Phase angle (astronomy)

Phase angle (astronomy)

Phase angle in astronomical observations is the angle between the light incident onto an observed object and the light reflected from the object. In the context of astronomical observations, this is usually the angle Sun-object-observer.

For terrestrial observations, "Sun-object-Earth" is a simplified model, since the precise angle would vary to a degree depending on the parallax between the two distant positions of the observer on the rotating surface of the Earth, which, in the case of observations of the Moon can be as much as 1°, or two full Moon diameters. With the development of the space travel, as well as in hypothetical observations from other points in space, the notion of phase angle became independent of Sun and Earth.

The etymology of the term is related to the notion of planetary phases, since the brightness of an object and its appearance as a "phase" is the function of the phase angle.

The phase angle varies from 0° to 180°. The value of 0° corresponds to the position where the illuminator, the object and the observer are collinear, with the illuminator and the observer on the same side of the object. The value of 180° is the position where the object is between the illuminator and the observer, known as the astronomical opposition. Values less than 90° represent backscattering; values greater than 90° represent forward scattering.

For some planets, such as Moon (see lunar phases), Venus and Mercury the phase angle (as seen from the Earth) covers the full 0-180 range. The superior planets cover shorter ranges. For example, for Mars the maximum phase angle is about 45°.

The brightness of an object is function of the phase angle, which is generally smooth, except for the so-called opposition spike near 0° and with the object generally becoming brighter when the angle is approaching 0° and 180°.

ee also

*Illumination angle.

Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

  • Phase angle (disambiguation) — Phase angle may refer to one of the following.*Phase angle vectors, phasors, and periodic phenomena *Phase angle (astronomy) the angle between the incident light and reflected light …   Wikipedia

  • phase angle — noun 1》 Physics a phase difference expressed as an angle, 360 degrees corresponding to one complete cycle. 2》 Astronomy the angle between the lines joining a given planet to the sun and to the earth …   English new terms dictionary

  • phase — phaseless, adj. phasic, phaseal, adj. /fayz/, n., v., phased, phasing. n. 1. any of the major appearances or aspects in which a thing of varying modes or conditions manifests itself to the eye or mind. 2. a stage in a process of change or… …   Universalium

  • X-ray astronomy — X rays start at 0.008 nm and extend across the electromagnetic spectrum to 8 nm, over which the Earth s atmosphere is opaque. X ray astronomy is an observational branch of astronomy which deals with the study of X ray observation and detection… …   Wikipedia

  • List of astronomy acronyms — This is a compilation of acronyms commonly used in astronomy. Most of the acronyms are drawn from professional astronomy and are used quite frequently in scientific publications. However, a few of these acronyms are frequently used by the general …   Wikipedia

  • Precession (astronomy) — In astronomy, precession refers to the movement of the rotational axis of a body, such as a planet, with respect to inertial space. In particular, it refers to the precession of the Earth s rotational axis, also called the precession of the… …   Wikipedia

  • Lunar phase — Moon phase redirects here. For the manga and anime series, see Tsukuyomi: Moon Phase. Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere. The apparent wobbling of the Moon is known as libration. The… …   Wikipedia

  • Orbital plane (astronomy) — All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi flat disk called the orbital plane. The orbital plane of an object orbiting another is the… …   Wikipedia

  • Opposition surge — The opposition effect brightens the area around Buzz Aldrin s shadow due to retroreflective properties of lunar regolith. The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect[1]) is the brig …   Wikipedia

  • Albedo — The albedo of an object is the extent to which it diffusely reflects light from the sun. It is therefore a more specific form of the term reflectivity. Albedo is defined asthe ratio of diffusely reflected to incident electromagnetic radiation. It …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”