全文获取类型
收费全文 | 89745篇 |
免费 | 1738篇 |
国内免费 | 868篇 |
专业分类
测绘学 | 2205篇 |
大气科学 | 6354篇 |
地球物理 | 18395篇 |
地质学 | 32123篇 |
海洋学 | 7654篇 |
天文学 | 19901篇 |
综合类 | 240篇 |
自然地理 | 5479篇 |
出版年
2021年 | 774篇 |
2020年 | 879篇 |
2019年 | 909篇 |
2018年 | 2090篇 |
2017年 | 1953篇 |
2016年 | 2534篇 |
2015年 | 1521篇 |
2014年 | 2464篇 |
2013年 | 4678篇 |
2012年 | 2627篇 |
2011年 | 3637篇 |
2010年 | 3046篇 |
2009年 | 4099篇 |
2008年 | 3808篇 |
2007年 | 3583篇 |
2006年 | 3454篇 |
2005年 | 2847篇 |
2004年 | 2778篇 |
2003年 | 2591篇 |
2002年 | 2430篇 |
2001年 | 2211篇 |
2000年 | 2114篇 |
1999年 | 1775篇 |
1998年 | 1843篇 |
1997年 | 1790篇 |
1996年 | 1438篇 |
1995年 | 1472篇 |
1994年 | 1268篇 |
1993年 | 1182篇 |
1992年 | 1138篇 |
1991年 | 1023篇 |
1990年 | 1144篇 |
1989年 | 960篇 |
1988年 | 930篇 |
1987年 | 1107篇 |
1986年 | 912篇 |
1985年 | 1237篇 |
1984年 | 1342篇 |
1983年 | 1302篇 |
1982年 | 1161篇 |
1981年 | 1066篇 |
1980年 | 1029篇 |
1979年 | 928篇 |
1978年 | 939篇 |
1977年 | 847篇 |
1976年 | 857篇 |
1975年 | 788篇 |
1974年 | 815篇 |
1973年 | 784篇 |
1972年 | 512篇 |
排序方式: 共有10000条查询结果,搜索用时 317 毫秒
571.
G. M. Petrov 《Astrophysics and Space Science》1991,177(1-2):269-270
The classical method of determination of the absolute azimuth (or Bessel's parameter n) can secure sufficiently precision for RA from observations of stars at high geographical latitudes during polar night only. 相似文献
572.
W. R. Dick S. Hirte E. Schilbach R. -D. Scholz 《Astrophysics and Space Science》1991,177(1-2):229-234
The observations and the plate reduction technique for the determination of positions and absolute proper motions which is used in Potsdam are described. Recent results have shown that an accuracy of about 0
.
1 for positions and 0
.
7
cent
.
–1
for proper motions can be achieved both for bright (8m–12m) and faint (16m–18m) stars. Three astrometric programmes using the Tautenburg plates are presented. 相似文献
573.
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
574.
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
575.
Glenn M. Frye Jr. Philip P. Dunphy Edward L. Chupp Paul Evenson 《Solar physics》1988,118(1-2):321-346
576.
The surface distribution of M stars is studied by differentiating them according to whether they show a circumstellar dust shell (CS) or not. Analysis shows that galactic latitudinal and longitudinal distributions are not determined by spectral subclasses alone. The study also indicates that M-type stars with CS have higher luminosities in the K band than those without CS. The M stars used in the study are obtained from theTwo-Micron Sky-Survey Catalogue (IRC) which is a most unbiased sample with respect to the interstellar extinction. The CS feature is identified by the ratio of flux densities at 12 and 25 m in the IRAS point source catalog. 相似文献
577.
M. J. Hagyard 《Solar physics》1988,115(1):107-124
We have analyzed the vector magnetic field of an active region at a location of repeated flaring to determine the nature of the currents flowing in the areas where the flares initiated. The component of electric current density crossing the photosphere along the line-of-sight was derived from the observed transverse component of the magnetic field. The maximum concentrations of these currents occurred exactly at the sites of flare initiation and where the photospheric field was sheared the most. The calculated distribution of current density at the flare sites suggested that currents were flowing out of an area of positive magnetic polarity and across the magnetic inversion line into two areas of negative polarity. This interpretation was reinforced by a calculation of the source field, the magnetic field produced in the photosphere by the electric currents above the photosphere. In the vicinity of the flare sites, the calculated source field exhibited three particular characteristics: (1) maximum magnitudes at the sites of flare initiation, (2) a rotational direction where the vertical current density was concentrated, and (3) a fairly constant angular orientation with the magnetic inversion line. The source field was thus very similar to the field produced by two arcades of currents crossing the inversion line at the locations of greatest magnetic shear with orientations of about 60° to the inversion line. With this orientation, the inferred arcades would be aligned with the observed chromospheric fibrils seen in the H data so that the currents were field-aligned above the photosphere. The field thus exhibited a vertical gradient of magnetic shear with the shear decreasing upward from the photosphere. We estimated the currents in the two arcades by matching the source field derived from observations with that produced by a model of parallel loops of currents. We found that the loops of the model would each have a radius of 4500 km, a separation of 1830 km, and carry a current of 0.15 × 1012 A. Values of vertical current densities and source fields appearing in the umbrae of the two large sunspots away from the flare sites were shown to lie at or below the level of uncertainty in the data. The main source of this uncertainty lay in the method by which the 180° ambiguity in the azimuth of the transverse field is resolved in umbral areas. We thus concluded that these quantities in large umbrae should be treated with a healthy skepticism. Finally, we found that the source field at the flare sites was produced almost entirely by the angular difference between the observed and potential field and not by the difference in field intensity. 相似文献
578.
The transport of thermal radiation has been considered within a finite slab which absorb and scatter anisotropically. The problem involves the space-dependent single-scattering albedow(x). Two approximations are taken forw(x). In the first it is represented in exponential form asw(x)=w
0 exp(–x/s), wherew
0 ands are given constants andx is the optical variable. The second approximation assumes the formw(x) =
r=0
R
d
r
*
p
r
(x/a), whered
r
*
are known expansion coefficients anda is the half optical thickness of the slab. Analytic expressions for the forward, backward radiation intensities and fluxes are given in each approximation. The solution of the linear transport equation is performed on the basis of integral Fourier transforms. 相似文献
579.
M. Zabierowski 《Astrophysics and Space Science》1988,141(2):333-338
The property of anthropomorphism as it is held in fractal cosmography is reconstructed.Man is only interested in the probability distribution of mass in cases where mass does not vanish, and he finds no help at all to be told that such cases almost never occur. 相似文献
580.
A survey of the 4(04)-3(03) and 1(01)-0(00) transitions of HOCO+ has been made toward several molecular clouds. The HOCO+ molecule was not observed in any sources except Sgr B2 and Sgr A. The 5(05)-4(04) and 4(14)-3(13) transitions were also detected toward Sgr B2. The results indicate that gas phase CO2 is not a major carbon reservoir in typical molecular clouds. In Sgr B2, the HOCO+ antenna temperature exhibits a peak approximately 2' north of the Sgr B2 central position (Sgr B2[M]) and the 4(04)-3(03) line emission is extended over a approximately 10' x 10' region. The column density of HOCO+ at the northern peak in Sgr B2 is approximately 3 x 10(14) cm-2, and the fractional abundance relative to H2 > or = 3 x 10(-10), which is about 2 orders of magnitude greater than recent predictions of quiescent cloud ion-molecule chemistry. 相似文献