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   68019篇
  免费   1232篇
  国内免费   672篇
测绘学   1690篇
大气科学   5407篇
地球物理   13697篇
地质学   23712篇
海洋学   5936篇
天文学   15092篇
综合类   138篇
自然地理   4251篇
  2021年   527篇
  2020年   561篇
  2019年   640篇
  2018年   1375篇
  2017年   1262篇
  2016年   1697篇
  2015年   1013篇
  2014年   1594篇
  2013年   3360篇
  2012年   1764篇
  2011年   2487篇
  2010年   2207篇
  2009年   3001篇
  2008年   2512篇
  2007年   2482篇
  2006年   2446篇
  2005年   2067篇
  2004年   2009篇
  2003年   1944篇
  2002年   1934篇
  2001年   1685篇
  2000年   1646篇
  1999年   1373篇
  1998年   1328篇
  1997年   1379篇
  1996年   1161篇
  1995年   1116篇
  1994年   1016篇
  1993年   929篇
  1992年   878篇
  1991年   860篇
  1990年   881篇
  1989年   846篇
  1988年   796篇
  1987年   968篇
  1986年   870篇
  1985年   1027篇
  1984年   1151篇
  1983年   1074篇
  1982年   990篇
  1981年   1008篇
  1980年   870篇
  1979年   847篇
  1978年   820篇
  1977年   799篇
  1976年   709篇
  1975年   700篇
  1974年   728篇
  1973年   738篇
  1972年   446篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
32.
33.
The growth of Glasgow from a small town into a heavily industrialised conurbation depended greatly on its local geological riches. Extensive mining and quarrying of a range of minerals took place from the 18th century onwards. The early underground stoop and room (pillar and stall) workings, and the backfilled quarries, together with variably consolidated natural superficial deposits, have bequeathed to the city a heritage of unwanted engineering problems which cannot easily be quantified. Recent work by the British Geological Survey (BGS) illustrates the scale of both geological and man-made problems.  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.
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   
36.
37.
38.
39.
Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (~0.1 ms?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators.  相似文献   
40.
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ~2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ~0.04 at 1.25 μm and ~0.05 at 2.2 μm.  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