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   61929篇
  免费   1301篇
  国内免费   1350篇
测绘学   1492篇
大气科学   4488篇
地球物理   12124篇
地质学   23186篇
海洋学   5863篇
天文学   13767篇
综合类   302篇
自然地理   3358篇
  2022年   538篇
  2021年   851篇
  2020年   892篇
  2019年   962篇
  2018年   1838篇
  2017年   1744篇
  2016年   1990篇
  2015年   1188篇
  2014年   1904篇
  2013年   3293篇
  2012年   2293篇
  2011年   2962篇
  2010年   2613篇
  2009年   3272篇
  2008年   2808篇
  2007年   2901篇
  2006年   2765篇
  2005年   1840篇
  2004年   1709篇
  2003年   1588篇
  2002年   1578篇
  2001年   1366篇
  2000年   1305篇
  1999年   1066篇
  1998年   1130篇
  1997年   1049篇
  1996年   916篇
  1995年   857篇
  1994年   810篇
  1993年   687篇
  1992年   674篇
  1991年   626篇
  1990年   730篇
  1989年   569篇
  1988年   554篇
  1987年   690篇
  1986年   528篇
  1985年   704篇
  1984年   769篇
  1983年   674篇
  1982年   688篇
  1981年   589篇
  1980年   623篇
  1979年   525篇
  1978年   520篇
  1977年   481篇
  1976年   447篇
  1975年   450篇
  1974年   449篇
  1973年   484篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
311.
?????HYPODD???????????????о????λ????????????CAP?????????????????????????????5??????????????????????????ε??????????????????????????????????????????????е??????????????????????????????????????????????????????????????????-??????????????????????????????????Σ????????о??и??μ????λ????5.5?????????????????????????????????????λ???λ??7??5.5???????????????4????????????????3?????????????????????????-??????????-????4?????????????????????  相似文献   
312.
Coral-reef terraces on tectonically elevated coasts provide important data on variations of sea level. In this paper a systematic approach is presented for the construction of synthetic reef terraces. Idealized reef terraces are generated assuming a constant rate of tectonic uplift ν and a harmonic variation of sea level with amplitudehwo and period τ. It is further assumed that coral terraces grow only during periods of relative sea-level rise. The behavior is governed by the parameter u = τυ/2π hwo. If u > 1 no reef terraces form. If 0.217 < u < 1 a discontinuous series of terraces form on the uplifting basement. If 0 < u < 0.217 the basement is covered with coral and a series of coral terraces forms on this coral. Uplifted coral-reef terraces on the Huon Peninsula, Papua New Guinea, compare reasonably well with terraces generated by our model with τ = 20,000 years andhwo = 21.4 m. We also compare the observed terraces with model terraces using the paleo sea level inferred from18O/16O data. The observed terraces are qualitatively similar to those predicted using the oxygen isotope data, however, the model terraces require larger relative sea-level variations with a near 20,000 year period than those inferred directly from the18O/16O data.  相似文献   
313.
Pulkovo astrometric observations began in the 1840s using the Repsold transit instrument in the prime vertical and Ertel vertical circle. The first observers on these instruments were W.I. Struve, 1840–1856, and Kh.I. Peters, 1842–1849. In the present work, we collected and analyzed different series of latitude variations from observations made by M.O. Nuren, B. Wanach, A.A. Ivanov, I.N. Bonsdorf, and A.Ya. Orlov. In addition, results are given of investigations of a specific behavior of the Chandler polar motion in this interval, obtained by C. Chandler, Ivanov, Kh. Kimura, Orlov, and N. Sekiguchi. The aim of this paper is to search for and analyze the earliest series of Pulkovo latitudes, in order to evaluate the possibility of their use to study the motion of the pole at the maximum available range of observations. Different methods were used to isolate and analyze the sum of Chandler and annual latitude variations. The annex provides a series of Pulkovo latitude variations for 1840–1848, which may be used to extend latitude variation back to 1840.  相似文献   
314.
Herein, the relationship between soil radon behavior and groundwater recharge in the town of Hosha, a semi-arid area of Jordan is explored. The annual rainfall average in this area is between 60–100 mm year−1. Soil radon concentration levels, using CR-39 detectors, have been measured at several depths. Nine sampling sites on a hill slope, in the summer and winter 2006 were chosen. Other soil parameters such as moisture, soil type and porosity were also measured. Correlation between the latter and the calculated effective diffusion coefficients, based on a linear approximation, was done in order to explore water movement. A weak correlation between the soil radon concentration and water recharge in this particular semi-arid area is observed. The results show that water recharge takes place vertically, i.e., water moves downward and in one of the nine sampling sites this movement seems to be more rapid compared to the other sites.  相似文献   
315.
We have searched for very high energy (VHE) gamma rays from four blazars using the CANGAROO-III imaging atmospheric Cherenkov telescope. We report the results of the observations of H 2356-309, PKS 2155-304, PKS 0537-441, and 3C 279, performed from 2005 to 2009, applying a new analysis to suppress the effects of the position dependence of Cherenkov images in the field of view. No significant VHE gamma ray emission was detected from any of the four blazars. The GeV gamma-ray spectra of these objects were obtained by analyzing Fermi/LAT archival data. Wide range (radio to VHE gamma-ray bands) spectral energy distributions (SEDs) including CANGAROO-III upper limits, GeV gamma-ray spectra, and archival data, even though they are non-simultaneous, are discussed using a one-zone synchrotron self-Compton (SSC) model in combination with a external Compton (EC) radiation. The HBLs (H 2356-309 and PKS 2155-304) can be explained by a simple SSC model, and PKS 0537-441 and 3C 279 are well modeled by a combination of SSC and EC model. We find a consistency with the blazar sequence in terms of strength of magnetic field and component size.  相似文献   
316.
We report on the discovery of 25 variable stars plus 13 suspected variables found in the field of the open cluster NGC 6819. The stars were identified from time-series photometric data obtained on the Isaac Newton Telescope, La Palma, during two observing runs covering the 19 nights between 1999 June  22–30  and 1999 July  22–31  . The variables found include 12 eclipsing binaries with an additional three suspected, nine BY Draconis systems, plus four variables of other types, including one star believed to be a Cepheid. Three of the 15 eclipsing binaries are believed to be cluster members. Details of a further 10 suspected variable stars are also included.  相似文献   
317.
SEM back-scattered electron images of zircon separates from a 2.9 Ga granitic dyke from the Acasta Gneiss Complex, Slave Province, reveal primary igneous oscillatory zoning which is transgressively overprinted by irregular alteration domains. Electron microprobe analyses show that altered zircon domains are depleted in Zr and Si relative to unaltered zircon while retaining a constant Zr/Si ratio. SIMS trace element analyses indicate that LREE are preferentially enriched in the altered domains. Altered zircon contains elevated concentrations of Ba (up to 580 ppm), Ca (up to 2% weight of the element) and common Pb (>50 ppb). The presence of LREE, Ca, Ba and common Pb in the zircons is interpreted to be the result of secondary alteration by an aqueous fluid, and not by primary incorporation during crystallization. We propose that the alteration-related elements are located in amorphous zircon domains.  相似文献   
318.
An elasto‐viscoplastic constitutive model for asphaltic materials is presented within the context of bounding surface plasticity theory, taking into account the effects of the stress state, void binder degree of saturation, temperature and strain rate on the material behaviour. A stress state dependent non‐linear elasticity model is introduced to represent time‐independent recoverable portion of the deformation. The consistent visco‐plasticity framework is utilised to capture the rate‐dependent, non‐recoverable strain components. The material parameters introduced in the model are identified, and their determination from conventional laboratory tests is discussed. The capability of the model to reproduce experimentally observed response of asphaltic materials is demonstrated through numerical simulations of several laboratory test data from the literature. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
319.
We have monitored S Ori 45, a young, low-mass (20 M j up) brown dwarf of the σ Orionis cluster (~3 Myr, 352 pc), using optical and near-infrared filters. S Ori 45 (spectral type M8.5) is found to be multi-periodic with a dominant modulation at 2.5–3.5 h, and a short modulation at about 46 min. We ascribe the longer of these modulations to a rotation period. After comparing these results with observations of more massive cluster brown dwarfs and field brown dwarfs, we conclude that substellar objects present rotational and angular momentum evolution. We have also obtained intermediate-resolution near-infrared spectroscopy of S Ori 70, which is a T-class, free-floating planetary candidate member in the σ Orionis cluster. Its observed spectrum has been compared to data of field brown dwarfs of similar types and to theoretical spectra computed for different surface temperatures and gravities. We conclude that S Ori 70 has a significantly cool, low-gravity atmosphere. This supports the young age of this object and its membership in the cluster. From state-of-the-art evolutionary models, the mass of S Ori 70 is estimated at 3 times the Jovian mass (+5 ?2 M j up), challenging current stellar/substellar formation models. S Ori 70 remains the lowest mass object so far identified in any open cluster.  相似文献   
320.
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