全文获取类型
收费全文 | 46906篇 |
免费 | 977篇 |
国内免费 | 374篇 |
专业分类
测绘学 | 1469篇 |
大气科学 | 3451篇 |
地球物理 | 10502篇 |
地质学 | 15552篇 |
海洋学 | 3846篇 |
天文学 | 10914篇 |
综合类 | 145篇 |
自然地理 | 2378篇 |
出版年
2021年 | 429篇 |
2020年 | 475篇 |
2019年 | 530篇 |
2018年 | 1205篇 |
2017年 | 1116篇 |
2016年 | 1504篇 |
2015年 | 904篇 |
2014年 | 1356篇 |
2013年 | 2347篇 |
2012年 | 1496篇 |
2011年 | 1854篇 |
2010年 | 1709篇 |
2009年 | 2262篇 |
2008年 | 1958篇 |
2007年 | 1947篇 |
2006年 | 1833篇 |
2005年 | 1386篇 |
2004年 | 1408篇 |
2003年 | 1300篇 |
2002年 | 1311篇 |
2001年 | 1170篇 |
2000年 | 1101篇 |
1999年 | 955篇 |
1998年 | 959篇 |
1997年 | 957篇 |
1996年 | 754篇 |
1995年 | 727篇 |
1994年 | 702篇 |
1993年 | 585篇 |
1992年 | 519篇 |
1991年 | 525篇 |
1990年 | 515篇 |
1989年 | 523篇 |
1988年 | 471篇 |
1987年 | 534篇 |
1986年 | 499篇 |
1985年 | 572篇 |
1984年 | 690篇 |
1983年 | 610篇 |
1982年 | 604篇 |
1981年 | 542篇 |
1980年 | 480篇 |
1979年 | 474篇 |
1978年 | 493篇 |
1977年 | 409篇 |
1976年 | 367篇 |
1975年 | 381篇 |
1974年 | 353篇 |
1973年 | 400篇 |
1972年 | 288篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
O vi ( = 1032 Å) profiles have been measured in and above a filament at the limb, previously analyzed in H i, Mg ii, Ca ii resonance lines (Vial et al., 1979). They are compared to profiles measured at the quiet Sun center and at the quiet Sun limb.Absolute intensities are found to be about 1.55 times larger than above the quiet limb at the same height (3); at the top of the prominence (15 above the limb) one finds a maximum blue shift and a minimum line width. The inferred non-thermal velocity (29 km s–1) is about the same as in cooler lines while the approaching line-of-sight velocity (8 km s–1) is lower than in Ca ii lines.The O vi profile recorded 30 above the limb outside the filament is wider (FWHM = 0.33 Å). It can be interpreted as a coronal emission of O vi ions with a temperature of about 106 K, and a non-thermal velocity (NTV) of 49 km s–1. This NTV is twice the NTV of quiet Sun center O vi profiles. Lower NTV require higher temperatures and densities (as suggested by K-coronameter measurements). Computed emission measures for this high temperature regime agree with determinations from disk intensities of euv lines. 相似文献
952.
On March 1, 1980, observations of Saturn from Pic-du-Midi Observatory using a Lallemand electronographic camera led to the discovery of a new satellite (V magnitude ? + 17.5) whose orbital period is surprisingly similar to that of Dione. 相似文献
953.
Photographs in the vicinity of the Earth-Moon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits (A. A. Kamel, 1969, Ph.D. dissertation, Stanford University), were made during August–September 1979, using the 30-in Cassegrain telescope at Leuschner Observatory, Lafayette, California. An effective 2° square field was covered at each position. No discrete objects, either natural or artificial, were found. The detection limit was about 14th magnitude. The present work extends traditional SETI observations to include the search for interstellar probes (R. A. Freitas, Jr., 1980a, J. Brit. Interplanet. Soc.33, 95–100). 相似文献
954.
This article is an English translation of the work cited in 46: 3092. The authors review the conclusions of Pivovarov (see 44: Title 3118) concerning the nature of the urbanization process and optimal city size in the USSR. The relationship between urbanization and the characteristics of a given socioeconomic system is examined, and the possibility of determining optimal size for cities of different functional types is evaluated 相似文献
955.
956.
In the first part, the authors review a group of studies dealing with the identification of regional differences in Italian migratory patterns, especially using the concept of migration field. The second part is devoted to a study of the hierarchical clustering of Italian provinces using 1973 inter-provincial migratory flow data. (summary in ENG, FRE) 相似文献
957.
958.
959.
John P. Vinti 《Celestial Mechanics and Dynamical Astronomy》1977,16(4):391-406
Newtonian cosmology is developed with the assumption that the gravitational constantG diminishes with time. The functional form adopted forG(t), a modification of a suggestion of Dirac, isG=A(k+t)
–1, wheret is the age of the Universe and a small constantk is inserted to avoid a singularity in the two-body problem. IfR is the scale factor, normalized to unity at an epoch time , the differential equation is then
. Here 0 is the mean density at the epoch time. With the above form forG(t), the solution is reducible to quadratures.The scale factorR either increases indefinitely or has one and only one maximum. LetH
0 be the present value of Hubble's constant
/R and 0c
the minimum density for a maximum ofR, i.e., for closure of the Universe. The conditions for a maximum lead to a boundary curve of 0c
versusH
0 and the numbers indicate strongly that thisG-variable Newtonian model corresponds to an open universe. An upward estimate of the age of the Universe from 1010 yr to five times such a value would still lead to the same conclusion.The present Newtonian cosmology appears to refute the statement, sometimes made, that the Dirac model forG necessarily leads to the conclusion that the age of the Universe is one-third the Hubble time. Appendix B treats this point, explaining that this incorrect conclusion arises from using all the assumptions in Dirac (1938). The present paper uses only Dirac's final result, viz,G(k+t)–1, superposing it on the differential equation
. 相似文献
960.
P. Maltby 《Solar physics》1977,55(2):335-346
The effects of the magnetic field as well as the velocity field on sunspot equilibrium are discussed. The gas pressure difference, P, between a spot and the environments in the same horizontal layer is primarily determined by the magnetic field. Using recent model atmospheres we find that P shows a maximum value, P
max, at a depth of 650 ± 150 km below the photosphere. The value of P
max suggests that the curvature of the field lines is important for the equilibrium.It appears that, at an optical depth of unity in the umbra, the density has a value close to that of the environment at the same geometric depth (see Figure 4). If such is the case the expression for the umbra temperature (Equation (15)) may be considerably simplified (Equations (17) and (18)) and compared with observations. 相似文献