The apparent change in direction of a celestial source of light caused by an observer's component of motion perpendicular to the impinging rays. In astronomy, aberration manifests, for example, as a star appearing aberrantly from what its true position on the sky would be because of the motion of the Earth. The star in question appears to move in the same direction of motion as an observer on the Earth, an effect which would not occur if Earth were stationary with respect to the star. Because all astronomical bodies are in motion relative to one another, aberration is experienced universally, but is only subjectively relevant, based on the observer. For humankind as observers, aberration has played a historically significant role in astronomy and physics, specifically in the development of theories regarding light, electromagnetism, and relativity. See also: Astronomy; Astrophysics; Electromagnetism; Light; Motion; Physics; Star

A book that contains astronomical or meteorological data arranged according to days, weeks, and months of a given year and may also include diverse information of a nonastronomical character. This article is restricted to astronomical and navigational almanacs.

The closest spiral galaxy to the Milky Way Galaxy. The name of the Andromeda Galaxy (Fig. 1) derives from its appearance in the constellation of Andromeda. The galaxy is also known as M31 and NGC 224, based on its entries in two widely used catalogs. See also: Andromeda; Astronomical catalogs; Constellation; Messier Catalog; Milky Way Galaxy

The interdisciplinary study that attempts to determine how much astronomy prehistoric people knew and how it influenced their lives. Archeoastronomy (Fig. 1) involves multiple disciplines: astronomy to chart the heavens, archeology to probe the cultural context, engineering to survey sites, and anthropology and ethnology to provide clues to the cultural past. Archeoastronomy has prompted valuable insights into the astronomy of the past and has even revolutionized some models of prehistoric cultures, given that the sky can perform a special role in the scheme of cultural systems. Archeoastronomy and its loose family of disciplines are sometimes subsumed under a broader field of study called cultural astronomy. See also: Anthropology; Archeology; Astronomy; Engineering

One of the many hundreds of thousands of small planets (technically called minor planets) revolving around the Sun, mainly between the orbits of Mars and Jupiter. Asteroids are rocky bodies varying greatly in size from less than 10 meters across (33 feet) to about 530 km (330 miles) across in the case of the largest asteroid, Vesta (Fig. 1). Unlike planets, asteroids are irregularly shaped and nonspherical because they do not possess enough mass to generate sufficient gravity to pull themselves into hydrostatic equilibrium. Asteroid surfaces also characteristically display many impact craters. This heavy cratering is indicative of the old age of asteroids, with most of the bodies dating back to the formation of our solar system 4.6 billion years ago. Asteroids are the remnants of the planet formation process, representing material that did not end up comprising a planet or moon. Asteroids are also present in exosolar systems, evidencing the general universality of planetary formation. See also: Gravity; Mass; Planet; Solar system; Vesta

That part of astronomy dealing with the position, distance, and motion of celestial objects, including solar system objects, stars, radio sources, and galaxies.

Books, CD-ROMs, or computer-accessible sets showing stars, constellations, or other astronomical phenomena and their locations in the sky. Astronomical atlases provide two-dimensional views of the distributions of objects.

Lists or enumerations of astronomical data relevant to astronomy, navigation, geodesy, and space science applications. Astronomical catalogs (see figure) vary a great deal in form and content depending upon the type of the data and the objects to which the catalogs are referring. The essential data may, for instance, be positions and motions of celestial objects, magnitudes, spectra, radial velocities of stars, or energy fluxes. The celestial objects may be exoplanets, stars, galaxies, or other distant phenomena. The objects may also be local phenomena such as bodies in our solar system, including asteroids and comets, or sunspots appearing on the Sun. The celestial objects in question are generally ordered by increasing right ascensions (the angular distance taken along the celestial equator from the vernal equinox eastward to the hour circle of a given celestial body) and identified by a catalog number. Catalogs are either derived directly from observations or are compiled from different sources. See also: Asteroid; Exoplanet; Geodesy; Sun

Schemes for locating astronomical objects in space. To an observer on the Earth's surface, the stars of the night sky appear to be placed upon a spherical shell of infinite radius with the observer at the center. Celestial objects appear to move with respect to the stars, and at any given time their position on this imaginary sphere, called the celestial sphere, can be specified by two angles, called celestial coordinates, whose values depend upon what coordinate system is used. See also: Celestial sphere

The use of spectroscopy (the analysis of light as a function of wavelength) as a tool for obtaining observational data on the chemical compositions, physical conditions, and radial velocities of astronomical objects. Astronomical applications of optical spectroscopy from ground-based observatories cover the electromagnetic spectrum from the near-ultraviolet [wavelengths around 0.3 micrometers (μm)] through the visible (0.4–0.7 μm) and into the near-infrared (2 μm). Space-based observatories, such as the Hubble Space Telescope and the Herschel Space Observatory, extend spectroscopic observations from the far-ultraviolet (0.1 μm) to the far-infrared (200 μm). Work at shorter wavelengths (x-ray and gamma-ray spectroscopy) and longer wavelengths (submillimeter and radio wavelengths) requires techniques other than those discussed here. See also: Gamma-ray astronomy; Hubble Space Telescope; Radio astronomy; Submillimeter astronomy; Ultraviolet astronomy; X-ray astronomy