全文获取类型
收费全文 | 100101篇 |
免费 | 1930篇 |
国内免费 | 951篇 |
专业分类
测绘学 | 2702篇 |
大气科学 | 7116篇 |
地球物理 | 20521篇 |
地质学 | 35007篇 |
海洋学 | 8643篇 |
天文学 | 22918篇 |
综合类 | 297篇 |
自然地理 | 5778篇 |
出版年
2021年 | 924篇 |
2020年 | 1036篇 |
2019年 | 1131篇 |
2018年 | 2546篇 |
2017年 | 2377篇 |
2016年 | 3018篇 |
2015年 | 1754篇 |
2014年 | 2889篇 |
2013年 | 5206篇 |
2012年 | 3068篇 |
2011年 | 4165篇 |
2010年 | 3579篇 |
2009年 | 4821篇 |
2008年 | 4358篇 |
2007年 | 4221篇 |
2006年 | 3976篇 |
2005年 | 3146篇 |
2004年 | 3089篇 |
2003年 | 2874篇 |
2002年 | 2760篇 |
2001年 | 2523篇 |
2000年 | 2398篇 |
1999年 | 1977篇 |
1998年 | 2071篇 |
1997年 | 2007篇 |
1996年 | 1614篇 |
1995年 | 1613篇 |
1994年 | 1397篇 |
1993年 | 1292篇 |
1992年 | 1190篇 |
1991年 | 1142篇 |
1990年 | 1208篇 |
1989年 | 1103篇 |
1988年 | 983篇 |
1987年 | 1185篇 |
1986年 | 1017篇 |
1985年 | 1264篇 |
1984年 | 1417篇 |
1983年 | 1316篇 |
1982年 | 1270篇 |
1981年 | 1124篇 |
1980年 | 1024篇 |
1979年 | 964篇 |
1978年 | 972篇 |
1977年 | 856篇 |
1976年 | 824篇 |
1975年 | 799篇 |
1974年 | 771篇 |
1973年 | 785篇 |
1972年 | 538篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
101.
U. C. Joshi A. K. Sen M. R. Deshpande J. S. Chauhan 《Journal of Astrophysics and Astronomy》1992,13(3):267-277
Photopolarimetric observations of comet Austin with the IAU/IHW filter system were obtained on the 2.34 m Vainu Bappu Telescope
(VBT) of the Indian Institute of Astrophysics, at Kavalur, India, during pre-perihelion phase on February 20,1990 and on the
1.2 m telescope of the Physical Research Laboratory at Gurusikhar, Mount Abu during postperihelion phase on May 2 and 4, 1990.
The comet appeared bluer than a solar analog during post-perihelion phase on May 2 and 4. The percent polarization shows a
sharp increase towards the red on May 2 and 4. The dominant sizes of the dust particles appear to lie in a narrow range of
0.1 to 0.5 Μm. Regarding the molecular band emission, CN and C2 bands are quite strong; C3 emission was also found to be strong though the observations on May 2 and 4 show significant variation as compared to C2 emission. Molecular band polarization for CN, C3, C2 and H2 O+ have been calculated. It has been found that emission polarization in CN, C2 and C3 is between 1–7% (phase angle between 107.4–109 degrees). For CN and C2 the polarization values are close to the theoretically predicted values, but for C3 the polarization value falls much below the theoretically predicted value. A similar result was found for comet Halley. 相似文献
102.
103.
104.
105.
We analyzed data from 23 boreholes at 19 sites in central and eastern Canada, for the purpose of estimating ground surface temperature (GST) histories. These boreholes were logged down to at least 550 m depth with thermistor probes. Thermal conductivity measurements had been previously made at small depth intervals for the entire depth ranges of most of the boreholes. The temperature profiles of these boreholes do not indicate water disturbance. We estimated terrain effects for each borehole using a time dependent solid-angle method. The thermal perturbations caused by lakes or deforestation near the borehole sites are insignificant in most cases. However, four of the holes were found to be severely influenced by terrain effects. GSTs estimated from the borehole data less influenced by the terraineffects form two groups. The first group, which are generally from data of better quality, show a cold period near the end of the last century before the recent warming trend; the second show it 80–100 years earlier. We consider the former typical of the climate of the Boreal climatic region of Canada. The difference between the two groups may reflect the spacial variability of the climate. Four GST estimates do not belong to either type, and the reasons are discussed. 相似文献
106.
We report intermediate resolution H spectroscopy of the black hole candidate Nova Muscae 1991 during quiescence. We classify the companion star as a K3-K4V which contributes 85±6 percent to the total flux from the binary. The photospheric absorption lines are broadened by 106±13 kms−1 with respect to template field stars, leading to a system mass ratio of q =M1/M2 = 7.8−2.0+3.4. Doppler imaging of the H line shows strong emission coming from the secondary star (EW=3.1±0.6Å) which we associate with chromospheric activity. However, the hot-spot is not detected and this may indicate a lower mass transfer rate than in other X-ray transients of comparable orbital periods. The surface brightness distribution of the accretion disk in H follows a relation I∝R−1.1, less steep than typically observed in cataclysmic variables. Updated system parameters are also presented. 相似文献
107.
The dynamics of the vortex lattice in the inner crust of a neutron star is considered. A general equation of motion is obtained
and solved under the assumption that there are regions of pinned and of free vortices. By comparing these solutions with observational
data for the Vela pulsar, the relative moments of inertia of regions of relaxation with the corresponding characteristic times
are calculated for two model stars with different equations of state. It is shown that the theory can be reconciled with observations
of the relaxation of pulsar angular velocity only for model stars with extremely stiff equations of state.
Translated from Astrofizika, Vol. 40, No. 1, pp. 67–76, January–March, 1997. 相似文献
108.
R. Sarmiento M. Gallardo A. Laquidara J. Reyna Almandos 《Astrophysics and Space Science》1997,256(1-2):359-363
This work presents preliminaries spectroscopic results about ionized xenon and xenon-helium mixture using a capillary pulsed discharge under several experimental results. 相似文献
109.
The evolutionary status of a sample of 31 W UMa-type systems (20 of W, 9 of A, and 2 of A/W sub-class) is examined and discussed on the basis of their positions in the H-R, mass-radius and mass-luminosity diagrams. The absolute elements of these systems have been evaluated by use of their spectroscopic and photometric data as they were found in the literature. Since different analysis methods have been used by the various investigators this gives the opportunity to make different H-R, mass-radius and mass-luminosity diagrams, according to the method used, and compare the results.Moreover, from the spectral type-period diagram of those systems that have spectral types from O to G2 it was found that all of them lie in the evolved region. Finally, the log (L
l/L
s) log (M
l/M
s) diagrams are presented and discussed. 相似文献
110.
Several temperature-depth profiles measured in Kasai and in Shaba provinces of Zaire using mining exploration boreholes exhibit a significant negative temperature gradient near the surface. This anomalous curvature which extends to 100–200 m depth could reflect the effect of variations in surface conditions. Applying the theory of heat conduction in a semi-infinite homogeneous medium, these profiles indicate a surface warming by 3–4°C. This warming is related to the effect of the environmental changes associated with the mining exploitation and the urbanization during the last 40–90 years. 相似文献