全文获取类型
收费全文 | 37468篇 |
免费 | 967篇 |
国内免费 | 1048篇 |
专业分类
测绘学 | 949篇 |
大气科学 | 2877篇 |
地球物理 | 7684篇 |
地质学 | 13809篇 |
海洋学 | 3346篇 |
天文学 | 8159篇 |
综合类 | 240篇 |
自然地理 | 2419篇 |
出版年
2022年 | 291篇 |
2021年 | 474篇 |
2020年 | 457篇 |
2019年 | 495篇 |
2018年 | 914篇 |
2017年 | 875篇 |
2016年 | 1050篇 |
2015年 | 723篇 |
2014年 | 1051篇 |
2013年 | 1869篇 |
2012年 | 1348篇 |
2011年 | 1790篇 |
2010年 | 1566篇 |
2009年 | 2023篇 |
2008年 | 1706篇 |
2007年 | 1773篇 |
2006年 | 1700篇 |
2005年 | 1224篇 |
2004年 | 1142篇 |
2003年 | 1042篇 |
2002年 | 1012篇 |
2001年 | 850篇 |
2000年 | 830篇 |
1999年 | 674篇 |
1998年 | 719篇 |
1997年 | 692篇 |
1996年 | 572篇 |
1995年 | 562篇 |
1994年 | 479篇 |
1993年 | 421篇 |
1992年 | 421篇 |
1991年 | 386篇 |
1990年 | 458篇 |
1989年 | 373篇 |
1988年 | 356篇 |
1987年 | 438篇 |
1986年 | 346篇 |
1985年 | 430篇 |
1984年 | 532篇 |
1983年 | 451篇 |
1982年 | 453篇 |
1981年 | 404篇 |
1980年 | 420篇 |
1979年 | 361篇 |
1978年 | 345篇 |
1977年 | 341篇 |
1976年 | 310篇 |
1975年 | 296篇 |
1974年 | 312篇 |
1973年 | 342篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
We present optical broadband photometry for the satellites J6, J7, J8, S7, S9, U3, U4, N1, and polarimetry for J6, obtained between 1970 and 1979. The outer Jovian satellites resemble C-type asteroids; J6 has a rotational lightcurve with period ~9.5 hr. The satellites beyond Jupiter also show C-like colors with the exception of S7 Hyperion. S9 Phoebe has a rotational lightcurve with period near either 11.25 or 21.1 hr. For U4 and N1 there is evidence for a lightcurve synchronous with the orbital revolution. The seven brighter Saturnian satellites show a regular relation between the ultraviolet dropoff and distance to the planet, probably related with differences in the rock component on their surfaces. 相似文献
92.
L. Trafton 《Icarus》1980,44(1):53-61
The presence of CH4 ice on Pluto implies that Pluto may have a substantial atmosphere consisting of heavy gases. Without such an atmosphere, sublimation of the CH4 ice would be so rapid on a cosmogonic time scale that either such an atmosphere would soon develop through the exposure of gases trapped in the CH4 ice or else the surface CH4 ice would soon be all sublimated away as other, more stable, ices became exposed. If such stable ices were present from the beginning, the existence of CH4 frosts would also imply that Pluto's present atmosphere contains a remnant of its primordial atmosphere. 相似文献
93.
The sizes, composition, and number of particles comprising the rings of Saturn may be meaningfully constrained by a combination of radar- and radio-astronomical observations. In a previous paper, we have discussed constraints obtained from radar observations. In this paper, we discuss the constraints imposed by complementary “passive” radio observations at similar wavelengths. First, we present theoretical models of the brightness of Saturn's rings at microwave wavelengths (0.34–21.0 cm), including both intrinsic ring emission and diffuse scattering by the rings of the planetary emission. The models are accurate simulations of the behavior of realistic ring particles and are parameterized only by particle composition and size distribution, and ring optical depth. Second, we have reanalyzed several previously existing sets of interferometric observations of the Saturn system at 0.83-, 3.71-, 6.0-, 11.1-, and 21.0-cm wavelengths. These observations all have spatial resolution sufficient to resolve the rings and planetary disk, and most have resolution sufficient to resolve the ring-occulted region of the disk as well. Using our ring models and a realistic model of the planetary brightness distribution, we are able to establish improved constraints on the properties of the rings. In particular, we find that: (a) the maximum optical depth in the rings is ~ 1.5 ± 0.3 referred to visible wavelengths; (b) a significant decrease in ring optical depth from λ3.7 to λ21.0 cm allows us to rule out the possibility that more than ~30% of the cross section of the rings is composed of particles larger than a meter or so; this assertion is essentially independent of uncertainties in particle adsorption coefficient; and (c) the ring particles cannot be primarily of silicate composition, independently of particle size, and the particles cannot be primarily smaller than ~0.1 cm, independently of composition. 相似文献
94.
According to radiometric dating of lunar rocks, meteoroidal bombardment and accompanying cratering on the Moon were intensive in the first 0.7×109 y, the so-calledterra stage. Recently the hypothesis of a terminal cataclysm has been gaining acceptance, meaning that a sharp increase in the bombardment followed by a steep decay occurred at the end of theterra stage.The purpose of this paper is to investigate possible variations in the intensity of the bombardment during theterra stage by analyzing the population of large (3–1000 km)terra craters and comparing it with results obtained by theoretical models. The proportion of fresh craters is specifically used.Observational data were obtained by studying the craters on an oldterra surface photographed by Zond 8 and/or covered by LTO and LM maps and by conducting a statistical study of theterra listed in the Lunar and Planetary Laboratory Catalog. Mathematical models were developed in such a way as to make them dimensionless, and as such applicable even without the knowledge of the physical constants and variables involved. Particularly powerful is the method of measuring time as multiples of the average lifetime of a crater of that size.The following conclusions and/or interpretations are reached. (1) A crater remains fresh for less than 20% of the average life of a crater of that size. (2) A condition of equilibrium (i.e., on the average, a new impact does not cause a net increase in the total number of craters) is reached or almost reached on lunarterrae. (3) The age of theterra surface is more than three average lifetimes of the 2 km to at least 20 km size craters (4) The observedterra crater population isnot the result of two cataclysms, one at the beginning of the stage and one at the end. (5) This population, however, could be the result of a constant or slowly decaying flux continuing until the end of the stage, when the terminal cataclysms occurred. This sequence of events cannot be proven or disproven. (6) If no terminal cataclysm occurred, then the meteoroidal flux during theterra stage was slowly decaying or constant. (7) The formation of all the large multi-ringed basins essentially contemporaneously is doubtful. 相似文献
95.
Series of white light heliograms and oft- and on-band H filtergrams have been obtained, with an average spatial resolution of 1, to study the flare active McMath region 15403 on 11 July, 1978. A great number of accurate heliographic positions were determined for the umbrae, the white light flare patches and several bright H flare knots, as well as along the principal zero filament and an arch prominence. Using the measured heliographic coordinates of these objects their motions could be analyzed in some detail. The velocities of several different objects could be deduced from the coordinates. Since the heliocentric angle of the region was about 45°, the variation in apparent heliographic coordinates also enabled some variations in heights to be determined.It is pointed out that the flare when fully developed, consisted almost entirely of loops. The zero filament which was activated prior to the flare ran between two umbrae of common penumbra and opposite polarity, one belonging to an old, the other to a new spot group. The white light flare developed on both sides of the filament where it passed between these two umbrae; it was also the place where the flare started. Observational evidence appears to indicate that the erupted filament re-formed from below.An indication has been found that there was a link between the motion of some umbrae and the major flare occurrence. 相似文献
96.
The radiative heat flux from 0.1 to 10 bars is estimated on the basis of a “two-cloud” scattering model that fits available spectral data and Pioneer photometry. Deeper than a few bars, the flux is 4.5 W m?2, compared with the 18.8 W m?2 used in an earlier study by Trafton and Stone. A temperature profile is computed, with the H2 pressure-induced opacity; the temperature at 1 bar is found to be 156°K, rather than the commonly accepted 170°K. An additional optical depth of unity at the 0.67-bar level could restore the conventional value; otherwise a considerably cooler atmosphere is a serious possibility. 相似文献
97.
Green's Theorem is developed for the spherically-symmetric steady-state cosmic-ray equation of transport in interplanetary space. By means of it the momentum distribution functionF
o(r,p), (r=heliocentric distance,p=momentum) can be determined in a regionr
arrbwhen a source is specified throughout the region and the momentum spectrum is specified on the boundaries atr
a
andr
b
. Evaluation requires a knowledge of the Green's function which corresponds to the solution for monoenergetic particles released at heliocentric radiusr
o
, Examples of Green's functions are given for the caser
a
=0,r
b
= and derived for the cases of finiter
a
andr
b
. The diffusion coefficient is assumed of the form = o(p)r
b
. The treatment systematizes the development of all analytic solutions for steady-state solar and galactic cosmic-ray propagation and previous solutions form a subset of the present solutions. 相似文献
98.
C. D'uston J. M. Bosqued F. Cambou V. V. Temny G. N. Zastenker O. L. Vaisberg E. G. Eroshenko 《Solar physics》1977,51(1):217-229
Solar wind and interplanetary magnetic field data were obtained by the PROGNOZ 1 and PROGNOZ 2 satellites during the period following the August 4, 1972 (0621 UT) solar flare. A thermalized plasma was recorded one hour after the shock followed two hours later by the plasma piston with a bulk velocity higher than 1700 km s-1. The comparison between the PROGNOZ and PIONEER 9 solar wind data shows an attenuation of the plasma properties with the deflection from the flare's meridian. 相似文献
99.
L. Losco 《Celestial Mechanics and Dynamical Astronomy》1977,15(4):477-488
This paper is the text of a course offered at Cortina d'Ampezzo (NATO Advanced Study Institute-Long time prediction in dynamics 1975). The integral invariants were introduced by H. Poincaré and E. Cartan. This mathematical notion is very tractable by the use of exterior algebra of different forms. Work at the University of Besançon introduces again this theory, in Celestial Mechanics. This paper has two parts: mathematical tools, then an application to then-body problem where, by the use of the 11th local integral, I found some new equations and developed them for the collinear triple collision of three bodies. 相似文献
100.
A two-dimensional brightness distribution of the old supernova remnant G 160.5+2.8 (HB-9) has been measured at a frequency of 25 MHz with the T-Shaped UTR-2 radio telescope, with the resolution 29×27. The source has a distinctly pronounced, almost spherical envelope. At the same time, individual features located near the internal boundary of the source are observable.Certain considerations are put forward to validate a suggestion that the brightest feature in the decameter range (maximum at=4h55m.3 and =46°.37) might be of an extragalactic origin.From structure considerations, the velocity of the pulsar PSR 0459+47 projected on HB-9 is estimated. The estimate does not contradict the hypothesis of Damasheket al. (1978) concerning a possible genetic relation between HB-9 and PSR 0459+47. 相似文献