全文获取类型
| 收费全文 | 36539篇 |
| 免费 | 3354篇 |
| 国内免费 | 3675篇 |
专业分类
| 测绘学 | 1820篇 |
| 大气科学 | 2319篇 |
| 地球物理 | 3008篇 |
| 地质学 | 10152篇 |
| 海洋学 | 3043篇 |
| 天文学 | 17439篇 |
| 综合类 | 1327篇 |
| 自然地理 | 4460篇 |
出版年
| 2024年 | 89篇 |
| 2023年 | 236篇 |
| 2022年 | 657篇 |
| 2021年 | 717篇 |
| 2020年 | 680篇 |
| 2019年 | 882篇 |
| 2018年 | 656篇 |
| 2017年 | 704篇 |
| 2016年 | 755篇 |
| 2015年 | 926篇 |
| 2014年 | 1090篇 |
| 2013年 | 968篇 |
| 2012年 | 1271篇 |
| 2011年 | 1265篇 |
| 2010年 | 1180篇 |
| 2009年 | 2676篇 |
| 2008年 | 2501篇 |
| 2007年 | 3016篇 |
| 2006年 | 3020篇 |
| 2005年 | 2746篇 |
| 2004年 | 2749篇 |
| 2003年 | 2457篇 |
| 2002年 | 2160篇 |
| 2001年 | 1794篇 |
| 2000年 | 1463篇 |
| 1999年 | 1368篇 |
| 1998年 | 1447篇 |
| 1997年 | 592篇 |
| 1996年 | 449篇 |
| 1995年 | 580篇 |
| 1994年 | 549篇 |
| 1993年 | 343篇 |
| 1992年 | 274篇 |
| 1991年 | 253篇 |
| 1990年 | 176篇 |
| 1989年 | 275篇 |
| 1988年 | 150篇 |
| 1987年 | 132篇 |
| 1986年 | 93篇 |
| 1985年 | 60篇 |
| 1984年 | 37篇 |
| 1983年 | 35篇 |
| 1982年 | 17篇 |
| 1981年 | 9篇 |
| 1980年 | 6篇 |
| 1979年 | 11篇 |
| 1978年 | 6篇 |
| 1977年 | 9篇 |
| 1897年 | 7篇 |
| 1877年 | 4篇 |
排序方式： 共有10000条查询结果，搜索用时 281 毫秒
41.
42.
V2213 Cyg was discovered as a variable star by Pavlenko (1999) in 1998. We present our photometry of V2213 Cyg from 1998–2003
based on CCD observations with the K-380 Cassegrain telescope of CrAO and the 60 cm Zeiss telescope of SAI. Observations have
been carried out mostly in R and sometimes in B and V Johnson system. The total amount of data is 2270 points, covering ∼50 nights. We classify this binary as a W UMa-type contact
system. Using all data we determined the orbital period to be 0.350079 ± 0.000007 day. The mean brightness varies between
R = 14.35 and 14.05. The mean 1999–2003 orbital light curve has two humps and a primary minimum (I), which is 0.04 mag brighter
than the deeper secondary one (II). The mean humps have slightly different height. The difference between two individual maxima
varies within 0.1 mag, which may indicate an activity of the components. The highest hump is an asymmetrical one: it has sort
of a shoulder at phases 0.75–0.80, before entering the less deep primary minimum (phase 0.0). The system is rather reddened,
its colour indices are: B−V ∼ 0.8 and V−R ∼ 0.7, and give a spectral class of V2213 Cyg earlier than K. 相似文献
43.
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
44.
45.
A highly automated moving object detection package 总被引：1，自引：0，他引：1
46.
47.
Cygnus A was the first hyper-active galaxy discovered, and it
remains by far the closest of the ultra-luminous radio galaxies. As
such, Cygnus A has played a fundamental role in the study of virtually
all aspects of extreme activity in galaxies. We present a review of jet
theory for powering the double-lobed radio emitting structures in
powerful radio galaxies, followed by a review of observations of Cygnus
A in the radio, optical, and X-ray relevant to testing various aspects
of jet theory. Issues addressed include: jet structure from pc- to
kpc-scales, jet stability, confinement, composition, and velocity, the
double shock structure for the jet terminus and the origin of multiple
radio hotspots, the nature of the filamentary structure in the radio
lobes, and the hydrodynamic evolution of the radio lobes within a dense
cluster atmosphere, including an analysis of pressure balance between
the various gaseous components. Also discussed are relativistic
particle acceleration and loss mechanisms in Cygnus A, as well as
magnetic field strengths and geometries both within the radio source,
and in the intracluster medium. We subsequently review the
classification, cluster membership, and the emission components of the
Cygnus A galaxy. The origin of the activity is discussed.
Concentrating on the nuclear regions of the galaxy, we review the
evidence for an obscured QSO, also given the constraints on the
orientation of the radio source axis with respect to the sky plane. We
present an overview of models of central engines in AGN and observations
of Cygnus A which may be relevant to testing such models. We conclude
with a brief section concerning the question of whether Cygnus A is
representative of powerful high redshift radio galaxies.
Received October 10, 1995 相似文献
48.
莱州湾南岸潍河下游地区咸水入侵灾害成因及特征 总被引：8，自引：1，他引：7
通过对莱州湾南岸咸水入侵较严重地潍河下游地区晚更新世以来沉积特征及现代自然环境条件变化的分析，探讨了沉积相对咸水入侵产生及空间范围特征的环境机理。晚更新世以来的三次海平面升降变化造成了潍河下游地区海陆沉积环境交替，形成了巨厚的海陆交互相沉积层。海进时期，大面积的滨海平原被淹没，在近海平原洼地滞留的海水经过蒸发、浓缩变为卤水，成为咸水入侵的物源；海退后陆源碎屑在滨海地区沉积形成了巨厚的古河道砂层。20世纪70年代末期以来，随着对地下淡水的过度开采，淡咸水水头压力差减小．卤水通过古河道砂层快速南侵。通过对潍河下游地区100余个地质钻孔水化学连续监测资料分析，阐明了咸水入侵的特征，有针对性地提出了咸水入侵的防治措施。 相似文献
49.
50.