Telescope Observation
Although any telescope (or steadily-held binoculars) will allow the phases of Venus to be seen, the planet is a notoriously difficult one to observe, for three main reasons. Firstly, whenever the planet is visible (shortly before sunrise or after sunset) it is positioned at a low altitude (angle above the horizon) where it is immersed in haze and atmospheric turbulence, which adversely affects the quality of the image seen in the telescope. Consequently most telescopic observers prefer to view the planet in full daylight - when its altitude is much higher - taking special care to shield the Sun from view. Secondly, the planet's brilliance - although making it a very obvious and beautiful object to the naked-eye - causes a menacing glare when
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seen through a telescope. Thirdly, the planet itself is permanently cloaked in thick cloud, so its surface features are never visible through telescopes. Most amateur astronomers can therefore only expect to observe its characteristic phase changes, but little else.
Experienced observers using medium and larger-sized telescopes often observe detail in its cloud features by attaching ultraviolet (UV), infrared (IR) or colour filters to the telescope eyepiece. In the latter category, yellow, blue and violet filters help to reduce the glare of the planet and improve the contrast of its cloud features whilst orange and red filters help to reduce the brightness of the sky during daylight observations. Even when using large telescopes, observers often report that Venus does not take high magnification well; magnifications over 300x are rarely beneficial, the image succumbing to the effects of low contrast and atmospheric turbulence.
Features observed in the planet's atmosphere include 'Y', 'Greek letter Psi' or 'C' shaped cloud patterns, bright spots, cusp caps, cusp extensions and irregularities along the terminator (the line separating the light and dark sides).
With patience, perseverance and considerable experience, detailed images of the planet can be obtained using CCD cameras and modified webcams (excellent examples of which can be seen at Damian Peach's website and on the ESA site). Although modern technology has allowed much detail to be resolved and captured electronically, many observers prefer to record their observations of the planet with more traditional sketches and drawings made at the eyepiece, a time-consuming but rewarding experience (see for example a selection of sketches produced by observers at the Astronomy Sketch of the Day website).
Apart from slight variations caused by light-scattering in the Earth's atmosphere, Venus normally appears white through telescopes. Its clouds are actually pale lemon-yellow but this can be difficult to discern because the coloration is subtle and is easily lost in the planet's …show more content…
glare.
There are two frequently observed but anomalous effects which are well-known to observers of Venus. They occur at very different times during the planet's apparition; namely, when it reaches half-phase (around the time of its greatest elongation) and when it displays a large, thin crescent phase (in the days leading up to - or following - inferior conjunction). More details about Venus' varying orbital configurations and phases can be found on the Planet Movements page.
The moment at which Venus attains an exact half-phase is referred to as the dichotomy, at which point the terminator appears perfectly straight.
In theory, Venus should appear exactly half-phase (phase = 0.50 or 50%) when it reaches greatest elongation, however it frequently does not because of eccentricities in the orbits of Venus and the Earth. Even when these eccentricities have been factored in, the date of the predicted dichotomy and that of the observed dichotomy often differ by up to several days; this is known as Schröter's effect (after the German astronomer Johann Schröter, who first noted the phenomenon in 1793). The date of dichotomy for evening elongations often occurs earlier than predicted, whilst that of morning elongations occurs later than predicted. The exact reason for the phenomenon is not known; some attribute it to the scattering of sunlight in Venus' atmosphere, others simply to errors in observation. It is however worth noting that the terminator normally appears straight for about four days in each
elongation.
seen through a telescope. Thirdly, the planet itself is permanently cloaked in thick cloud, so its surface features are never visible through telescopes. Most amateur astronomers can therefore only expect to observe its characteristic phase changes, but little else.
Experienced observers using medium and larger-sized telescopes often observe detail in its cloud features by attaching ultraviolet (UV), infrared (IR) or colour filters to the telescope eyepiece. In the latter category, yellow, blue and violet filters help to reduce the glare of the planet and improve the contrast of its cloud features whilst orange and red filters help to reduce the brightness of the sky during daylight observations. Even when using large telescopes, observers often report that Venus does not take high magnification well; magnifications over 300x are rarely beneficial, the image succumbing to the effects of low contrast and atmospheric turbulence.
Features observed in the planet's atmosphere include 'Y', 'Greek letter Psi' or 'C' shaped cloud patterns, bright spots, cusp caps, cusp extensions and irregularities along the terminator (the line separating the light and dark sides).
With patience, perseverance and considerable experience, detailed images of the planet can be obtained using CCD cameras and modified webcams (excellent examples of which can be seen at Damian Peach's website and on the ESA site). Although modern technology has allowed much detail to be resolved and captured electronically, many observers prefer to record their observations of the planet with more traditional sketches and drawings made at the eyepiece, a time-consuming but rewarding experience (see for example a selection of sketches produced by observers at the Astronomy Sketch of the Day website).
Apart from slight variations caused by light-scattering in the Earth's atmosphere, Venus normally appears white through telescopes. Its clouds are actually pale lemon-yellow but this can be difficult to discern because the coloration is subtle and is easily lost in the planet's …show more content…
glare.
There are two frequently observed but anomalous effects which are well-known to observers of Venus. They occur at very different times during the planet's apparition; namely, when it reaches half-phase (around the time of its greatest elongation) and when it displays a large, thin crescent phase (in the days leading up to - or following - inferior conjunction). More details about Venus' varying orbital configurations and phases can be found on the Planet Movements page.
The moment at which Venus attains an exact half-phase is referred to as the dichotomy, at which point the terminator appears perfectly straight.
In theory, Venus should appear exactly half-phase (phase = 0.50 or 50%) when it reaches greatest elongation, however it frequently does not because of eccentricities in the orbits of Venus and the Earth. Even when these eccentricities have been factored in, the date of the predicted dichotomy and that of the observed dichotomy often differ by up to several days; this is known as Schröter's effect (after the German astronomer Johann Schröter, who first noted the phenomenon in 1793). The date of dichotomy for evening elongations often occurs earlier than predicted, whilst that of morning elongations occurs later than predicted. The exact reason for the phenomenon is not known; some attribute it to the scattering of sunlight in Venus' atmosphere, others simply to errors in observation. It is however worth noting that the terminator normally appears straight for about four days in each
elongation.