page_title: Phase curve (astronomy)
section_title: Venus
hierarchical_section_title: Phase curve (astronomy) / Venus
caption_attribution_description: English: Phase curve of Venus
context_page_description: 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 astronomical unit from the Earth and Sun. The phase angle equals the arc subtended by the observer and the sun as measured at the body.
  The phase curve is useful for characterizing an object's regolith and atmosphere. It is also the basis for computing the geometrical albedo and the Bond albedo of the body. In ephemeris generation, the phase curve is used in conjunction with the distances from the object to the Sun and the Earth to calculate the apparent magnitude.
context_section_description: The relatively flat phase curve of Venus is characteristic of a cloudy planet. In contrast to Mercury where the curve is strongly peaked approaching phase angle zero (full phase) that of Venus is rounded. The wide illumination scattering angle of clouds, as opposed to the narrower scattering of regolith, causes this flattening of the phase curve. Venus exhibits a brightness surge near phase angle 170°, when it is a thin crescent, due to forward scattering of sunlight by droplets of sulfuric acid that are above the planet's cloud tops. Even beyond 170° the brightness does not decline very steeply.
  The history of observation and analysis of the phase curve of Venus is similar to that of Mercury. The best set of modern observations and interpretation was reported by A. Mallama, D. Wang and R. Howard. They used the LASCO instrument on SOHO and ground-based, CCD equipment to observe the phase curve from 2 to 179°. As with Mercury, these new data are the major source of the phase curve used in the Astronomical Almanac for predicting apparent magnitudes.
  In contrast to Mercury the maximal apparent brightness of Venus as seen from Earth does not occur at phase angle zero. Since the phase curve of Venus is relatively flat while its distance from the Earth can vary greatly, maximum brightness occurs when the planet is a crescent, at phase angle 125°, at which time Venus can be as bright as magnitude −4.9. Near inferior conjunction the planet typically fades to about magnitude −3 although the exact value depends on the phase angle. The typical range in apparent brightness for Venus over the course of one apparition is less than a factor of 10 or merely 1% that of Mercury.
caption: The phase curve of Venus[15] compared to Mercury,[1] and the brightness excess of Venus.
exif: {}