriate due to the cometary activity of this object. 相似文献
103.
104.
The observed times of minimum light derived from the photometry of the Wolf-Rayet eclipsing binary stars CQ Cep and V444 Cyg are used to estimate the mass-loss rate of the Wolf-Rayet components in several modes of mass-loss and mass-exchange. 相似文献
105.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques. 相似文献
106.
107.
Apostolos A. Christou 《Earth, Moon, and Planets》2004,95(1-4):425-431
Based on the number of planet-approaching cometary orbits at Mars and Venus relative to the Earth, there should be ample opportunities
for observing meteor activity at those two planets. The ratio of planet-approaching Jupiter family comets (JFCs) at Mars,
Earth, and Venus is 4:2:1 indicating that JFC-related outbursts would be more frequent at Mars than the Earth. The relative
numbers of planet-approaching Halley-type comets (HTCs) implies that the respective levels of annual meteor activity at those
three planets are similar. We identify several instances where near-comet outbursts (Jenniskens, P.: 1995, Astron. Astrophys. 295, 206–235) may occur. A possible double outburst of this type at Venus related to 45P/Honda-Mrkos-Padjusakova may be observable
by the ESA Venus Express spacecraft in the summer of 2006. Similarly, the Japanese Planet-C Venus orbiter may observe an outburst related to 27P/Crommelin’s perihelion passage in July 2011. Several additional opportunities
exist to observe such outbursts at Mars from 2019 to 2026 associated with comets 38P/Stephan-Oterma, 13P/Olbers and 114P/Wiseman-Skiff. 相似文献
108.
109.
The 362.156 GHz absorption spectrum of H2O2 in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0-30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124-150). Our derived H2O2 abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037-1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (Ls) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HOx abundances above ∼10 km altitudes between the Ls=112° period of the Encrenaz et al. upper limit measurement and the current Ls=250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785-12590). The observed H2O2 line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037-1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315-325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H2O2. Observation of Mars H2O2 also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own. 相似文献
110.
The elm decline of 5000 14C yr ago has been the most widely discussed phenomenon in post‐glacial vegetation history. This pan‐European reduction of elm populations, echoed in the decimation of elmwoods in Europe during the twentieth century, has attracted a series of interrelated hypotheses involving climate change, human activity, disease and soil deterioration. The elm bark beetle (Scolytus scolytus L.) is an essential component of disease explanations. We present evidence for the presence of the beetle over a prolonged period (ca. 7950–4910 yr BP [8800–5660 cal. yr BP]) from a lowland raised mire deposit in northeast Scotland, with its final appearance at this site, and the first and only appearance in another mire of a single scolytid find, around the time of the elm decline. The subfossil S. scolytus finds are not only the first from Scotland, but they also represent the most comprehensive sequence of finds anywhere. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献