全文获取类型
收费全文 | 136497篇 |
免费 | 3024篇 |
国内免费 | 1261篇 |
专业分类
测绘学 | 3287篇 |
大气科学 | 9851篇 |
地球物理 | 28182篇 |
地质学 | 48295篇 |
海洋学 | 12215篇 |
天文学 | 29829篇 |
综合类 | 546篇 |
自然地理 | 8577篇 |
出版年
2022年 | 755篇 |
2021年 | 1316篇 |
2020年 | 1494篇 |
2019年 | 1641篇 |
2018年 | 3603篇 |
2017年 | 3333篇 |
2016年 | 3918篇 |
2015年 | 2234篇 |
2014年 | 3696篇 |
2013年 | 6962篇 |
2012年 | 4358篇 |
2011年 | 5582篇 |
2010年 | 4825篇 |
2009年 | 6298篇 |
2008年 | 5560篇 |
2007年 | 5457篇 |
2006年 | 5273篇 |
2005年 | 4645篇 |
2004年 | 4626篇 |
2003年 | 4227篇 |
2002年 | 3751篇 |
2001年 | 3219篇 |
2000年 | 3153篇 |
1999年 | 2608篇 |
1998年 | 2615篇 |
1997年 | 2460篇 |
1996年 | 2176篇 |
1995年 | 2114篇 |
1994年 | 1860篇 |
1993年 | 1745篇 |
1992年 | 1633篇 |
1991年 | 1572篇 |
1990年 | 1694篇 |
1989年 | 1492篇 |
1988年 | 1334篇 |
1987年 | 1606篇 |
1986年 | 1416篇 |
1985年 | 1728篇 |
1984年 | 2002篇 |
1983年 | 1906篇 |
1982年 | 1744篇 |
1981年 | 1649篇 |
1980年 | 1445篇 |
1979年 | 1402篇 |
1978年 | 1411篇 |
1977年 | 1263篇 |
1976年 | 1218篇 |
1975年 | 1185篇 |
1974年 | 1167篇 |
1973年 | 1219篇 |
排序方式: 共有10000条查询结果,搜索用时 541 毫秒
881.
We have obtained velocity-resolved spectra of the 12C II 157.8 microns 2P3/2-2P1/2 fine structure line in the M42 region of Orion. Observations at 0.8 km s-1 resolution with a laser heterodyne spectrometer show multiple velocity components in some locations, with typical linewidths of 3-5 km s-1. Spectra of theta 1C and BN-KL also show weak emission from the F = 2-1 hyperfine component of the equivalent 13C II line. From the observed 12C II/13C II line intensity ratios, we deduce that the 12C II emission is optically thick with tau approximately 5 at both positions. Excitation temperatures of 128 K and 90 K, together with column densities of approximately 1 x 10(19) cm-2 and approximately 4 x 10(18) cm-2, are derived for theta 1C and BN-KL, respectively. 相似文献
882.
An observational test--the detection of a hydrogen coma around comets at large heliocentric distances--is proposed for determining whether comets were formed by the agglomeration of unaltered, ice-coated, interstellar grains. Laboratory experiments showed that amorphous water ice traps H2, D2, and Ne below 20 K and does not release them completely until the ice is heated to 150 K. Gas/ice ratios as high as 0.63 are obtainable. Thus, if the ice-coated interstellar grains were not heated above approximately 110 K, prior to their agglomeration into cometary nuclei, the inward propagating heat waves should release from the comets a continuous flux of molecular hydrogen. This flux would exceed that of water molecules at approximately 3 AU preperihelion and approximately 4 AU postperihelion. 相似文献
883.
The following physical parameters have been computed for the Jovian atmosphere between 270 and ?300 km: (1) Pressure, (2) Density, (3) Speed and sound, (4) Number density, (5) Density scale. It has considered that the top of the clouds is at 0 km. For the calculations of these parameters we have used:
- for the altitudes 270-0 km data from Voyager I and II.
- for the altitudes ?300–0 km data from Voyager II and spectroscopic observations.
884.
R. A. Howland 《Celestial Mechanics and Dynamical Astronomy》1988,44(3):209-226
The librational motion of the Ideal Resonance Problem (Garfinkel, 1966, Jupp, 1969) is treated through an initialnon-canonical transformation which, however, leaves the equations of motion in a quasi-canonical form, with Hamiltonian expressed in standard trigonometric functions amenable to traditional averaging techniques. The perturbed solutions, similarly expressed intrigonometric near-identity transformations, and their frequencies can be found to arbitrary order, with the elliptic integrals expected of the system introduced only in a final explicit quadrature for a Kepler-type equation in the angular variable. The specific transformations and resulting equations of motion are introduced, and explicit solutions for the original variables are found to second order, with mean motion accurate to fifth. Limitation of the present solution to the librational region, the extension of that solution to higher order, and observations on the form of the associated Hamiltonian are also discussed. 相似文献
885.
Tepev Mons is a large volcanic structure of about 250 km in diameter with an elevation of 5 km above the surroundings, located at the southwestern edge of Bell Regio. It is surrounded by a moat with a depth of about 0.5 km. If this moat is considered to be caused by bending of the lithosphere due to the load of the volcano, then elastic bending models give limits for the effective flexural rigidity FR and the effective elastic thickness of the lithosphere L: 2 x 1023 Nm FR 3 x 1024 Nm and 30 km L 100 km. High flexural rigidities are associated with small depressions and large thicknesses of the lithosphere and vice versa.Contribution No 345, Institut für Geophysik der Universität Kiel, F.R.G. 相似文献
886.
A. O. Allakhverdiyev O. H. Guseinov F. K. Kasumov I. M. Yusifov 《Astrophysics and Space Science》1986,121(1):21-44
A new way of choosing of the calibrators for constructing (D)-dependence of supernova remnants (SNR) is suggested. A knowledge of the distance of an SNR is not sufficient to consider it as a calibrator. It is shown that neither more accurate determination of distance to calibrators, nor an increase in their number, nor an increase in the precision of the remnant angular size and radio flux determination can lead to a considerable improvement of simple (D)-dependence which is suggested in the given work.For remnants with small surface brightnesses there is no (D)-dependence at all. The most accurate remnant distances currently known, are given: these are liable to change only after a large amount of additional observational data accurately compiled. 相似文献
887.
The similarity in properties of GK Per and BV Cen suggests that the latter may be a classical nova remnant rather than a dwarf nova. Our H and V CCD photometry show no sign of a nova shell but disclose the presence of a close and probably physical companion. BV(RI)
c
CCD photometry of the companion, assumed to be a dwarf, givesM
v
=3.1±0.3 for BV Cen at quiescence. Even though half of the light in BV Cen comes from the secondary this is much brighter than expected for a dwarf nova and supports the conclusion that BV Cen was an unrecorded nova. 相似文献
888.
SeveralN-body experiments were performed in order to simulate the dynamical behaviour of systems of galaxies gravitationally dominated by a massive dark background.We discuss mass estimates from the dynamics of the luminous component (M
VT) under the influence of such a background, assuming a constant dark/luminous mass ratio (M
D/M
L) and plausible physical conditions. We extend in this way previous studies (Smith, 1980, 1984) about the dependence ofM
VT on the relative distributions of dark and luminous matter (Limber, 1959). We found that the observed ratio of the virial theorem mass to luminosity (M
VT/L) in systems of galaxies of different sizes could be the result of different stages of their post-virialisation evolution as was previously suggested by White and Rees (1978) and Barnes (1983). This evolution is mainly the result of the dynamical friction that dark matter exerts on the luminous component. Thus our results give support to the idea that compact groups of galaxies are dynamically more evolved than large clusters, which is expected from the hierarchical clustering picture for the formation of such structures. 相似文献
889.
An analysis of the effects of the mass transfer on the unsteady free-convection flow of a viscous incompressible fluid, past an impulsively started infinite porous vertical limiting surface with heat sources is presented, when the free-stream velocity and the suction velocity, are oscillating in the time about constant mean values. Approximate solutions for the coupled nonlinear equations are derived for the mean velocity, the mean temperature, the mean skin-friction, and the mean rate of heat transfer. All the above quantities are shown graphically followed, by a discussion. 相似文献
890.
The published photometric and spectroscopic data of the symbiotic binary V 1329 Cyg are interpreted. It is shown, that V 1329 Cyg is an eclipsing binary with an elliptical orbit orbit (e=0.28). The cooler component fills up the Roche-lobe at periastron. A model of moving gaseous structures in the system is proposed and their influence on the radial velocity curve is shown. The following characteristics of the system are derived: the cooler component is an M6 giant with mass 7.9M
, radius 339R
and luminosityM
bol=–5.42, the hot component is a white dwarf surrounded by an accretion disk. The mean distance between the components is 842R
and in periastron it decreases to 605R
. 相似文献