Twilight Viewing of Planets


by Wes Stone

Telescopes make it possible to see that which is invisible to the eye alone, and facilitate viewing many objects that are accessible to the naked eye. While most visual telescope use (excluding solar observation) takes place under dark skies, useful observations can be made at sunset and sunrise. The planets are quite bright and easily glimpsed during twilight. An unexpected bonus is that the detail seen on planetary disks during twilight may exceed that seen under darker skies. Venus may be seen with the naked eye during twilight or even during daylight. Its phases are its claim to fame as far as amateur telescope users are concerned, but these are often hard to see clearly. Venus is so bright in proportion to its size that atmospheric turbulence and aberrations in telescope lenses often combine to transform the planet into an amorphous, shimmering blob of light from which a focus is almost impossible to obtain. One solution to this problem is to cut down the glare by adding a polarizing filter. Another is to observe Venus during twilight. This second method is preferable for two reasons: 1) Venus shows its striking silvery-white hue much better without a filter; and 2) During twilight, Venus is higher in the sky, where there is less atmospheric turbulence to contend with.

A good opportunity for using this technique came on December 21, 1992. On this date, Venus and Saturn appeared close together in the sky (such a pairing of planets is called a conjunction). At sunset, Venus was already visible. I pointed my scope at Venus, and switched eyepieces to get a magnification of 79x. At greatest elongation from the Sun, Venus appears half-lit, like a quarter Moon. Three weeks remained until this date, but only a slight bulge was visible in the semicircle. The phase was very clean. Although my Orthoscopic eyepiece is excellent, the objective of the old K-Mart 60mm refractor is far from optical perfection. Despite this, only a little color fringing was visible. The air was not extremely steady, but only a few slight ripples distorted the image, and these never threatened the visibility of the phase.

As an added bonus, Saturn was on the edge of the same eyepiece field as Venus. This planet was fairly faint, and not yet visible to the naked eye, but in the scope the rings were clearly visible and easily separated from the planetary disk. I probably saw at least as much detail on Saturn as if I had waited until later to look at it; the planet was well past opposition and sank to the horizon quite early on that night.

Other planets can be observed during twilight with favorable results. Jupiter and its moons are bright enough to see in fairly bright twilight, and the most prominent markings will be visible on Jupiter's disk. During the Comet Shoemaker-Levy 9 impacts, Jupiter was around 90 degrees from the Sun. To get the best view of the impact sites, I often observed in late afternoon, using Venus to position the 11-inch telescope's setting circles and then swinging over to Jupiter. Jupiter was invisible to the naked eye, but apparent in the 8x50 finder scope. It appeared washed out in the main scope, but a polarizing filter fitted to the eyepiece did the trick. Rotated to the correct angle, it dimmed the glare from the sunlight while allowing Jupiter to shine through.

It is almost imperative to observe Mercury during twilight, since this planet almost never rises in a dark sky or sets after evening twilight's end. On any given night, the best time to observe detail on a planet is when it is high in the sky, away from the thick, turbulent atmosphere near the horizon. If this favorable position occurs during dawn or dusk, don't fret. You may be rewarded with a great view.



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