Phase angle ( Eng. Phase angle ) - the angle between the light incident on the observed object and the light reflected from the object received by the observer. [1] [2] In the framework of astronomical observations, it is usually an angle in the Sun-object-observer system.
For observers on Earth, the angle “Sun-object-Earth” is approximately equal to the angle “Sun-object-observer”, the difference between them depends on the diurnal parallax , which in the case of observations of the moon can be about 1 ° or two diameters of the full moon. [3] With the development of space flights, the concept of a phase angle can become more general and not depend on the Sun and the Earth.
The origin of the name is associated with the concept of the phase of the planet, since the brightness of the object and the proportion of the visible illuminated surface is a function of the phase angle.
The phase angle varies from 0 ° to 180 °. The value 0 ° corresponds to the position at which the illuminating object, observer and the observed object are on one straight line, while the observer and the illuminating object are located on one side of the observed object. This configuration is called confrontation. The value of 180 ° corresponds to the location of the observed object between the illuminating object and the observer; this configuration is called a connection.
For some objects, such as the Moon (see phases of the Moon ), Venus and Mercury, the phase angle, when observed from the Earth, can take on any value in the range 0–180 °. On upper planets, the interval of phase angle values is narrower. For example, for Mars, the maximum value of the phase angle is 45 ° for an observer on Earth.
The apparent brightness of the object is a function of the phase angle, usually this function is smooth except for a peak near 0 °, which does not affect gas giants or bodies with an extended atmosphere; also smoothness can be broken near 180 °. [4] The dependence of gloss on phase is called the phase curve .
Notes
- ↑ Glossary: Planet Rings Node .
- ↑ Martinez, Patrick. The Observer's Guide to Astronomy. - Cambridge University Press, 1994. - P. 557. - ISBN 978-0-5213-79458 .
- ↑ Seager, Sara. Exoplanet Atmospheres: Physical Processes. - Princeton University Press, 2010 .-- P. 33. - ISBN 978-0-6911-4645-4 .
- ↑ Karttunen, Hannu. Fundamental Astronomy. - Springer Science & Business Media, 2007. - P. 153. - ISBN 978-3-540-34143-7 .