Printer Friendly

Observing minor moons: look for some of the solar system's more challenging objects.

My earliest memories out under the stars are of observing Mars during its close approach in 1954. That's 60 years ago, and when I think back to that time, I don't think anyone I knew ever thought of trying to see Phobos or Deimos--or, for that matter, any other of the smaller solar system moons. Back then, we knew that all the planets except Mercury, Venus, and Pluto had moons, but we also knew you were limited to viewing Earth's moon, the four brighter moons of Jupiter, and Saturn's largest moon, Titan. Maybe your telescope was large enough to allow you to identify a few more of Saturn's moons as well, but I don't remember anyone trying for Neptune's Triton or any of the moons of Uranus.

Times have changed. The smaller-sized telescopes of visual observers have been supplanted by large Dobsonians. And while light pollution has limited the value of urban/ suburban observing sites, many amateurs haul large light buckets to remote dark sky sites and see many objects considered impossible by amateurs in the 1950s.

Thus, the observation of solar system moons has progressed from the large, bright moons, such as the Galilean moons of Jupiter, to the medium-bright moons, like Titan and Rhea, to dim ones such as the Uranian moons and Triton, and on to challenging moons like Phobos, Deimos, Himalia, and Phoebe. As amateur instruments increased in size, visual limits dissolved, and dimmer targets became more possible.

What Have I Seen?

I've observed 22 solar system moons: 1 of Earth (of course), 2 of Mars, 5 of Jupiter, 8 of Saturn, 5 of Uranus, and 1 of Neptune. These are visual observations, using just a telescope and an eyepiece. No cameras or image intensifies were involved.

The first truly minor moons I observed were Phobos and Deimos. They were shown to me through the 11-inch Brashear Refractor of Wagman Observatory, owned and operated by the Amateur Astronomers Association of Pittsburgh, Pennsylvania, during the close approach of Mars in 2003. The club member running the telescope at the time employed an occulting bar eyepiece to help defeat Mars's glare. Later I found Deimos with ray 8-inch LX-200 while using an occulting filter eyepiece to snag it. (See p. 33 for more on occulting bars and filters.) Both observations were conducted in the light-polluted skies of suburban Pittsburgh. Over the years, I've also observed Uranus's challenging moon Miranda and Jupiter's challenging moon Himalia in the very dark skies of the Okie-Tex Star Party.

How Do You Find Them? What Do You See?

As you can imagine, observing minor moons is much like observing asteroids. The fun and challenge is not in the details you see; it's in proving that you successfully observed your target, i.e., proving which of the small objects in the field of view is that target. And how do you do that? At first I drew what I saw and compared that to data from planetarium programs. I now use the asteroid-hunting method described by the Astronomical League ( On Day 1, locate an asteroid (or moon) and mark its location on a star chart. On Day 2 (perhaps the next night or the first clear night after Day 1, which could be many days later), locate the same object and mark its new location on the star chart. Finally, using the object's original marked position, return your telescope to that spot and verify that it's no longer there.

A year ago, Tom Dobbins wrote about the visual observation of Galilean moon details (S&T: Jan. 2014, p. 54). Nothing like that is even remotely possible with these much smaller bodies. The following are some examples of what I've seen as described in my logs.

When I observed Deimos with my 8-inch scope, I used a 25-mm Orthoscopic eyepiece with a violet occulting filter and 3x Barlow, resulting in 240x. I drew a map of the field of view (FOV), showing Mars, a few field stars, and a very faint dot about three Martian diameters from the planet. Comparing the angle of lines going from the dot to Mars, and from Mars to the brightest nearby star, I saw the same pattern in my planetarium software. Deimos was seen right where it was predicted to be. There was no sign of Phobos, which my software indicated should have been between Deimos and Mars, about one planetary diameter from Mars. I imagine it was buried in light pollution and/or Mars's glare.

I found Uranus's moon Miranda by a somewhat similar technique. While using my 17.5-inch Dobsonian telescope at the 2009 Okie-Tex Star Party, a 9-mm Nagler eyepiece resulted in 223x. I observed Uranus and made a sketch of the FOV. The very few visible field stars were later confirmed by my planetarium software. Miranda's predicted magnitude that night was +16.5. That's dim, but the skies were very dark. Nevertheless, I was surprised to later find a dot in my drawing right where Miranda was predicted to be. There were no field stars displayed near Miranda in my drawing or by the software.

Jupiter's VI moon, Himalia, was my next target after I failed on several attempts to find the planet's V moon, Amalthea. Amalthea is somewhat brighter than Himalia, but is very near the planet, masked in glare. My Himalia observations were also made at the Okie-Tex Star Party, this time in 2010. We've had good luck with Okie-Tex weather over the years, maybe losing only four nights of the 25 or 30 we've been there. I used a 20-mm eyepiece for 100x and a 12-mm eyepiece for 167x with my 17.5-inch Dobsonian. The beginning of the week suffered from wind gusts. Sunday night, I had several weak "maybes" for Himalia. Monday was similar. Tuesday, I was sure I'd observed Himalia, and it was a dim dot, slowly blinking on and off. I assumed the wind gusts were causing much of that. On Wednesday, I confirmed Tuesday's Himalia observations by returning to my previous position and verifying nothing was visible there. I then went to Himalia's next predicted position and easily located it. I showed it to two of my friends. Thursday, one of my friends used my charts to find Himalia with his 12.5-inch f/6 Dobsonian.

When printing charts for Himalia, I inserted a circle that represented the FOV for my 20-mm eyepiece and my f/4.5 17.5-inch Dobsonian. Himalia was about 3A of the FOV away from Jupiter. As soon as Jupiter was off the eyepiece, dim stars appeared, allowing me to star-hop to Himalia's location. This became easier as the days passed and I became more familiar with the patterns of these dim stars. Because we saw Himalia move from one day to the next, there is no question in our minds we observed Jupiter VI. But even though Himalia was fairly distant from Jupiter at the time, the glare was still significant.


There are many planetarium programs that give excellent asteroid position predictions. Add a FOV circle representing your eyepiece and scope to aid in the asteroid's identification and have at it. I learned the hard way that many software packages' minor moon predictions are not accurate at all, so I use the NASA/JPL HORIZONS Web-Interface ( to produce an ephemeris. If you're not familiar with the term "ephemeris," it's a table that lists an astronomical object's predicted positions. (See for more tips on generating ephemerides.)

Generate an ephemeris for your target and compare its output with what's predicted by your planetarium software. If you find it checks out, generate ephemerides for several moons on several different widely separated dates and again compare them with your software's predictions. You might be able to prove complete reliability for your program and not need to generate any future ephemerides.

I've successfully tested both SkyTools 3 Pro ( and Guide 9.0 ( They're very straightforward, and I use them without concern for predictive accuracy. The Himalia chart included here was produced with SkyTools. It shows a naked-eye diagram, a finder diagram and, most importantly, an eyepiece FOV diagram for the target moon, Himalia. A minor shortcoming of SkyTools is that it doesn't yet provide predictions for Jupiter's moon Elara or Saturn's moon Janus, but Version 4 will correct that. (The absence of Janus may not be an issue since it has never been visually observed to the best of my knowledge.) By the way, either moon can be manually plotted from ephemeris data for a given point in time by using what SkyTools calls "Sky Marks." Guide has a reputation for plotting solar system moons with extreme accuracy. It certainly does for me. While its graphics might not be as modern as those of some of its competitors, they are customizable and perform excellently. The authors of both software packages are easy to contact and provide excellent support.

Let's walk through a set-up for minor moon observations, assuming we want to observe Phoebe (Saturn IX). Saturn will be at opposition and high in the sky at midnight on May 22, 2015. Open Guide, set the date, time, and observing location as accurately as possible. Use the "Go to > Planets" option to retrieve positional data for Phoebe. Zoom in and out to get the desired level of chart detail and, finally, increase/decrease the number of shown stars to bring the dimmer ones to a magnitude comparable to Phoebe's. The results should resemble the Guide chart pictured here. (NB: Since I had previously successfully tested Guide against the NASA/JPL HORIZONS data, I saw no need to generate and compare an ephemeris to Guide for this observation attempt.)

The inner circle represents the FOV of a 9-mm 100[degrees] eyepiece with my 17.5-inch Dobsonian. Notice that Saturn is buried in the plots of its major moons in the lower left of the chart and out of the FOV. Thus, glare should be reduced as you search for Phoebe's dim dot. Theoretically, Phoebe should be easy to identify based on the pattern of stars in the eyepiece, but if it's not visible, the next step would be to go to a similar focal length Orthoscopic or Plossl eyepiece and try again. With much less glass, these eyepieces should absorb less light; that might help Phoebe's visibility. An eyepiece of higher magnification may also be of assistance, since it increases contrast. A benefit of both is that Saturn might be out of the FOV.

What Future Targets Remain?

When I first began this project, I thought it would be a nice challenge to observe all the solar system's moons that were known when I was a child: Earth 1, Mars 2, Jupiter 12, Saturn 9, Uranus 5, and Neptune 2.1 then noted the magnitudes of some of Jupiter's and Neptune's minor moons and very quickly ruled them out.

Jupiter and Saturn do, however, provide some moons I have yet to see that are possibly within the magnitude range of our current scopes. Jupiter has Amalthea (magnitude 14.1) and Elara (magnitude 16.3). Saturn has Janus (magnitude 14.4) and Phoebe (magnitude 16.5). Perhaps some are possible from a very dark site, but Amalthea never ventures much more than 1 Jovian diameter from Jupiter so perpetually resides in intense glare. Someone once related to me that no one has ever visually observed Amalthea except for E. E. Barnard when he first discovered it. In fact, there have been other sightings: it's been observed through the 18-1/2-inch Clark refractor at the Dearborn Observatory (Northwestern University); the 23-inch Clark at the Halsted Observatory (Princeton University); the 26-inch Clark at McCormick Observatory (University of Virginia); and the 26-inch Clark at the United States Naval Observatory. At least a few astronomers have tracked it down, thanks to Alvan Clark & Sons!

Another possibility is Janus, but it orbits in Saturn's ring system; even when the rings are edge-on, Saturn provides more than enough glare necessary to block its observation. So, Elara and Phoebe are probably the only targets that realistically remain. Both follow orbits fairly distant from their planets, but they're also very dim. I might need a larger telescope, but that certainly won't prevent me from trying.

Creating an Occulting Filter Eyepiece

Some moons orbit near their planets, so planetary glare is of great concern when trying to locate them. Phobos and Deimos are classic examples of this, but I had glare issues the first time I tracked down Himalia, even though it was distant from Jupiter at the time. Occulting bar or filter eyepieces may be of assistance with these problems.

While an occulting bar will entirely block out a planet, an occulting filter will provide a dim image of it. If you have an accurate predictive chart, this may help you locate a dim moon; you'll know where to look in relation to its planet. The bar or filter must be mounted at the eyepiece's focal plane, or a crisp edge won't be generated in the image.

I've created two occulting filter eyepieces. I removed a violet filter from its cell, scored it with a glass cutter and straight edge, broke it in half, and rubbed the "diameter edges" against emery cloth to eliminate small burrs. I then epoxied the two semicircular filter segments to the inside of the field stops of two old, low-power Orthoscopic eyepieces. I attached the pieces to the inside of the field stops in case the glue failed; this position lowered the chances of a loose filter falling onto a mirror surface. Be sure to use something like an Orthoscopic, Kellner, or Plossl. Most modern wide-field eyepieces are unsuitable for this project as their focal planes are between internal glass elements.

Terry N. Trees has been an amateur astronomer since he was a child, and has taught astronomy and other science courses at a small western Pennsylvania university. He and his wife travel extensively to star parties in Canada and the United States. Trees can be reached at

20 Challenging Moons Targetable
by Larger Amateur Telescopes

Moon            Diameter (km)    Mag (v)             Discovery


I--Phobos            22            11.4      A. Hall, 1877
II--Deimos           12            12.5      A. Hall, 1877


V--Amalthea          167           14.1      E. Barnard, 1892
VI--Himalia          170           14.2      C. Perrine, 1904
VII--Elara           85            16.3      C. Perrine, 1905


I--Mimas             396           12.8      W. Herschel, 1789
II--Enceladus        504           11.8      W. Herschel, 1789
III--Tethys         1066           10.3      J. Cassini, 1684
IV--Dione           1123           10.4      J. Cassini, 1684
V--Rhea             1529           9.7       J. Cassini, 1672
VII--Hyperion        270           14.4      Bond & Bond, Lassell, 1848
VIII--Iapetus       1471        11.0 (var)   J. Cassini, 1671
IX--Phoebe           220           16.5      W. Pickering, 1898
X--Janus             180           14.4      A. Dollfus, 1966


I--Ariel            1158           13.7      W. Lassell, 1851
II--Umbriel         1169           14.5      W. Lassell, 1851
III--Titania        1578           13.5      W. Herschel, 1787
IV--Oberon          1523           13.7      W. Herschel, 1787
V--Miranda           472           15.8      C. Kuiper, 1948


I--Triton           2706           13.5      W. Lassell, 1856

When to Seek Minor Moons in 2015-16

          Opposition                               Distance
Planet       Dates              New Moons         from Earth

Mars      May 21, 2016   May 6 & June 4, 2016      0.5 a.u.

Jupiter   Feb 6, 2015    Jan 19 & Feb 18, 2015     4.3 a.u.
          Mar 7, 2016    Feb 7 & Mar 8, 2016       4.4 a.u.

Saturn    May 22, 2015   May 17 & June 15, 2015    9.0 a.u.
          June 2, 2016   May 6 & June 4, 2016      9.0 a.u.

Uranus    Oct 11,2015    Sept 12 & Oct 12, 2015     19 a.u.
          Oct 14, 2016   Sept 30 & Oct 30, 2016     19 a.u.

Neptune   Aug 31, 2015   Aug 13 & Sept 12, 2015     29 a.u.
          Sept 2, 2016   Aug 31 & Sept 30, 2016   28.9 a.u.
COPYRIGHT 2015 All rights reserved. This copyrighted material is duplicated by arrangement with Gale and may not be redistributed in any form without written permission from Sky & Telescope Media, LLC.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2015 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Solar System Satellites
Author:Trees, Terry N.
Publication:Sky & Telescope
Date:Feb 1, 2015
Previous Article:Into the void: the emptiest spaces of the universe may hold clues to cosmic mysteries.
Next Article:Cosmic dread: the astronomy of H.P. Lovecraft.

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters |