全文获取类型
| 收费全文 | 7371篇 |
| 免费 | 1146篇 |
| 国内免费 | 1773篇 |
专业分类
| 测绘学 | 92篇 |
| 大气科学 | 202篇 |
| 地球物理 | 894篇 |
| 地质学 | 4864篇 |
| 海洋学 | 613篇 |
| 天文学 | 2125篇 |
| 综合类 | 314篇 |
| 自然地理 | 1186篇 |
出版年
| 2024年 | 26篇 |
| 2023年 | 107篇 |
| 2022年 | 321篇 |
| 2021年 | 335篇 |
| 2020年 | 310篇 |
| 2019年 | 305篇 |
| 2018年 | 291篇 |
| 2017年 | 296篇 |
| 2016年 | 282篇 |
| 2015年 | 326篇 |
| 2014年 | 333篇 |
| 2013年 | 391篇 |
| 2012年 | 346篇 |
| 2011年 | 365篇 |
| 2010年 | 306篇 |
| 2009年 | 575篇 |
| 2008年 | 469篇 |
| 2007年 | 514篇 |
| 2006年 | 559篇 |
| 2005年 | 432篇 |
| 2004年 | 421篇 |
| 2003年 | 452篇 |
| 2002年 | 376篇 |
| 2001年 | 309篇 |
| 2000年 | 313篇 |
| 1999年 | 268篇 |
| 1998年 | 292篇 |
| 1997年 | 195篇 |
| 1996年 | 151篇 |
| 1995年 | 134篇 |
| 1994年 | 129篇 |
| 1993年 | 86篇 |
| 1992年 | 85篇 |
| 1991年 | 45篇 |
| 1990年 | 45篇 |
| 1989年 | 25篇 |
| 1988年 | 19篇 |
| 1987年 | 22篇 |
| 1986年 | 10篇 |
| 1985年 | 3篇 |
| 1984年 | 6篇 |
| 1983年 | 3篇 |
| 1982年 | 1篇 |
| 1981年 | 4篇 |
| 1980年 | 1篇 |
| 1979年 | 1篇 |
| 1978年 | 1篇 |
| 1977年 | 1篇 |
| 1954年 | 3篇 |
排序方式： 共有10000条查询结果，搜索用时 15 毫秒
61.
62.
Ignacio Ferreras Alessandro Melchiorri Domenico Tocchini-Valentini 《Monthly notices of the Royal Astronomical Society》2003,344(1):257-261
The age of the Universe has been increasingly constrained by different techniques, such as the observations of type Ia supernovae (SNIa) at high redshift or dating the stellar populations of globular clusters. In this paper, we present a complementary approach using the colours of the brightest elliptical galaxies in clusters over a wide redshift range ( z ≲ 1) . We put new and independent bounds on the dark energy equation of state parametrized by a constant pressure-to-density ratio w Q and by a parameter (ξ) which determines the scaling between the matter and dark energy densities. We find that accurate estimates of the metallicities of the stellar populations in moderate and high-redshift cluster galaxies can pose stringent constraints on the parameters that describe dark energy. Our results are in good agreement with the analysis of dark energy models using SNIa data as a constraint. Accurate estimates of the metallicities of stellar populations in cluster galaxies at z ≲ 2 will make this approach a powerful complement to studies of cosmological parameters using high-redshift SNIa. 相似文献
63.
近年来月球探测已经进入了一个全新的时代。特别是 1 990年以来 ,多个月球探测计划已经被成功实现 ,而且另外还有多个探测计划也在准备当中 ,并将在未来的几年内发射升空。在这种背景之下 ,中国的航天机构和有关的科学家也开始积极酝酿和开发自己的月球探测计划。这些月球探测计划将利用卫星上搭载的各种仪器探测和测量月球的地质和地理特性、化学成分和矿物组成、月球物理学特征以及包含地球大气在内的地月空间环境和行星际空间环境 ;进一步研究月球的起源和演化 ,探明月面环境 ,研究太阳等离子体物理 ,提供月面天文台和月面长期科研基地的候选地址 ,调查月球上的可利用资源 ,为将来开发月球提供充实的背景资料。参与新一轮的月球探测同样也为中国天文学研究带来了新的机会。 相似文献
64.
L. A. Nolan J. S. Dunlop R. Jimenez A. F. Heavens 《Monthly notices of the Royal Astronomical Society》2003,341(2):464-476
We explore whether the rest-frame near-ultraviolet spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by first testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-ultraviolet spectra of F stars of known (subsolar and supersolar) metallicity. F stars are of particular interest because the rest-frame near-ultraviolet spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turn-off.
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3–5 . 相似文献
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3–5 . 相似文献
65.
66.
67.
68.
69.
70.