A series of images of Venus: January 19-September 11, 2001.
Click here to see these images as a movie.
Contents: Venus's dense atmosphere reflects sunlight well, but shows no features. However, the orbital geometry produces constantly changing phase and angular size, as this time series shows. Specifically, in the sun-centered model of Copernicus, Venus orbits the sun and should exhibit phases of illumination running from 0 to 100%. By contrast, in the earth-centered model of Ptolemy, Venus moves around an epicycle on the near side of the sun, and can never reach 50% illumination. The shape of Venus in our first image alone is enough to rule this model out. Galileo's telescopic observations of Venus were one of the reasons he thought he had proven the validity of Copernicus's model.
The details of each Venus observation (the angular diameter of the planet, its distance from earth, its angular distance from the sun, its illumination phase, its brightness, and the elevation above the horizon at the time of the observation) are tabulated in the Venus tables web page.
Our first observation was made just two days past greatest elongation, the time when Venus is at its greatest angular distance from the sun as viewed from earth. For a while, Venus is nearer to the earth in each successive image (and hence looks bigger) and closer in the sky to the sun (and hence shows a skinnier phase). The image nearest conjunction (March 27) was taken while Venus was quite low in the sky, so that atmospheric turbulence has broadened the razor thin crescent of that day. But Venus misses perfect alignment with the sun by passing a bit to its north. The part of Venus illuminated is determined by the direction to the sun. At first this is to the west (and the right side is illuminated), then to the south at closest approach (and the bottom is illuminated), and finally to the east (and the left is illuminated).
Whenever visible, Venus is the brightest object in the sky after the sun and moon. The total brightness changes remarkably little with time. While in the crescent phase only a small fraction of the planet is illuminated, this occurs when the planet is close and hence large. Furthermore, Venus' dense atmosphere is especially good at forward scattering (the glancing deflection of sunlight seen at crescent phase).
The angular size peaked at 59.2 arcsececonds at inferior conjunction on March 30. From there it will drop to a minimum size of 9.8 arcseconds at superior conjunction next year on January 14.
Processing: The most important limitation to taking sharp images of Venus is air turbulence. Our sharpest images are taken with the shortest possible exposure time (0.11 second) and when Venus is high in the sky (large elevation). However, since Venus is always close to the sun (elongation less than 48 degrees) this always occurs in bright daylight. This is not a show-stopper as Venus is even bright enough to be seen against the blue sky with the unaided eye. (Try this at home! You need to know where to look. The wunderground online planetarium will give you the direction. A pair of binoculars will help. Be sure first to focus it on the furthest thing you can see.)
For our images, one practical problem is avoiding saturating the camera with too much light. This can be done by using filters that only let through light at wavelengths where the sun is not very bright (such as ultraviolet) or that only let through a very narrow range of wavelengths (such as a hydrogen alpha filter, which lets through only red light of a special wavelength). Since the sky is blue and Venus is not, red filters have the additional advantage of emphasizing Venus relative to the sky.
For each image dark subtraction and flat fielding corrections were made. For some, unsharp masking was also applied. Finally, adjustment of the brightness and contrast allows us to subtract the uniform skylight and show only Venus in the final images. The time of each observation, the duration of each exposure, the filter used, and the name of the observer (Calvin students and faculty) are tabulated in the Venus tables web page.
Orientation and scale: North is up and East is to the left. Each image 125 by 111 arcseconds. The linear diameter of Venus is 12,100 km.
Content updated 6/25/01