全文获取类型
收费全文 | 26139篇 |
免费 | 401篇 |
国内免费 | 272篇 |
专业分类
测绘学 | 637篇 |
大气科学 | 2027篇 |
地球物理 | 5601篇 |
地质学 | 9132篇 |
海洋学 | 2131篇 |
天文学 | 5497篇 |
综合类 | 42篇 |
自然地理 | 1745篇 |
出版年
2020年 | 141篇 |
2019年 | 135篇 |
2018年 | 304篇 |
2017年 | 282篇 |
2016年 | 429篇 |
2015年 | 314篇 |
2014年 | 446篇 |
2013年 | 1213篇 |
2012年 | 521篇 |
2011年 | 812篇 |
2010年 | 676篇 |
2009年 | 945篇 |
2008年 | 873篇 |
2007年 | 827篇 |
2006年 | 861篇 |
2005年 | 754篇 |
2004年 | 769篇 |
2003年 | 734篇 |
2002年 | 735篇 |
2001年 | 599篇 |
2000年 | 620篇 |
1999年 | 587篇 |
1998年 | 565篇 |
1997年 | 577篇 |
1996年 | 473篇 |
1995年 | 475篇 |
1994年 | 455篇 |
1993年 | 421篇 |
1992年 | 393篇 |
1991年 | 338篇 |
1990年 | 386篇 |
1989年 | 306篇 |
1988年 | 348篇 |
1987年 | 389篇 |
1986年 | 336篇 |
1985年 | 497篇 |
1984年 | 550篇 |
1983年 | 550篇 |
1982年 | 445篇 |
1981年 | 436篇 |
1980年 | 450篇 |
1979年 | 389篇 |
1978年 | 408篇 |
1977年 | 357篇 |
1976年 | 385篇 |
1975年 | 350篇 |
1974年 | 390篇 |
1973年 | 373篇 |
1972年 | 238篇 |
1971年 | 193篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Eric W. Elst 《Earth, Moon, and Planets》1995,71(3):275-277
During 1987–1994, observational campaigns with different telescopes at several observatories have been initiated by the author in order to discover new Trojans. The importance of Trojan asteroids comes from celestial mechanics, where they represent the physical solution of the famous Lagrange triangular problem. Their importance lies also in the fact, that they may have some relation with comets. Furthermore, the Trojan belt may be as large as the belt of asteroids. Moreover, recently families have been discovered between the already well known Trojans. Enough reasons to continue to search for these interesting objects. 相似文献
62.
H. W. Duerbeck 《Astrophysics and Space Science》1984,99(1-2):93-94
A catalogue of galactic novae and an atlas of finding charts are under preparation and will be published in 1984. The status of the project is described.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia, 3–7 June, 1983. 相似文献
63.
Photochemical calculations indicate that in the prebiotic atmosphere of the Earth ammonia would have been irreversibly converted to N2 in less than 40 years if the ammonia surface mixing ratio were ≤ 10?4. However, if a continuous outgassing of ammonia were maintained, radiative equilibrium calculations indicate that a surface mixing ratio of ammonia of 10?5 or greater would provide a sufficient greenhouse effect to keep the surface temperature above freezing. With a 10?4 mixing ratio of ammonia, 60 to 70% of the present day solar luminosity would be adequate to maintain surface temperatures above freezing. A lower limit to the time constant for accumulation of an amount of nitrogen equivalent to the present day value is 10 my if the outgassing were such as to provide a continuous surface mixing ratio of ammonia ≥ 10?5. 相似文献
64.
Since the original suggestion by Gillett et al. (1969) it has generally been assumed that the region of partial transparency near 5 μm in Jupiter's atmosphere (the 5-μm window) is bounded by the v4 NH3 at 6.1 μm and the v3 CH4 band at 3.3 μm. New measurements of Jupiter and of laboratory phosphine (PH3) samples show that PH3 is a significant contributor to the continuum opacity in the window and in fact defines its short-wavelength limit. This has important implications for the use of 5-mu;m observations as a means to probe the deep atmospheric structure of Jupiter. The abundance of PH3 which results from a comparison of Jovian and laboratory spectra is about 3 to 5 cm-am. This is five to eight times less than that found by Larson et al. [Astrophys. J. (1977) 211, 972–979] in the same spectral region, but is in good agreement with the result of Tokunaga et al. [Astrophys. J. (1979) 232, 603–615] from 10-μm observations. 相似文献
65.
An axisymmetric model for approximate solution of the magnetospheric Alfvén wave problem at latitudes above the plasmapause is proposed, in which a realistic dipole geometry is combined with finite anisotropic ionosphere conductivities, thus bringing together various ideas of previous authors. It is confirmed that the axisymmetric toroidal and poloidal modes interact via the ionospheric Hall effect, and an approximate method of solution is suggested using previously derived closed solutions of the uncoupled wave equations.A solution for zero Hall conductivity is obtained, which consists of sets of independent shell oscillations, regardless of the magnitude of the Pedersen conductivity. One set reduces to the classical solutions for infinite Pedersen conductivity, while another predicts a new set of harmonics of a quarter-wave fundamental, with longer eigenperiods than the classical solutions for a given L-shell. 相似文献
66.
Photoelectric observations on five consecutive nights yield a period of rotation of 80 ± 2 hr with an amplitude of 0.7 magnitude for 182 Elsa, making it the longest period of rotation known to date. 182 Elsa is classed as an S object with a diameter of 48 km. 相似文献
67.
The methane abundance in the lower Jovian stratosphere is measured using Galilean satellite eclipse light curves. Spectrally selective observations in and between absorption bands are compared. An average mixing ratio at the locations measured is [CH4]/[H2] ~ 1.3 × 10?3, larger than the value 0.9 × 10?3 expected for a solar abundance of carbon. Some zenographic variation of the mixing ratio may occur. Observationally compatible values are 1.3–2.0 × 10?3 in the STZ, 1.3– 2.6 × 10?3 on the GRS/STrZ edge, and 0.7–1.3 × 10?3 in the GRS. 相似文献
68.
69.
W.B. Hubbard 《Icarus》1977,30(2):305-310
A theory which is almost fully analytic is used to investigate Jupiter's cooling rate. We find that a simple model of contraction with adiabatic interior structure gives a total cooling time to the present which is in good agreement with the age of the solar system. The interplay between the surface condition and the cooling rate is exhibited and discussed. The current rate of change of the effective temperature is calculated to be ?1°K/0.145×109 yr. Discrepancies with fully numerical investigations of the Jovian age and cooling rate are noted. 相似文献
70.
W. J. Maciel 《Astrophysics and Space Science》1992,196(1):23-28
The oxygen abundance gradient relative to hydrogen is considered, as derived for galactic Hii regions and type II planetary nebulae. The so-called simple model for the chemical evolution of the Galaxy is shown to explain well the observed gradients, provided some reasonable assumptions are made regarding the gas distribution in the galactic disk. 相似文献