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Observing Comet 45P/Honda-Mrkos-Pajdusakova

Seeing a comet visually is more difficult than the pretty pictures make it appear. Standing next to the photographer, few people would have noticed the comet above, and binoculars were required for most to see it at all. This image of comet PANSTARRS was captured by SkyTools user Ernie Parker on March 14, 2013 at StargateCOlorado Observatory in Trinidad, CO.

All You Need is a Small Telescope or Binoculars

Observing 45P from the northern hemisphere through January 5th will be relatively easy. This comet will not be visible to the naked eye, but if you know where to look, it will be fairly easy to spot in a small telescope or binoculars. 

Timing is Everything

45P is visible right after sunset, low in the southwestern sky, so a location with a relatively clear western horizon is required. The less light pollution the better. The tricky part is the timing. The comet is following the setting sun, so it will be highest in the sky as twilight begins, but very difficult to see because of the bright sky. As the minutes pass the sky will become darker, but the comet will move closer to the horizon, making it harder to see. There are few magic minutes in between when the comet will be most easily seen. 

Unfortunately, the exact time depends on your latitude and how bright your sky is. If I knew exactly where you would be observing from I could tell you exactly when and where to look. But we can get pretty close. The chart below shows the position of the comet on the evening of January 2. Note the position of Venus and Mars and how they point out where the star Theta Cap is. The comet will be near this star, so finding this star will be most of the battle.

45P on the evening of January 2, as seen for New Mexico 

The chart below shows the approximate position of the comet each night until the moon begins to interfere too much. It shows the field of view and stars that you would see from a dark location in typical 7x50 binoculars. The date and best time to view is marked for each night. The times depend on your latitude. If you are in the south, say at 24N in Florida, it may be as late as 7PM before you get the best view. Farther north, at say 50N, the best view may be as early as 5:50 PM. 

Rather than go by the time, It is best to go out as the sun is setting and start looking for Venus and Mars. As the sky darkens, more stars will appear. Looking at the naked-eye chart above, try looking where you think the star Theta Cap is. This is the brighter star that 45P is moving toward as each night passes. It will pass close to this star on the nights of the 4th and 5th. Holding your gaze steady, bring the binoculars up to your eyes. The brightest star you see will likely be Theta Cap. Use the binocular finder chart provided below to spot the comet relative to this star on the night you are looking.

On New Year's Eve the moon will be to the right of the comet, which may help in spotting it. But the moon will be a very thin crescent and difficult to spot itself. It will also be outside of the comet field of view in typical binoculars. 

The comet will appear as a faint round smudge of light. In very dark skies, or in a small telescope, a faint tail may be apparent, pointing up and to the left, away from the sun. 

Behind the Scenes -- How I Made the Calculations

You may notice that the predictions of other web sites and various software are different from those presented here. Others have focused on the close pass to the earth in February, but I think this is a mistake. Unless it brightens dramatically, the best opportunity to see this comet is in early January. 

I follow the observations that are reported by observers around the world. Based on these observations I compute magnitude, coma diameter, and "degree of concentration" parameters. These parameters are made available to SkyTools users when they update their Current Comets observing list, which is always kept up to date with the latest bright comets. Using these parameters, SkyTools can accurately predict the magnitude and size of the comet for a month or two in advance, presuming that the comet doesn't suddenly change. If the comet does change suddenly, the parameters are quickly updated. 

But there is more to being able to detect a comet than its magnitude alone. In fact, the magnitude of a comet is a poor predictor of visibility in the eyepiece. This is because comets vary greatly in size and how concentrated their light is. A large diffuse comet will be much more difficult to spot than a small one, or one that has a bright center, even though the magnitudes are the same. To make things even more interesting, a bright twilight sky will affect the visibility of the large or diffuse comet to a greater degree than that of the small or concentrated one. It is at this point that most people simply throw up their hands.

For my SkyTools software, I developed a scientific model for the detectability of extended objects in the eyepiece, based on the sky brightness and visual contrast. Given the magnitude, size, and degree of concentration, this model can be successfully applied to comets.

As the altitude of the comet and brightness of the sky changes during the night, SkyTools samples the visual detectability of the comet, selecting the time of night when the contrast is at its highest. When this is done for successive nights, a Nightly Optimum Viewing Ephemeris can be computed. It is this ephemeris that was used to make the predictions above, and no other software can compute it. 

The ephemeris can be displayed on a chart, like the binocular finder chart above, for any location and telescope. 

The Story of Comet 45P

Sixty-eight years ago, in December 1948, Moniru Honda discovered a new comet. Antonín Mrkos, and Ľudmila Pajdušáková also shared in the discovery. 

45P started way out in the Oort cloud, where millions of comets circle the sun, so distantly that they cannot be detected. Out that far they are easily deflected, and sometimes they fall toward the sun and the inner solar system. Most make a pass and then head back out to where they came from, perhaps never to be seen again. But a few will pass close enough to the gravity of a planet, usually Jupiter, such that the orbit is altered. These comets can be deflected so that they no longer go far away from the sun, but orbit the sun regularly, and because they periodically appear in our skies, they are called periodic comets. Each periodic comet is given a number. The most famous is the first one that was recognized, comet 1P/Halley. The comet discovered by Honda is the 45th numbered periodic comet. The orbit of 45P stretches out to Jupiter, but every 5.3 years it passes closer to the sun, between the orbits of Venus and Mercury. It is at these times that it can be spotted in backyard telescopes, although some opportunities are better than others. This is one of the better opportunities.

The next time 45P/Honda-Mrkos-Pajdusakova will be close to the earth and sun will be in early April 2022, when it is predicted to reach a maximum brightness of magnitude 8.5, more than a magnitude fainter than now.

Greg Crinklaw — Astronomer and Developer of SkyTools 

SkyTools 3, because the astronomy matters.