Mohawk Valley Astronomical Society

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Twilight Spark

by Perry Pezzolanella

Mercury is one of the brightest planets in the sky, often brighter than Saturn, but it is one of the most elusive. The difficulty with Mercury is that even under the best observing conditions it can only be observed low in the twilight sky where atmospheric turbulence from the cooling evening causes blurring. It also has to compete with local topography such as mountains, hills, trees, and buildings. The best observing time usually finds Mercury as a brassy spark in the bright twilight barely above the trees and rooftops, and more often between tree branches and houses. Mercury provides a rewarding challenge since it is seldom seen; legend has it that Copernicus never saw Mercury.

Under the best conditions, Mercury never appears more than 12 arcseconds in diameter at which point it is nearly at its dimmest, appearing as a fingernail-thin crescent too close to the Sun to safely observe. Mercury can shine as bright as magnitude -2, but during that time it is very tiny, nearly full, and also too close to the Sun for serious observing. In between these two extremes is a compromise of viewing Mercury going through phases from gibbous to half and to a nice crescent, increasing from 6 to 10 arcseconds across and shining around magnitude 0.

Mercury is actually best seen during spring evenings and autumn mornings given a clear sky and good horizon. During those times of day at those times of year the path in the sky that the planets follow known as the ecliptic is nearly perpendicular to the horizon. This places Mercury higher above the horizon than at any other time of the year. The fun part is finding Mercury and following it each evening. A small telescope will add to the fun by resolving the phases as Mercury goes from a small, nearly full disc to a larger, thinning crescent in the evening and from a large, thickening crescent to a nearly full disc in the morning. In spite of the effort to find and follow Mercury, very few observe it long enough to observe surface detail.

Is it really possible to see any surface detail on Mercury? With the advancement of telescopes, optics, and filters, direct observing through a telescope is yielding more convincing views of dusky patches. Use of an orange filter increases the odds of seeing detail along with observing around half phase when there is still enough surface area visible and a reasonable size. To increase the odds of observing detail, it is best to use at least 180x to resolve the phases. Then use an orange (W21) filter to help steady the image and darken the bright twilight background. Viewing Mercury higher up before the Sun sets will increase luck and the steadier air may allow for a tantalizing glimpse of Mercury’s delicate markings. Caution must be exercised to not accidentally sweep up the Sun when locating Mercury if not using a programmable go-to mount. Tracking Mercury during the morning as it rises will help in following it well after sunrise high into the sky above the haze and turbulence. The sky must be absolutely free of haze and high clouds, and the optics must be spotless.

The best way of seeing surface detail is through photography. There have been many successful images of Mercury showing detail that can be matched to a map. Some photographers lucky enough to observe Mercury in dry climates with bountiful clear skies have been able to photograph Mercury over the course of an entire evening or morning apparition and combine the photos into a time lapse. The result is a short video of a beautiful view of Mercury’s changing size and phase while dusky surface details slowly rotate to boot!

What color is Mercury? Most observers describe Mercury appearing with a copperish or brassy hue, which could be due to observing it low towards the horizon where the Earth’s thicker atmosphere can redden it. However, Mercury is a rocky world with no clouds, so it certainly will never look as brilliant or as white as Venus. It is amazing what can be seen with Mercury if the weather cooperates and patience is exercised. A planet so elusive can end up being quite rewarding to novice and experienced observers alike!