全文获取类型
收费全文 | 41298篇 |
免费 | 716篇 |
国内免费 | 317篇 |
专业分类
测绘学 | 959篇 |
大气科学 | 2944篇 |
地球物理 | 8001篇 |
地质学 | 14215篇 |
海洋学 | 3848篇 |
天文学 | 9740篇 |
综合类 | 74篇 |
自然地理 | 2550篇 |
出版年
2021年 | 403篇 |
2020年 | 401篇 |
2019年 | 453篇 |
2018年 | 940篇 |
2017年 | 843篇 |
2016年 | 1090篇 |
2015年 | 608篇 |
2014年 | 1001篇 |
2013年 | 2073篇 |
2012年 | 1132篇 |
2011年 | 1619篇 |
2010年 | 1470篇 |
2009年 | 1983篇 |
2008年 | 1645篇 |
2007年 | 1730篇 |
2006年 | 1587篇 |
2005年 | 1311篇 |
2004年 | 1270篇 |
2003年 | 1241篇 |
2002年 | 1193篇 |
2001年 | 1056篇 |
2000年 | 980篇 |
1999年 | 810篇 |
1998年 | 813篇 |
1997年 | 829篇 |
1996年 | 668篇 |
1995年 | 656篇 |
1994年 | 616篇 |
1993年 | 570篇 |
1992年 | 526篇 |
1991年 | 495篇 |
1990年 | 506篇 |
1989年 | 496篇 |
1988年 | 474篇 |
1987年 | 553篇 |
1986年 | 489篇 |
1985年 | 604篇 |
1984年 | 650篇 |
1983年 | 592篇 |
1982年 | 527篇 |
1981年 | 578篇 |
1980年 | 469篇 |
1979年 | 458篇 |
1978年 | 433篇 |
1977年 | 433篇 |
1976年 | 383篇 |
1975年 | 378篇 |
1974年 | 377篇 |
1973年 | 384篇 |
1971年 | 224篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
311.
The assumption that the very low albedo determined for Halley's comet is typical of all short period comets, taken together with the assumption that the average sizes of long and short period comets are approximately equal, leads to an increase in the total mass of comets in the solar system by almost two orders of magnitude. If gravitational ejection from the Uranus - Neptune zone during the later phases of planet formation is indeed responsible for the classical Oort cloud between 104–1015 AU, then the mass of comets in this transplanetary region during cosmogonie times has to exceed the combined masses of Uranus and Neptune by over an order of magnitude. Furthermore, if the recent arguments for as many as 1014 comets in an inner Oort cloud between ~40– 104AU are valid, then the total mass of comets in the solar system approaches 2% of a solar mass. 相似文献
312.
Regional variations in the incidence of premature deaths between 1971 and 1981 arc examined by mapping standardised mortality ratios for deaths under the age of 65. Premature mortality is also examined with regards to the number of years of normal life lost by those who die prematurely, using a specially defined unfulfilled life index. Unfulfilled life and premature deaths arc found to have a higher incidence in the major urban areas and a lower incidence in western counties, although there are some interesting differences in their spatial distributions. Differences with regard to sex. age at death and cause of death are also examined. 相似文献
313.
M. Golay N. Cramer D. Huguenin B. Nicolet A. Blecha 《Astrophysics and Space Science》1985,109(1):191-212
The present paper is the introduction to a systematic analysis of 123 6-degree fields near the galactic plane, recorded in the medium ultraviolet (2000 Å) by the ballon-borne experiment SCAP 2000 of the Laboratoire d'Astronomie Spatiale, Marseille, and Geneva Observatory. The available data are presented and the general properties of the images are briefly discussed.It is shown that the high selectivity of the UV passband regarding spectral type, together with the strong interstellar extinction at that wavelength, provide the necessary conditions for an efficient application of Wolf's method to study the distribution of interstellar matter in the solar neighbourhood. The results of a fast analysis of the available data are presented here. 相似文献
314.
The continuum emission of an X1 flare on 26 March, 1970 observed close to the solar limb (N 05 E 64) was analyzed by a photometric determination of the contrast I()/I
0() in the wavelength range 3558–5920 Å. Two possible mechanisms for the emission were investigated, namely hydrogen Paschen and H– continua. We show the unlikeness of the Paschen possibility and derive strong constraints on the temperature structure and energy deposition mechanism imposed by the H– continuum process.We conclude that the emission must have originated in deep atmospheric layers. The relevance of this result with respect to flare energy release and transport processes is also discussed. 相似文献
315.
"The paper reports the use of Australian Resources Information System to calculate two proximate populations (people living within (i) 150 and (ii) 500 km) for the centroids of each of 3,000 half by half degree geodetic grid cells covering Australia. The use of proximate population data is exemplified by computing a map of the eighteen regions collectively containing a maximum fraction of the Australian population." The data used are from the 1981 census. 相似文献
316.
Using observations obtained with the Clark Lake radioheliograph we determined the diameter of the Sun in the decameter wavelength range. Both equatorial and polar diameters increase with decreasing frequency, as D=Af
. The eccentricity of the brightness distribution appears to remain constant in the frequency range (30–74 MHz) in good agreement with the optical results in a corresponding height range. The smaller size of the polar diameter is attributed to coronal holes covering the poles during the period of our observations, while streamers were observed at the equator most of the time. 相似文献
317.
The effect of the Coriolis force on the Kelvin-Helmholtz instability has been investigated, in which basic velocities of fluid are varying slowly with lateral coordinatey. The problem is solved by J.W.K.B. approximation method. Parabolic and other types of profiles for wide long channel flow have also been studied in detail. 相似文献
318.
On the coronograph spectrophotographic records taken on 31.372 UT, August 1979, some faint emission features were found which can be ascribed to Siii and Niii. These emissions were obviously a transient phenomena which were detected only 10 hr after the supposed fall of Comet 1979 XI in the Sun's photosphere. It cannot be excluded that the appearance of Si and Ni lines was triggered by the evaporation of dust particles with a high abundance of heavier elements in the solar corona. This assumption is also supported by intensity distribution of the Fex coronal line around the Sun's limb. The maximum coincide with the position angle of the projected path of the comet. 相似文献
319.
We examine the possibility that the observed cosmic-ray protons are of primary extragalactic origin. The present \(\bar p\) data are consistent with a primary extragalactic component having \(\bar p\) /p?3.2±0.7 x 10-4 independent of energy. Following the suggestion that most extragalactic cosmic rays are from active galaxies, we propose that most of the observed \(\bar p\) 's are alos from the same sites. This would imply the possibility of destroying the corresponding \(\bar \alpha \) 'sat the source, thus leading to a flux ratio \(\bar \alpha \) /α< \(\bar p\) /p. We further predict an estimate for \(\bar \alpha \) α~10-5, within the range of future cosmic-ray detectors. the cosmological implications of this proposal are discussed. 相似文献
320.
A series of hydrodynamical models of type-II supernova outbursts (SNII) has been calculated. Approximate relations connecting the total outburst energy ε, the mass of envelope ejectedM, the presupernova radiusR, and the amount of ionizing quanta radiated by the supernovaeN H with such values as the duration of the light curve plateau Δt, and absolute magnitude in the wavelength bandV and photospheric velocityU PH observed near the middle of the plateau have been established. Advantage has been taken of the relations to obtain a preliminary evaluation for the characteristics of the average SN II: ε=7×1050 erg,M=6M ⊙,R=500R ⊙,N H=2×1058. The SNIIs with plateau-like light curves seem to be accounted for by thermonuclear explosions of degenerate cores of red giant stars and result in a total disruption of the star without any stellar remnant. To the contrary, SNIIs with linear light curves have substantially different properties (in particular, they throw considerably less massive envelopes off). These SNII must signify the birth of collapsed objects—neutron stars (pulsars) or black holes. 相似文献