首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   181326篇
  免费   19868篇
  国内免费   43302篇
测绘学   6297篇
大气科学   31635篇
地球物理   41880篇
地质学   85257篇
海洋学   28987篇
天文学   34925篇
综合类   4389篇
自然地理   11126篇
  2022年   1097篇
  2021年   1959篇
  2020年   3353篇
  2019年   6930篇
  2018年   9122篇
  2017年   8605篇
  2016年   9525篇
  2015年   7008篇
  2014年   8215篇
  2013年   12542篇
  2012年   8785篇
  2011年   9781篇
  2010年   9305篇
  2009年   10274篇
  2008年   8761篇
  2007年   8634篇
  2006年   7872篇
  2005年   6745篇
  2004年   6971篇
  2003年   6512篇
  2002年   6065篇
  2001年   5372篇
  2000年   4904篇
  1999年   4469篇
  1998年   4550篇
  1997年   4535篇
  1996年   3866篇
  1995年   3623篇
  1994年   3309篇
  1993年   3108篇
  1992年   2800篇
  1991年   2588篇
  1990年   2611篇
  1989年   2299篇
  1988年   2117篇
  1987年   2286篇
  1986年   2025篇
  1985年   2382篇
  1984年   2654篇
  1983年   2453篇
  1982年   2382篇
  1981年   2151篇
  1980年   1904篇
  1979年   1815篇
  1978年   1724篇
  1977年   1580篇
  1976年   1485篇
  1975年   1397篇
  1974年   1464篇
  1973年   1498篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
RATS is an Italian project devoted to Hot Jupiter search with the transit method. A planet transiting in front of a host star can be mimed by several, and well defined, astrophysical phenomena (Brown, 2003). In order to recognize these false alarms we can utilize a preventive strategy to limit false alarm rates and a spectroscopic follow up to refuse no transit candidates. As preventive strategy it is important to develop an accurate target field selection, with well defined requisites, in order to maximize the solar type star numbers and to minimize the risk of possible astrophysical false alarms.  相似文献   
872.
We carried out optical and Hα photometry of two contact binaries (V861 Herculis, EQ Tauri). The light curve modeling revealed stellar spots in both contact systems and strong Hα excess in the position of the observed stellar spots. A correlation was found between the VR and R−Hα colour indices of V861 Her.  相似文献   
873.
In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water-ice in subsurface layers and about peculiarities of the CO2 seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water-ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10-20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO2 deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3×1015 kg, which is larger than the total mass of the seasonal deposition at the north by 40-50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO2 occurred 10-15 degrees of Ls later than is predicted by the GCM.  相似文献   
874.
Armoured jawless fish, or 'ostracoderms', lived 450–360 million years ago, and display unusual morphologies, unlike any modern fish group. Since they left no living descendants, their mode of swimming has, until recently, remained speculative, although this is a crucial question as the first true pectoral fins evolved within the 'ostracoderms'. The discovery of the oldest-known fish trails, from the Early Devonian (400 million year old) 'Lower Old Red Sandstone' of south-east Wales offers new insights into the swimming behaviour of these early fish, notably the osteostracan 'ostracoderms' (or cephalaspids), whose horseshoe-shaped head and paddle-shaped pectoral fins have remained a functional riddle.  相似文献   
875.
The new polyenzyme method for making gravy fromHarengula zunasi offal involves protein enzymolysis with flavorase after proper alkaline and neutral protease levels were established by orthogonal trials to select the best hydrolytic conditions for processing offal with alkaline and neutral protease. The conditions for the polyenzyme method were pH of 7.0, temperature of 50°C, alkaline and neutral protease concentration of 1.5% respectively, hydrolysis time of 120 min, and flavorase concentration of 2.0%, for 60 min. The new gravy-making technology yields a nutritious and delicious gravy containing 40.3% of total essential amino acids, with delicious amino acids Glu, Asp, Gly, Ala, Pro and Ser comprising 49.5%, total and amino nitrogen being respectively 1.9 and 1.1 g/100 g (amino acid nitrogen being 61.0% of total nitrogen), The polyenzyme method was used to make 14.8% protein gravy fromHarengula zunasi offal. In addition, inorganic elements, the phosphorus content is the highest. This project was aided financially by the Guangdong Science and Technology Office (No. 2KM06002S).  相似文献   
876.
877.
878.
879.
880.
an extension of the Gylden-Meshcherskii problem when the mass depends both on time and distance between two bodies is considered. certain mass loss laws as well as the Meshcherskii position vector and time transformation are used to convert the problem into the cases with equations of motion arising from integrable potentials.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号