全文获取类型
收费全文 | 60802篇 |
免费 | 10761篇 |
国内免费 | 15193篇 |
专业分类
测绘学 | 6359篇 |
大气科学 | 11442篇 |
地球物理 | 11968篇 |
地质学 | 27371篇 |
海洋学 | 9275篇 |
天文学 | 6824篇 |
综合类 | 4125篇 |
自然地理 | 9392篇 |
出版年
2024年 | 155篇 |
2023年 | 710篇 |
2022年 | 1915篇 |
2021年 | 2260篇 |
2020年 | 2465篇 |
2019年 | 2702篇 |
2018年 | 2419篇 |
2017年 | 2671篇 |
2016年 | 2732篇 |
2015年 | 3119篇 |
2014年 | 3788篇 |
2013年 | 3951篇 |
2012年 | 4031篇 |
2011年 | 4276篇 |
2010年 | 3649篇 |
2009年 | 4537篇 |
2008年 | 4222篇 |
2007年 | 4694篇 |
2006年 | 4451篇 |
2005年 | 3854篇 |
2004年 | 3473篇 |
2003年 | 3076篇 |
2002年 | 2684篇 |
2001年 | 2192篇 |
2000年 | 2017篇 |
1999年 | 1814篇 |
1998年 | 1569篇 |
1997年 | 1258篇 |
1996年 | 1100篇 |
1995年 | 989篇 |
1994年 | 872篇 |
1993年 | 754篇 |
1992年 | 532篇 |
1991年 | 448篇 |
1990年 | 283篇 |
1989年 | 294篇 |
1988年 | 201篇 |
1987年 | 135篇 |
1986年 | 83篇 |
1985年 | 87篇 |
1984年 | 52篇 |
1983年 | 32篇 |
1982年 | 32篇 |
1981年 | 28篇 |
1980年 | 25篇 |
1979年 | 24篇 |
1978年 | 19篇 |
1977年 | 27篇 |
1976年 | 13篇 |
1954年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
AGGLOMERATION AND RADIATION EFFECT OF THE PULL OF URBANIZATION 总被引:2,自引:1,他引:1
QIJin-li 《中国地理科学(英文版)》2003,13(3):224-227
In order to explore the train of thought for China‘s urbanizing development and coordinated rural eco-nomic development, and to find good ways of solving rural problems through urbanization, this paper absorbs the push-and-pull forces theory and the systematic dynamic theory in the traditional population migration theories, views urbanization as a dynamic system, makes research on the push-and-pull mechanism of urbanization. The pulling power of urbanization is analyzed according to two aspects, the agglomeration effect and the radiation effect of cities. The agglomeration effect provides continuous propelling force for urbanization, and the radiation effect further accelerates the urbanization process by pushing forward the development of rural economy. Of course, the slow de-velopment of urbanization can result in the hindrance to rural economic development. 相似文献
82.
83.
84.
85.
86.
87.
88.
Two small satellites of Pluto, S/2005 P1 (hereafter P1) and S/2005 P2 (hereafter P2), have recently been discovered outside the orbit of Charon, and their orbits are nearly circular and nearly coplanar with that of Charon. Because the mass ratio of Charon-Pluto is ∼0.1, the orbits of P2 and P1 are significantly non-Keplerian even if P2 and P1 have negligible masses. We present an analytic theory, with P2 and P1 treated as test particles, which shows that the motion can be represented by the superposition of the circular motion of a guiding center, the forced oscillations due to the non-axisymmetric components of the potential rotating at the mean motion of Pluto-Charon, the epicyclic motion, and the vertical motion. The analytic theory shows that the azimuthal periods of P2 and P1 are shorter than the Keplerian orbital periods, and this deviation from Kepler's third law is already detected in the unperturbed Keplerian fit of Buie and coworkers. In this analytic theory, the periapse and ascending node of each of the small satellites precess at nearly equal rates in opposite directions. From direct numerical orbit integrations, we show the increasing influence of the proximity of P2 and P1 to the 3:2 mean-motion commensurability on their orbital motion as their masses increase within the ranges allowed by the albedo uncertainties. If the geometric albedos of P2 and P1 are high and of order of that of Charon, the masses of P2 and P1 are sufficiently low that their orbits are well described by the analytic theory. The variation in the orbital radius of P2 due to the forced oscillations is comparable in magnitude to that due to the best-fit Keplerian eccentricity, and there is at present no evidence that P2 has any significant epicyclic eccentricity. However, the orbit of P1 has a significant epicyclic eccentricity, and the prograde precession of its longitude of periapse with a period of 5300 days should be easily detectable. If the albedos of P2 and P1 are as low as that of comets, the large inferred masses induce significant short-term variations in the epicyclic eccentricities and/or periapse longitudes on the 400-500-day timescales due to the proximity to the 3:2 commensurability. In fact, for the maximum inferred masses, P2 and P1 may be in the 3:2 mean-motion resonance, with the resonance variable involving the periapse longitude of P1 librating. Observations that sample the orbits of P2 and P1 well on the 400-500-day timescales should provide strong constraints on the masses of P2 and P1 in the near future. 相似文献
89.
Measurements of 18O concentrations in precipitation, soil solution, spring and runoff are used to determine water transit time in the small granitic Strengbach catchment (0·8 km2; 883–1146 m above sea level) located in the Vosges Mountains of northeastern France. Water transit times were calculated by applying the exponential, exponential piston and dispersion models of the FlowPC program to isotopic input (rainfall) and output (spring and stream water) data sets during the period 1989–95. The input function of the model was modified compared with the former version of the model and estimated by a deterministic approach based on a simplified hydrological balance. The fit between observed and calculated output data showed marked improvements compared with results obtained using the initial version of the model. An exponential piston version of the model applied to spring water indicates a 38·5 month mean transit time, which suggests that the volume in the aquifer, expressed in water depth, is 2·4 m. A considerable thickness (>45 m) of fractured bedrock may be involved for such a volume of water to be stored in the aquifer. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
90.