全文获取类型
收费全文 | 50601篇 |
免费 | 849篇 |
国内免费 | 614篇 |
专业分类
测绘学 | 1282篇 |
大气科学 | 3326篇 |
地球物理 | 9457篇 |
地质学 | 18540篇 |
海洋学 | 4905篇 |
天文学 | 12038篇 |
综合类 | 115篇 |
自然地理 | 2401篇 |
出版年
2022年 | 361篇 |
2021年 | 604篇 |
2020年 | 681篇 |
2019年 | 790篇 |
2018年 | 1552篇 |
2017年 | 1486篇 |
2016年 | 1716篇 |
2015年 | 884篇 |
2014年 | 1591篇 |
2013年 | 2779篇 |
2012年 | 1780篇 |
2011年 | 2256篇 |
2010年 | 2054篇 |
2009年 | 2572篇 |
2008年 | 2251篇 |
2007年 | 2313篇 |
2006年 | 2169篇 |
2005年 | 1443篇 |
2004年 | 1444篇 |
2003年 | 1331篇 |
2002年 | 1299篇 |
2001年 | 1147篇 |
2000年 | 1122篇 |
1999年 | 942篇 |
1998年 | 964篇 |
1997年 | 909篇 |
1996年 | 742篇 |
1995年 | 704篇 |
1994年 | 737篇 |
1993年 | 572篇 |
1992年 | 577篇 |
1991年 | 507篇 |
1990年 | 601篇 |
1989年 | 468篇 |
1988年 | 466篇 |
1987年 | 527篇 |
1986年 | 425篇 |
1985年 | 589篇 |
1984年 | 593篇 |
1983年 | 560篇 |
1982年 | 566篇 |
1981年 | 440篇 |
1980年 | 491篇 |
1979年 | 396篇 |
1978年 | 409篇 |
1977年 | 370篇 |
1976年 | 335篇 |
1975年 | 353篇 |
1974年 | 336篇 |
1973年 | 325篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We give the results of photographic, photoelectric, and spectral observations of the flare star PP Ori. The 109 photographic observations used, which were obtained on the 40 Schmidt telescope of the Byurakan Observatory over a period of about 20 years, and four spectra obtained on the same telescope with a4° objective prism show no variation in brightness. Photoelectric observations in the UBVR bands using the 50cm and 60cm telescopes of the high-altitude Maidanak station of the Tashkent Astronomical Institute in 1987 and 1989 give grounds for suspecting a variation in brightness much larger than observational errors. Spectral observations of the star PP Ori made on the 2.6m telescope of the Byurakan Observatory show weak H emission. The results of all these observations show that PP Ori is an Orion variable of spectral class K7-M0 with absolute visual magnitude7
m
5–8
m
5.Translated fromAstrofizika, Vol. 38, No. 2, 1995. 相似文献
992.
Sh. T. Habibullin 《Earth, Moon, and Planets》1995,71(1-2):43-58
According to A.A. Khentov Venus' rotation is in the quasi-stationary state as a result of the balance interaction of the solar tidal torque with the aerodynamical torque of the rotating Venus' atmosphere. In case of the nonconservative forces are negligible and the solar attraction is the stabilizing factor, the rotation of the rigid Venus may be assumed as the first approximation. The theory of the rotation of the rigid Venus in the coordinates,, had been constructed. It have been found that Venus rotates almost uniformly and the libration harmonics are negligible. 相似文献
993.
Summary Cosmic dust grains play an important role for the thermal, dynamical, and chemical structure of the interstellar medium. This is especially true for the star formation process and the late stages of stellar evolution. Dust grains determine the spectral appearance of protostars, very young stellar objects with disk-like structures as well as of evolved stars with circumstellar envelopes.In this review, we will demonstrate that solid particles in interstellar space are both agent and subject of galactic evolution. We will especially discuss the different dust populations in circumstellar envelopes, the diffuse interstellar medium, and the molecular clouds with strong emphasis on the evolutionary aspects and the metamorphosis of these populations. 相似文献
994.
The zooplankton of two salt ponds at Aveiro was studied to evaluate its density and diversity. Samples were collected biweekly from the salt ponds Esmolas and Tanoeiras. Samples were first separated into Holoplankton (Copepoda, nauplii,Acartia, Ostracoda and Anostraca) and Meroplankton (Mollusca, Insecta, annelidan larvae and Ichthyoplankton). The Holoplankton was mainly composed of:Acartia tonsa, Acartia sp.,Eurytemora velox, Artemia sp., and harpacticoids and calanoids. In both salt ponds, species diversity was identical, but total zooplankton density was higher in the Tanoeiras salt pond, probably because its physical and chemical characteristics allowed the development of stable communities. 相似文献
995.
Heleniak T 《Post-Soviet geography》1995,36(10):644-648
"Since the Russian economy recently has shown sings of stabilization, there now is evidence that demographic trends are beginning to improve somewhat. Although the population declined by 164,200, to 148.1 million at the beginning of 1996, the release of new statistical compendia by the Russian statistical agency (Goskomstat Rosii) in 1995 and early 1996 makes it possible to identify several positive trends.... Life expectancy increased for the first time since 1985. The infant mortality rate declined for the second year in a row. The crude death rate declined for the first time in a decade. The birth rate remained at about the same level for the third consecutive year. The rate of natural increase (births minus deaths), although still negative, recorded a moderate increase." 相似文献
996.
We have calculated the circumstellar extinction curves produced by dust grains which absorb and scatter the stellar radiation in the shells of pre-main-sequence stars. A Monte Carlo method was used to model the radiative transfer in non-spherical shells. The dependence on the particle size distribution and the dust shell parameters has been examined.The application of the theoretical results to explain the extinction and polarization of the Herbig Be star HD 45677 shows that the dust shell is not disk-like and that very small grains are absent in it. 相似文献
997.
N. N. Morozhenko 《Solar physics》1984,92(1-2):153-160
The paper deals with the excitation of the helium singlet level 21
P in the homogeneous and filamentary models of quiescent prominences with following parameters: the optical thickness at the limit of helium Lyman continuum
1c
M
= 0.1–100, T
e
= 7000 K, n
e
= 5 × 1010 cm–3. Assuming a model He atom with seven discrete levels (11
S, 23
S, 21
S, 23
P, 21
P, 33
D, 31
D) and the continuum the steady state equations for the levels 23
S, 21
P and the continuum have been solved together with the radiative transfer equations for the line 584 Å and the continuum 504 Å. The variations with depth of the functions n
2
3
S
/n
1
1
S
(1
c), n
2
1
P
/n
1
1
S
(1c
), and n
+
He
n
e
/n
1
1
S(1c
) as well as the intensities of the triplet (D3, 10830 Å) and singlet (16678, 20581 Å) lines have been calculated. Comparison with observations leads to the following conclusions: (1) The line intensities calculated for filamentary models of prominences agree better with observations than those for homogeneous ones. (2) The helium level 21
P is excited by diffuse field 584 Å being formed by recombinations and spontaneous transitions 21
P – 11
S and escaping from the prominence into the space between the filaments and to the surface. (3) Underpopulation of the singlet level 21
P may be explained by combination of weak excitation mechanism (recombinations and formation of the diffuse field 584 Å) and strong deexcitation mechanism (spontaneous transitions into the level 11
S). 相似文献
998.
The effect of Faraday rotation is shown to lead to the appearance of linear polarization of stellar radiation scattered in an optically-thin circumstellar electron-magnetized shell, even in the case when the shell is spherical. The spectral dependence of the polarization degree is evaluated for scattering in (i) a spherically-symmetric magnetized shell with a power-law radial dependence of the electron density, and (ii) a non-spherical ellipsoidal uniform envelope. The position of maximum in the polarization spectrum permits us to determine the magnetic field magnitude on a star surface. If the rotational and magnetic axes do not coincide, the periodic variability of the polarization will be observed with the period of stellar rotation. Some Be-stars, such as Cas, 48 Lib, EW Lac, Aqr, HD 45677, X Per, are proposed as candidates to be investigated for magnetic fields, as well as some stars of the T Tau-type. This method may be also applied to supernovae shells. 相似文献
999.
The radial velocity and profile variations of UV lines of the shell star Tau have been examined in the IUE spectra obtained in 1978–1982. The neutral atoms, and once or twice-ionized ions (exceptCii, Aliii, Siiii resonance lines) follow the same velocity variations as in the visual spectra, while the Siiv andCiv resonance lines show a constant negative velocity (–50 km s–1 at the core). The Aliii,Cii resonance lines and probably Feiii (mult. No. 34) are formed in both regions, i.e., in lowly-ionized and highly-ionized regions and the Siiii resonance line is formed in a highly-ionized region.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia, 3–7 June, 1983. 相似文献
1000.
H. C. Bhatt T. Chandrasekhar N. M. Ashok J. N. Desai 《Astrophysics and Space Science》1984,104(2):293-296
Near-infrared photometric and low-resolution spectroscopic observations of Aurigae at two phases during the current eclipse are presented. The eclipse depths are found to be wavelength-independent in the infrared right up to 2.5 m. There is no infrared excess at wavelengths shorter than 2.5 m. The light absorbing particles in the eclipsing body must be larger than 10 m in size. 相似文献