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A & G example articles

Astronomy & Geophysics is published by Blackwell's for the RAS. Many major articles are available only to members of the RAS, but these pages provide free access to selected reviews, chosen to be of broad interest, to illustrate the nature of those articles.
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[X-ray binary]

Evidence for the Black Hole Horizon by Ramesh Narayan
(A&G 44, 6.22, 2003; The 2002 George Darwin Lecture)

Astronomers have discovered many candidate black holes in X-ray binaries and in the nuclei of galaxies. The candidate objects are too massive to be neutron stars and for this reason they are considered to be black holes. While the evidence based on mass is certainly strong, there is no proof yet that any of the objects possesses the defining characteristic of a black hole, namely an event horizon. Type I X-ray bursts, which are the result of thermonuclear explosions when gas accretes on to the surface of a compact star, may provide important evidence in this regard. Type I bursts are commonly observed in accreting neutron stars, which have surfaces, but have never been seen in accreting black hole candidates. It is argued that the lack of bursts in black hole candidates is compelling evidence that these objects do not have surfaces. The objects must therefore possess event horizons.

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[Solar eclipse]

Historical eclipses and Earth's rotation by F. Richard Stephenson
(A&G 44, 2.22, 2003; The 2002 Harold Jeffreys Lecture)

The Earth, in its diurnal rotation, acts as a remarkably accurate timekeeper. However, small variations in the length of the day occur at the millisecond level. Historical eclipse observations, recorded by various ancient and medieval cultures, enable changes in the Earth's spin rate to be monitored with fair precision as far back as around 700 BC. Although lunar and solar tides are the main causes of long-term changes in the length of the day, the early observations reveal that non-tidal mechanisms are also important. In this paper both the historical development of this subject and recent advances are reviewed.

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[Inflationary bubbles]

Cosmology: a matter of all and nothing by John Barrow
(A&G 43, 4.8, 2002; The 2002 Gerald Whitrow Lecture)

The modern picture of the expanding Big Bang universe is described. Implications of the expansion for the evolution of life are highlighted, together with the new features contributed by the inflationary universe theory. Observational tests of inflation are described along with some of the possibilities introduced by new theories of strings and quantum gravity. These theories allow the numbers of dimensions of space and of time to be larger than the three and one we experience and permit the observed constants of Nature to vary slowly in time. Recent astronomical evidence that is consistent with small variations of the fine-structure constant is described, and some of its far-reaching implications discussed.

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[Sunspot image]

Solar variability and climate change: is there a link? by Sami K. Solanki
(A&G 43, 5.9, 2002; The 2001 Harold Jeffreys Lecture)

Radiation from the Sun makes Earth a habitable planet. Fluctuations in the solar output are therefore likely to affect the climate on Earth, but establishing both how the output of the Sun varies, and how such variations influence Earth's climate, have proved tricky. However, increasing quantities of data from the Sun and about the climate on Earth over recent years mean that rapid progress is being made. This paper reviews the current debate on the influence of the Sun and summarizes the state of play in this area of solar physics.

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[Galaxy survey]

The expansion rate of the universe by Wendy Freedman
(A &G 43, 1.10, 2002; The 2001 George Darwin Lecture)

Modern cosmology is undergoing an explosion of observational and experimental results that is in turn driving significant theoretical advances and a dynamic interface between theory and experiment. As a consequence, cosmological parameters are becoming much more precisely constrained. In this, the George Darwin lecture for 2001, Freedman looks back at the some of the advances made since Edwin Hubble presented his George Darwin lecture in 1953, and looks ahead to the resolution of significant cosmological uncertainties.

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[Martina valleys]

Martian oceans, valleys and climate by Michael Carr
(A&G 41, 3.20, 2000; The 2000 Harold Jeffreys Lecture)

Mars Global Surveyor altimetry shows that the heavily cratered southern hemisphere of Mars is 5 km higher than the sparely cratered plains of the northern hemisphere. Previous suggestions that oceans formerly occupied the northern plains, as evidenced by shorelines, are partly supported by the new data. A previously identified outer boundary has a wide range of elevations and is unlikely to be a shoreline; an inner contact with a narrow range of elevations is a more likely candidate. No shorelines are visible in the newly acquired, 1.5 metre/pixel imaging. Newly imaged valleys provide strong support for sustained or episodic flow of water across the Martian surface. A major surprise, however, is the near absence of valleys less than 100 m across. Martian valleys seemingly do not divide into ever smaller valleys, as terrestrial valleys commonly do. This could be due to lack of precipitation or lack of surface runoff because of high infiltration rates. High erosion rates and formation of valley networks supports warm climates and presence of large bodies of water during heavy bombardment. The climate history and fate of the water after heavy bombardment remain controversial.

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[Distant galaxies]

The story of galaxy evolution in full colour by Richard Ellis et al.
(A&G 41, 2.10, 2000)

A Holy Grail of modern astronomy is understanding the origin of Edwin Hubble's morphological sequence of galaxy types. What made some collapsing gas clouds turn into elegant spiral systems like our own Milky Way, whereas others became smooth, featureless ellipticals? More fundamentally, does the taxonomic scheme introduced by Hubble in the 1920s have any physical relevance? The Space Telescope that bears Hubble's name is providing answers to these and other questions in the context of modern theories of structure formation.

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