全文获取类型
收费全文 | 39251篇 |
免费 | 825篇 |
国内免费 | 602篇 |
专业分类
测绘学 | 976篇 |
大气科学 | 3258篇 |
地球物理 | 7994篇 |
地质学 | 13112篇 |
海洋学 | 3561篇 |
天文学 | 9027篇 |
综合类 | 123篇 |
自然地理 | 2627篇 |
出版年
2021年 | 232篇 |
2020年 | 292篇 |
2019年 | 308篇 |
2018年 | 695篇 |
2017年 | 610篇 |
2016年 | 950篇 |
2015年 | 682篇 |
2014年 | 912篇 |
2013年 | 2024篇 |
2012年 | 1124篇 |
2011年 | 1538篇 |
2010年 | 1280篇 |
2009年 | 1881篇 |
2008年 | 1672篇 |
2007年 | 1593篇 |
2006年 | 1510篇 |
2005年 | 1367篇 |
2004年 | 1292篇 |
2003年 | 1256篇 |
2002年 | 1171篇 |
2001年 | 1025篇 |
2000年 | 1041篇 |
1999年 | 956篇 |
1998年 | 882篇 |
1997年 | 876篇 |
1996年 | 748篇 |
1995年 | 673篇 |
1994年 | 592篇 |
1993年 | 547篇 |
1992年 | 540篇 |
1991年 | 503篇 |
1990年 | 497篇 |
1989年 | 448篇 |
1988年 | 431篇 |
1987年 | 482篇 |
1986年 | 459篇 |
1985年 | 550篇 |
1984年 | 612篇 |
1983年 | 589篇 |
1982年 | 560篇 |
1981年 | 486篇 |
1980年 | 470篇 |
1979年 | 420篇 |
1978年 | 428篇 |
1977年 | 386篇 |
1976年 | 337篇 |
1975年 | 349篇 |
1974年 | 354篇 |
1973年 | 359篇 |
1972年 | 205篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
Summary. Four box cores collected from the Ontong—Java plateau during the Eurydice expedition have been used to make relative geomagnetic palaeo-intensity measurements. Rock magnetic measurements on the sediments show that they are characterized by a uniform magnetic mineralogy, and that they are suitable for relative intensity estimates. These are obtained by normalizing the NRM by an ARM imparted in a low DC bias field. the palaeoceanographic event known as the preservation spike is used to establish a crude time-scale for the record so that it may be compared with other data from the same region, and also with global palaeointensity estimates. the marine sediment data are quite similar to Australian intensity data from lake sediments and archaeomagnetic sources, but as might be expected exhibit some obvious differences from the global record. 相似文献
22.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
23.
24.
25.
E. C. Hopewell M. J. Barlow J. E. Drew Y. C. Unruh Q. A. Parker M. J. Pierce P. A. Crowther C. Knigge S. Phillipps A. A. Zijlstra 《Monthly notices of the Royal Astronomical Society》2005,363(3):857-866
We report the discovery of five massive Wolf–Rayet (WR) stars resulting from a programme of follow-up spectroscopy of candidate emission-line stars in the Anglo-Australian Observatory United Kingdom Schmidt Telescope (AAO/UKST) Southern Galactic Plane Hα survey. The 6195–6775 Å spectra of the stars are presented and discussed. A WC9 class is assigned to all five stars through comparison of their spectra with those of known late-type WC stars, bringing the known total number of Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibit near-infrared (NIR) excesses characteristic of hot dust emission (as seen in the great majority of known WC9 stars), we find that two of the stars show no discernible evidence of such excesses. This increases the number of known WC9 stars without NIR excesses to seven. Reddenings and distances for all five stars are estimated. 相似文献
26.
27.
New determination of the Earth orientation parameters (EOP), based on optical astrometry observations since the beginning of the century, is now under preparation by the Working group established by Commission 19 of the IAU. The Hipparcos catalog is to define the celestial reference frame in which the new series of EOP are to be described, The novelties of the prepared solution are the higher resolution (5 days) and more parameters estimated from the solution (celestial pole offsets, rheological parameters of the Earth, certain instrumental constants). The mathematical model of the solution is described, and the results based on the observations made with 46 instruments at 29 observatories and a preliminary Hipparcos catalog are presented. 相似文献
28.
C. Beaugé 《Celestial Mechanics and Dynamical Astronomy》1996,64(4):313-350
Starting with a simple Taylor-based expansion of the inverse of the distance between two bodies, we are able to obtain a series expansion of the disturbing function of the three-body problem (planar elliptic case) which is valid for all points of the phase space outside the immediate vicinity of the collision points. In particular, the expansion is valid for very high values of the eccentricity of the perturbed body. Furthermore, in the case of an interior mean-motion resonant configuration, the above-mentioned expression is easily averaged with respect to the synodic period, yielding once again a global expansion of (R) valid for very high eccentricities.Comparisons between these results and the numerically computed exact function are presented for various resonances and values of the eccentricity. Maximum errors are determined in each case and their origin is established. Lastly, we discuss the applicability of the present expansion to practical problems. 相似文献
29.
The explicit forms of the metric as well as the equations of motion in the first-order post-Newtonian approximation are worked out under several gauge conditions. It is noted that the so-called EIH (Einstein, Infeld, and Hoffman) equation of motion for an assembly ofN finite mass points mutually interacting via gravitation is identically obtained under three different gauge conditions, namely the harmonic gauge, Chandrasekhar gauge and a composite Chandrasekhar gauge used by Misneret al. (1970), even though the solutions for the metric are found to be all different. In one case the metric has a component apparently diverging, but finally generates regular affine connections so that the equations of motions become free from any singularity. By use of the Chandrasekhar gauge and his formulation, the second-order contribution to the acceleration of planets in the limit of test particle motion around the Sun has been calculated, the inclusion of which in the EIH set of the equations of motion would extend the relative accuracy of computing the total acceleration of any planet to better than one part in 1017. 相似文献
30.
Daniela Lazzaro Marcos A. Florczak Alberto Betzler Othon C. Winter Silvia M. Giuliatti-Winter Claudia A. Angeli Dietmar W. Foryta 《Planetary and Space Science》1996,44(12):1547-1550
The results of photometric observations of comet/asteroid 2060 Chiron at the Observatório do Pico dos Dias (Brazil-OPD) and the Observatoire de Haute-Provence (France-OHP) during 1994 and 1995 are presented. The analysis of the data shows a decrease of 2060 Chiron brightness from its peak values of 1988–1991. The absolute magnitude, Hv, varies from a maximum of 6.6 in February 1994 up to a minimum of 6.8 in June 1995. Therefore 2060 Chiron is back to a minimum of activity close to that of 1983–1985. The slope parameter G is found to be G = 0.71 ± 0.15. It is suggested that the H-G magnitude system, generally adopted to present 2060 Chiron brightness, is not the most appropriate due to the cometary activity of this object. 相似文献