全文获取类型
收费全文 | 52598篇 |
免费 | 936篇 |
国内免费 | 421篇 |
专业分类
测绘学 | 1214篇 |
大气科学 | 3954篇 |
地球物理 | 10498篇 |
地质学 | 18995篇 |
海洋学 | 4400篇 |
天文学 | 11486篇 |
综合类 | 111篇 |
自然地理 | 3297篇 |
出版年
2021年 | 418篇 |
2020年 | 497篇 |
2019年 | 511篇 |
2018年 | 1167篇 |
2017年 | 1144篇 |
2016年 | 1364篇 |
2015年 | 879篇 |
2014年 | 1343篇 |
2013年 | 2631篇 |
2012年 | 1577篇 |
2011年 | 2118篇 |
2010年 | 1810篇 |
2009年 | 2431篇 |
2008年 | 2115篇 |
2007年 | 2114篇 |
2006年 | 2023篇 |
2005年 | 1557篇 |
2004年 | 1581篇 |
2003年 | 1485篇 |
2002年 | 1416篇 |
2001年 | 1318篇 |
2000年 | 1239篇 |
1999年 | 1053篇 |
1998年 | 1032篇 |
1997年 | 1063篇 |
1996年 | 850篇 |
1995年 | 835篇 |
1994年 | 794篇 |
1993年 | 668篇 |
1992年 | 662篇 |
1991年 | 596篇 |
1990年 | 655篇 |
1989年 | 586篇 |
1988年 | 550篇 |
1987年 | 641篇 |
1986年 | 567篇 |
1985年 | 723篇 |
1984年 | 762篇 |
1983年 | 782篇 |
1982年 | 716篇 |
1981年 | 642篇 |
1980年 | 680篇 |
1979年 | 577篇 |
1978年 | 581篇 |
1977年 | 531篇 |
1976年 | 511篇 |
1975年 | 509篇 |
1974年 | 478篇 |
1973年 | 475篇 |
1972年 | 315篇 |
排序方式: 共有10000条查询结果,搜索用时 562 毫秒
371.
The rigorous Kerker-Matijevi formulae for light scattering by co-axial double cylinders are used to calculate the extinction properties of hollow organic grains. A size distribution of such particles together with iron whiskers of radii 0.01 m, silica spheres of radius 0.03 m and free aromatic molecular clusters comprised of 50–100 atoms yield excellent agreement with data on the extinction of starlight. The mass ratios of silica to organics and of iron to organics are in good accord with cosmic abundance constraints. 相似文献
372.
The variations of the declinations of 36 Belgrade zenith stars from BZS170 Catalogue published in Astronomy and Astrophysics,177, 313–316 (1987) are examined by spectral analysis. The prospect is to evaluate multiples of the Chandler period of which the most prominent for all stars is the double one. 相似文献
373.
G. Z. Xie F. K. Liu Y. Y. Zhu J. H. Fan R. W. Lu 《Astrophysics and Space Science》1991,179(2):321-329
It has become clear in recent years that relativistic beaming is a good explanation for the BL Lac phenomenon. Of studies based on the relativistic beaming model of BL Lac objects, we note that the orientation of jet's axis to the line-of-sight is very small and, therefore, the observed flux emitted from a rapidly moving source is orders of magnitude higher than the flux in its rest-frame:F
obs = 3 +
F
intr, where is the bulk relativistic Doppler factor. Then the observed apparent magnitudem
v
must be corrected for this effect. For our 39 samples, the corrected apparent magnitudem
v
corr
and logZ have a good correlation. 相似文献
374.
E. F. Arias C. Boucher M. Feissel J. -F. Lestrade 《Astrophysics and Space Science》1991,177(1-2):187-192
The relative orientations of various VLBI celestial reference frames are evaluated on the basis of coordinate differences of common sources. It is shown that an accuracy better than 0.001 can be achieved. Possible regional deformations in the different catalogues are investigated; they are found to reach a few 0.001 in some restricted zones. The application of these studies to the realisation of a combined celestial reference frame consistent with the BIH Terrestrial System is outlined. 相似文献
375.
Quasi-steady high-temperature current sheets are an energy source during the main or hot phase of solar flares. Such sheets are shown to be stabilized with respect to the tearing instability by a small transverse component of magnetic field existing in the sheets. 相似文献
376.
B. Andersen V. Domingo A. Jiménez A. Jones S. Korzennik P. L. Pallé F. Pérez Hernández C. Régulo T. Roca Cortés L. L. Tomás 《Solar physics》1988,116(2):391-399
Monochromatic extinction coefficients at four wavelengths have been obtained over a period of more than two years at the Observatorio del Teide (Izaña Tenerife) using a full disc, direct sunlight, quadruple photometer devoted to the detection of integral luminosity oscillations of the Sun. The mean extinction coefficients (0.13 at 500 nm) show a seasonal variation of about 15%, the best atmospheric conditions being in winter and autumn. Moreover, in anyone day the extinction coefficient in the afternoon is always lower than the one in the morning by 7%. A one-year period fluctuation, with an amplitude of 0.035 mag, has been identified in the instrumental magnitudes outside the atmosphere, and is interpreted as the variation produced by the different Sun-Earth distance from winter to summer. Finally, the study made to detect periodic time fluctuations in both, Sun's magnitude and extinction coefficients, has given null results at levels of 0.04 and 1.8%, respectively. 相似文献
377.
F. P. Keenan 《Solar physics》1988,116(2):279-284
R-matrix calculations of electron impact excitation rates in S vi are used to determine the theoretical temperature sensitive emission line ratios
and
which are found to be up to 30% larger than the earlier results of Flower and Nussbaumer. A comparison of the present data with solar observations from the Harvard S-055 spectrometer on board Skylab implies that the 944.5 Å transition may be blended with lines from species with relatively low ionization potentials, in contrast to the findings of Flower and Nussbaumer. The 712.8 Å transition may also be similarly blended. 相似文献
378.
Spectrophotometric observations of the eclipsing binary system Capricorni, covering the wavelength interval 3300–7300 Å, have been presented. Comparison of the standard spectral scans of Cap with the spectral scans of the stars of known spectral types and luminosity classes taken from the Breger (1976) catalogue shows that, near the phase of secondary eclipse, Cap shows the spectral-luminosity type as A8-9III. This is in agreement with the photometric findings of Srivastava (1987b). H emission is not visible convincingly, however, some irregularities are apparent, at least in two scans, around the H region. 相似文献
379.
We present the results of the photometric observations, in theUBVRJHKLMNQ system, of CH Cyg, made in the period 1978–1987. They have shown that from 1985 to 1987 dust condensed in its circumstellar envelope have taken place the condensation of the dust matter. In 1987 its mass and temperature were about 3×10–8
M
and 800 K, respectively. 相似文献
380.
Liouville's theorem for radiation, of which the generalized étendue is a consequence, implies 2 d2
d2
A = constant along the ray path, where is the refractive index and d2
and d2
A are the ranges, respectively, of solid angle and of area that define a ray (actually a bundle of rays). Implications of this concept on the propagation of radio waves from the actual to the apparent source in the solar corona (i.e., the scatter image of the true source) are discussed. The implications for sources of fundamental plasma radiation include: (1)The observed solid angle (defining the directivity) and apparent area A of the source are compatible with Liouville's theorem only if the apparent source (the scatter image of the true source) corresponds to the envelope of subsources with a small filling factor f. (2) The brightness temperature T
Bof the actual source is greater than that of the apparent source by f
-1. (3) For sources of fundamental plasma radiation the factor f is very small ( 10-2). (4) A long-standing discrepancy between the observed low value of T
B at meter/decameter wavelengths for the quiet Sun and the known coronal temperature may be resolved by noting that the implied coronal temperature is given by T
B
f and that the factor f must be significantly less than unity.A brief discussion is included of the relation between Liouville's theorem, the generalized étendue, Milne's laws, occupation numbers, extension in phase, and suppression of emission by a medium with refractive index unequal to unity. 相似文献