全文获取类型
收费全文 | 106111篇 |
免费 | 1955篇 |
国内免费 | 904篇 |
专业分类
测绘学 | 2715篇 |
大气科学 | 7655篇 |
地球物理 | 21146篇 |
地质学 | 37203篇 |
海洋学 | 9063篇 |
天文学 | 24240篇 |
综合类 | 288篇 |
自然地理 | 6660篇 |
出版年
2021年 | 902篇 |
2020年 | 1014篇 |
2019年 | 1057篇 |
2018年 | 2418篇 |
2017年 | 2293篇 |
2016年 | 2921篇 |
2015年 | 1764篇 |
2014年 | 2943篇 |
2013年 | 5532篇 |
2012年 | 2979篇 |
2011年 | 4107篇 |
2010年 | 3553篇 |
2009年 | 4729篇 |
2008年 | 4360篇 |
2007年 | 4200篇 |
2006年 | 3966篇 |
2005年 | 3334篇 |
2004年 | 3253篇 |
2003年 | 3010篇 |
2002年 | 2826篇 |
2001年 | 2624篇 |
2000年 | 2466篇 |
1999年 | 2143篇 |
1998年 | 2191篇 |
1997年 | 2110篇 |
1996年 | 1718篇 |
1995年 | 1732篇 |
1994年 | 1530篇 |
1993年 | 1410篇 |
1992年 | 1370篇 |
1991年 | 1298篇 |
1990年 | 1450篇 |
1989年 | 1275篇 |
1988年 | 1177篇 |
1987年 | 1376篇 |
1986年 | 1169篇 |
1985年 | 1497篇 |
1984年 | 1681篇 |
1983年 | 1590篇 |
1982年 | 1522篇 |
1981年 | 1381篇 |
1980年 | 1253篇 |
1979年 | 1175篇 |
1978年 | 1192篇 |
1977年 | 1064篇 |
1976年 | 1045篇 |
1975年 | 941篇 |
1974年 | 1012篇 |
1973年 | 971篇 |
1972年 | 620篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
R. Surendiranath N. Kameswara Rao Ram Sagar J. S. Nathan K. K. Ghosh 《Journal of Astrophysics and Astronomy》1990,11(2):151-166
bdAbstract The open cluster NGC 2818 containing a planetary nebula has been observed inVRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope. The study extending to starsV ∼ 21 magnitude establishes the distance modulus as(m-M)
0 = 12.9 ±0.1 for the cluster. Based on the fitting of theoretical isochrones computed for solar metallicity, an age of 5(±1)
× 108 years has been assigned to the cluster. Association of the planetary nebula with the cluster indicates that the progenitor
mass of the planetary nebula on the main sequence is ≥2.5M⊙
Based on observations obtained with the Vainu Bappu Telescope. 相似文献
103.
A new procedure to separate the instrumental and atmospheric components of stray light is presented. It is based on the dependence of the aureole's atmospheric component on the air mass and is applied to measurements taken with the Vacuum Newton Telescope (VNT) at the Observatorio del Teide (Tenerife). The resulting instrumental part is independent of the air mass.The variation of both components with wavelength is also studied. The instrumental component shows no dependence on wavelength, in contrast to the atmospheric one which is greater in the blue than in the red.It is concluded that observations with air masses larger than two will probably be strongly affected by stray light. 相似文献
104.
The long-term modulation of galactic cosmic rays is investigated from 1957 up to 1992 analysing the dynamic and the quasi-stationary components, separately. It has been found that the dynamic component is characterized by the presence of two peaks at the maximum phase of each solar activity cycle. We infer that the time interval between the two peaks corresponds to a period (well-related to the polar heliomagnetic reversal) in which somewhat decreased activity occurs for intense and long-lasting solar events. In fact, a contemporary dip in the magnetic energy released from the Sun was observed, in agreement with the suggested double maximum displayed by the basic features of the 11-year solar-activity cycle (Gnevyshev, 1977, and references therein). Moreover, the dynamic component of cosmic-ray modulation often shows a multi-structured profile in both peaks of activity, fairly well-connected with the pattern of the green corona brightness. On the other hand, analysing the quasi-stationary long-term trend of cosmic-ray intensity we pick out a good relationship between periods of enhanced cosmic-ray modulation and the area expansion of coronal intensity levels. The relevance of our results for solar-terrestrial forecasting is underlined. 相似文献
105.
U. C. Joshi A. K. Sen M. R. Deshpande J. S. Chauhan 《Journal of Astrophysics and Astronomy》1992,13(3):267-277
Photopolarimetric observations of comet Austin with the IAU/IHW filter system were obtained on the 2.34 m Vainu Bappu Telescope
(VBT) of the Indian Institute of Astrophysics, at Kavalur, India, during pre-perihelion phase on February 20,1990 and on the
1.2 m telescope of the Physical Research Laboratory at Gurusikhar, Mount Abu during postperihelion phase on May 2 and 4, 1990.
The comet appeared bluer than a solar analog during post-perihelion phase on May 2 and 4. The percent polarization shows a
sharp increase towards the red on May 2 and 4. The dominant sizes of the dust particles appear to lie in a narrow range of
0.1 to 0.5 Μm. Regarding the molecular band emission, CN and C2 bands are quite strong; C3 emission was also found to be strong though the observations on May 2 and 4 show significant variation as compared to C2 emission. Molecular band polarization for CN, C3, C2 and H2 O+ have been calculated. It has been found that emission polarization in CN, C2 and C3 is between 1–7% (phase angle between 107.4–109 degrees). For CN and C2 the polarization values are close to the theoretically predicted values, but for C3 the polarization value falls much below the theoretically predicted value. A similar result was found for comet Halley. 相似文献
106.
107.
108.
109.
We analyzed data from 23 boreholes at 19 sites in central and eastern Canada, for the purpose of estimating ground surface temperature (GST) histories. These boreholes were logged down to at least 550 m depth with thermistor probes. Thermal conductivity measurements had been previously made at small depth intervals for the entire depth ranges of most of the boreholes. The temperature profiles of these boreholes do not indicate water disturbance. We estimated terrain effects for each borehole using a time dependent solid-angle method. The thermal perturbations caused by lakes or deforestation near the borehole sites are insignificant in most cases. However, four of the holes were found to be severely influenced by terrain effects. GSTs estimated from the borehole data less influenced by the terraineffects form two groups. The first group, which are generally from data of better quality, show a cold period near the end of the last century before the recent warming trend; the second show it 80–100 years earlier. We consider the former typical of the climate of the Boreal climatic region of Canada. The difference between the two groups may reflect the spacial variability of the climate. Four GST estimates do not belong to either type, and the reasons are discussed. 相似文献
110.
Gavin L. England Birger Rasmussen Neal J. McNaughton Ian R. Fletcher David I. Groves & Bryan Krapez 《地学学报》2001,13(5):360-367
SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime 207 Pb/206 Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event. 相似文献