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Phase angle (astronomy)

 

 
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Phase angle in astronomical observation
Observational astronomy

Observational astronomy is a division of the astronomy science that is concerned with getting data, in contrast with theoretical astrophysics which is mainly concerned with finding out the measurable implications of physical model s....
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 , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
-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
Parallax

Parallax is an apparent displacement or difference of orientation 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....
 between the two distant positions of the observer on the rotating surface of the Earth, which, in the case of observations of the Moon
Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
 can be as much as 1°, or two full Moon diameters.






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Phase angle in astronomical observation
Observational astronomy

Observational astronomy is a division of the astronomy science that is concerned with getting data, in contrast with theoretical astrophysics which is mainly concerned with finding out the measurable implications of physical model s....
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 , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
-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
Parallax

Parallax is an apparent displacement or difference of orientation 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....
 between the two distant positions of the observer on the rotating surface of the Earth, which, in the case of observations of the Moon
Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
 can be as much as 1°, or two full Moon diameters. With the development of the space travel
Spaceflight

Spaceflight is the use of space technology to achieve the flight of spacecraft into and through outer space.Spaceflight is used in space exploration, and also in commercial activities like space tourism and telecommunications satellite....
, 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 roots and history of words; and how their form and meaning have changed over time.In languages with a long detailed history, etymology makes use of philology, the study of how words change from culture to culture over time....
 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

Backscatter is the reflection of waves, particles, or signals back to the direction they came from. The term is used in astronomy and several fields of physics, as well as in photography and medical ultrasonography....
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 incident electromagnetic wave, in such a manner...
ing.

For some planets, such as Moon
Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
 (see lunar phases), Venus
Venus

Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus , the Roman mythology goddess of love....
 and Mercury
Mercury (planet)

Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 88 days. The orbit of Mercury has the highest Orbital eccentricity of all the Solar System planets, and it has the smallest axial tilt....
 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

In cryptography, MARS is a block cipher that was IBM's submission to the Advanced Encryption Standard process. MARS was selected as an AES finalist in August 1999, after the AES2 conference in March 1999, where it was voted as the fifth and last finalist algorithm....
 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°.

See also

  • Illumination angle.