Phase angle (astronomy)
Encyclopedia
Phase angle in astronomical observation
s 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 often nearly the same thing as "Sun-object-observer", since the difference depends on the parallax
, 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 phase
s, 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 backscatter
ing; values greater than 90° represent forward scatter
ing.
For some objects, 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 a function of the phase angle, which is generally smooth, except for the so-called opposition spike near 0° and when the object becomes fainter as the angle approaches 180°. This relationship is referred to as the phase curve
.
Observational astronomy
Observational astronomy is a division of the astronomical science that is concerned with getting data, in contrast with theoretical astrophysics which is mainly concerned with finding out the measurable implications of physical models...
s 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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
-object-observer.
For terrestrial observations, "Sun-object-Earth" is often nearly the same thing as "Sun-object-observer", since the difference depends on the parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
, which in the case of observations of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
can be as much as 1°, or two full Moon diameters. With the development of the space travel
Spaceflight
Spaceflight is the act of travelling into or through outer space. Spaceflight can occur with spacecraft which may, or may not, have humans on board. Examples of human spaceflight include the Russian Soyuz program, the U.S. Space shuttle program, as well as the ongoing International Space Station...
, as well as in hypothetical observations from other points in space, the notion of phase angle became independent of Sun and Earth.
The etymology
Etymology
Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...
of the term is related to the notion of planetary phase
Planetary phase
Planetary phase is the term used to describe the appearance of the illuminated section of a planet. Like lunar phases, the planetary phase depends on the relative position of the sun, the planet and the observer....
s, since the brightness
Brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...
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 backscatter
Backscatter
In physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...
ing; values greater than 90° represent forward scatter
Forward scatter
In telecommunication and astronomy, forward scatter is the deflection—by diffraction, nonhomogeneous refraction, or nonspecular reflection by particulate matter of dimensions that are large with respect to the wavelength in question but small with respect to the beam diameter—of a portion of an...
ing.
For some objects, such as Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
(see lunar phases), Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
and Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
the phase angle (as seen from the Earth) covers the full 0-180 range. The superior planets cover shorter ranges. For example, for Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
the maximum phase angle is about 45°.
The brightness of an object is a function of the phase angle, which is generally smooth, except for the so-called opposition spike near 0° and when the object becomes fainter as the angle approaches 180°. This relationship is referred to as the phase curve
Phase curve (astronomy)
In astronomy a phase curve describes the brightness of a reflecting body as a function of its phase angle. The brightness usually refers the object’s absolute magnitude which, in turn, is its apparent magnitude at a distance of one astronomical unit from the Earth and Sun...
.