全文获取类型
收费全文 | 91079篇 |
免费 | 5958篇 |
国内免费 | 7284篇 |
专业分类
测绘学 | 2829篇 |
大气科学 | 10955篇 |
地球物理 | 18844篇 |
地质学 | 38916篇 |
海洋学 | 8490篇 |
天文学 | 14402篇 |
综合类 | 3472篇 |
自然地理 | 6413篇 |
出版年
2023年 | 475篇 |
2022年 | 1346篇 |
2021年 | 1751篇 |
2020年 | 1569篇 |
2019年 | 1716篇 |
2018年 | 5257篇 |
2017年 | 4862篇 |
2016年 | 4208篇 |
2015年 | 1991篇 |
2014年 | 2724篇 |
2013年 | 3609篇 |
2012年 | 3621篇 |
2011年 | 5652篇 |
2010年 | 5526篇 |
2009年 | 6065篇 |
2008年 | 4971篇 |
2007年 | 5605篇 |
2006年 | 2774篇 |
2005年 | 2822篇 |
2004年 | 2444篇 |
2003年 | 2498篇 |
2002年 | 2243篇 |
2001年 | 1876篇 |
2000年 | 2011篇 |
1999年 | 2255篇 |
1998年 | 2013篇 |
1997年 | 2110篇 |
1996年 | 1745篇 |
1995年 | 1645篇 |
1994年 | 1499篇 |
1993年 | 1344篇 |
1992年 | 1151篇 |
1991年 | 940篇 |
1990年 | 815篇 |
1989年 | 838篇 |
1988年 | 762篇 |
1987年 | 748篇 |
1986年 | 644篇 |
1985年 | 718篇 |
1984年 | 748篇 |
1983年 | 666篇 |
1982年 | 602篇 |
1981年 | 642篇 |
1980年 | 532篇 |
1979年 | 485篇 |
1978年 | 449篇 |
1977年 | 439篇 |
1976年 | 392篇 |
1975年 | 382篇 |
1973年 | 386篇 |
排序方式: 共有10000条查询结果,搜索用时 656 毫秒
31.
32.
本文论述了珠穆朗玛峯地区土壤中稀土元素的含量,以及它们在土壤中的分布模式,并发现土壤中稀土元素的含量水平在很大程度上受土壤母质的制约。 相似文献
33.
在回转椭球状降水粒子的假设下求出了降水粒子的散射截面,并且考虑了粒子轴线铅垂和随机取向两种重要的典型情况;推导了双极化雷达的回波方程以及降雨量公式。该方程除包含降水粒子的信息外,关于雷达与降水粒子间的几何关系都以雷达天线仰角及雷达与降水粒子间的距离表示,同时还考虑了地球的球面效应和大气折射效应。由于采用了并矢方法,使推导简捷明了 相似文献
34.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during
1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal
that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles.
The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease
of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles
of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed. 相似文献
35.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem. 相似文献
36.
37.
W. H. Jefferys G. F. Benedict P. D. Hemenway P. J. Shelus R. L. Duncombe 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):299-305
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed. 相似文献
38.
Maarten A. Prins Hongbo Zheng Kay Beets Simon Troelstra Patrick Bacon Ilse Kamerling Wouter Wester Martin Konert Xiangtong Huang Wang Ke Jef Vandenberghe 《第四纪科学杂志》2009,24(1):75-84
The Mangshan Plateau is located on the south bank of the Huang He (Yellow River) just west of the city of Zhengzhou, well outside the Loess Plateau in central China. Mixing models of the grain‐size data indicate that the loess deposits are mixtures of three loess components. Comparison of the mixing model with existing models established for a series of loess–palaeosol sequences from the Loess Plateau indicates that the Mangshan loess has been supplied from a proximal dust source, the Huang He floodplain, during major dust outbreaks. The high accumulation rates, the composition of the loess components, and especially the high proportions of a sandy loess component support this. Owing to the exceptionally high accumulation rates, the Mangshan grain size, magnetic susceptibility and carbonate records provide a high‐resolution archive of environmental and climate change. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
39.
40.
Starfish oocytes with intact germinal vesicles (GVs) were cut along desired planes with glass needles or ligated using silk
thread loops into two parts and allowed to mature in vitro, and inseminated. The experimental results showed that (1) only
the parts with GVs or partial GV contents (PGVCs) cleaved, those without any GV materials did not; but nucleated and non-nucleated
fragments cut from mature eggs were able to divide; (2) the development of animal parts of oocytes containing GVs or PGVCs
was like that of animal fragments of matured oocytes with female pronuclei; most of them gave rise to permanent blastulae,
and just a few formed ectodermal vesicles with a little primary mesenchyme; (3) a large part of vegetal fragments with GVs
or PGVCs, and the vegetal parts of mature eggs without female pronuclei developed into small but normal embryos; (4) the fragments
containing GVs or PGVCs obtained from the oocytes along a plane parallel to the animal-vegetal (A-V) axis developed as normally
as the halves (with or without female pronuclei) severed from mature eggs along the same axis. Based on the data above, it
was concluded that (1) the non-chromatin materials in the oocyte GVs are indispensable for successful fertilization and cleavage
of starfish eggs; (2) some factor (s) located asymmetrically in the vegetal hemispheres of starfish oocytes is (are) responsible
for formation of the archenteron and primary mesenchyme. It is evident from the above findings that the oocyte cytoplasm of
the starfish had already regionalized before the GV break-down.
Contribution No. 1722 from the Institute of Oceanology, Academia Sinica 相似文献