全文获取类型
收费全文 | 79469篇 |
免费 | 1158篇 |
国内免费 | 495篇 |
专业分类
测绘学 | 1847篇 |
大气科学 | 5577篇 |
地球物理 | 15105篇 |
地质学 | 31017篇 |
海洋学 | 6428篇 |
天文学 | 16576篇 |
综合类 | 277篇 |
自然地理 | 4295篇 |
出版年
2021年 | 552篇 |
2020年 | 652篇 |
2019年 | 748篇 |
2018年 | 3932篇 |
2017年 | 3677篇 |
2016年 | 2905篇 |
2015年 | 1064篇 |
2014年 | 1554篇 |
2013年 | 2878篇 |
2012年 | 2708篇 |
2011年 | 4740篇 |
2010年 | 4271篇 |
2009年 | 4963篇 |
2008年 | 4148篇 |
2007年 | 4743篇 |
2006年 | 2391篇 |
2005年 | 2155篇 |
2004年 | 2107篇 |
2003年 | 2149篇 |
2002年 | 1836篇 |
2001年 | 1455篇 |
2000年 | 1356篇 |
1999年 | 1206篇 |
1998年 | 1217篇 |
1997年 | 1208篇 |
1996年 | 966篇 |
1995年 | 923篇 |
1994年 | 818篇 |
1993年 | 747篇 |
1992年 | 678篇 |
1991年 | 661篇 |
1990年 | 705篇 |
1989年 | 635篇 |
1988年 | 576篇 |
1987年 | 668篇 |
1986年 | 645篇 |
1985年 | 815篇 |
1984年 | 885篇 |
1983年 | 859篇 |
1982年 | 782篇 |
1981年 | 728篇 |
1980年 | 705篇 |
1979年 | 646篇 |
1978年 | 649篇 |
1977年 | 609篇 |
1976年 | 580篇 |
1975年 | 587篇 |
1974年 | 546篇 |
1973年 | 572篇 |
1972年 | 375篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
We present the results of a spectral study of Galaxy No. 47 of the list [1]. We determine the equivalent widths of the lines for the compact part of the galaxy and the relative intensities for both the compact part and the halo. We estimate the mass of the galaxy from the inclination of the H line. We obtain the electron density and the mass of the gas component of the compact part of the galaxy.Translated fromAstrofizika, Vol. 38, No. 2, 1995. 相似文献
902.
Image intensified video detection systems were used to observe the 1991 Perseid meteor shower from two locations in eastern Canada. In 29.6 hours of total observing time a total of 668 meteors were detected, of which 403 were Perseids. We derived a profile of TVZHR (television zenithal hourly rate) values for the 1991 Perseid shower over the solar longitude (epoch 2000) interval 138°51 to 141°01. The apparent limiting stellar magnitudes of the observing systems were +9.4 and +8.8 (corresponding to limiting meteor magnitudes for our geometry ranging from +8.7 to +7.0). Within the observing period, the maximum TVZHR rate was approximately 1600, and occurred at solar longitude 139.9°. This is in good agreement with the second peak observed by visual observers. The data suggest that TVZHR values should be divided by a factor of approximately 5 to compare TVZHR and ZHR values. 相似文献
903.
"While 'closed-door' immigration policies are adopted by most countries, 'exceptionalist' legislation is often made to permit entry of special immigrant groups. An example is the British Nationality (Hong Kong) Act 1990, which was passed in the run-up to the change in sovereignty of Hong Kong in 1997. Britain's increasingly restrictive immigration policies prior to 1990 [have] resulted in the majority of Hong Kong citizens having British nationality (as British Dependent Territories citizens) but without the right of abode in the U.K. The 1990 Act conferred full British citizenship status on 50,000 heads of households in Hong Kong." The authors conclude that "in a world of marked global inequalities, immigration pressure will become even more extreme and is likely to produce an increasing number of cases of exceptionalist immigration legislation in countries with both ?open' and 'closed'-door policies.' 相似文献
904.
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
905.
Green AE 《International journal of population geography : IJPG》1995,1(1):29-50
"Amongst the key dimensions of population and labour market change in most parts of western Europe and other similar economies in the 1990s are low fertility rates, rising numbers of smaller households, increasing female participation rates, growth in higher level non-manual occupations and the spread of flexible employment patterns. This paper explores how these trends have led to an increase in the number and significance of one particular subgroup: dual career households. An overview from available secondary sources of the geography and socio-economic characteristics of such households in Britain is presented, and directions for possible future research on dual career households are outlined." 相似文献
906.
"Debates concerning the origins and development of the late nineteenth- to early twentieth-century declines in marital fertility and infant mortality in England and Wales have been centred largely on the material provided by answers to the ?special' questions in the 1911 census. In their published form these figures have restricted researchers to an examination of large scale geographic and social class differences in the levels and rates of decline of the two phenomena. This paper outlines research conducted on a sample of individual census returns from the 1911 census. From this data it becomes clear that for Victorian and Edwardian England ?where one lived' was rather more important than ?who one was' in determining both family building strategies and the survival of those children born." 相似文献
907.
In this paper we present multicolor images of the Beta Pictoris disk, obtained with a new Coronograph, built at STScI, for ESO's New Technology Telescope. These B, V, and R observations probe the Beta Pictoris circumstellar disk in to a distance of 35 AU (2) from the central star. We derive surface brightness profiles for the disk and discuss the results in the context of two-component disk models (e.g. Backmanet al. 1992), which are based on fits to available optical and infrared data for Beta Pictoris's disk. These models generally imply that the disk is composed of 1) an outer disk extending from 100–1000 AU, with a midplane particle density decreasing as n(r) r–3, 2) a transition region extending from 10–100 AU with lower density and either smaller grains or a less steep spatial gradient than the outer component, 3) an innermost clear zone with even lower density. 相似文献
908.
Interpretation of X-ray eclipses in SS 433 binary system in the framework of wind-wind collision model yields high value of mass ratioq =m
x
/m
v
> 1 which implies high mass of relativistic objectm
x
> 8M
. 相似文献
909.
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
910.
Interaction of an ejecting pulsar in a binary with a matter surrounding it is considered. Around the pulsar a cavern is formed, and its shape calculated. Non-stationary regime of floating caverns filled with binary radioemission is discussed. This regime can lead to a special radio-burster. It is noticed that in binaries with rather large separation between components a pulsed radioemission from a pulsar can be observable. In this case, the pulsar will be a fine probe for exploration of physical characteristics of a stellar wind from a normal expansion. 相似文献