Planetary Observers Club - Inner Solar System

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Planetary Observers Club Chair:

Aaron Clevenson
19411 Cluster Oaks Drive
Humble, TX 77346




The Projects for the Inner Solar System

Mercury IconMERCURY

As an inner planet (closer to the Sun than the Earth), appearances of Mercury are fleeting, best seen just after sunset or just before sunrise. In compensation, this elusive planet can be seen, although sometimes with difficulty, several times a year. Mercury is never visible to the naked eye more than 28° above the horizon. Observations must therefore be accomplished during twilight, when Mercury is at or near its highest elevation for that particular apparition, or appearance. The result is we must observe through the thicker portion of Earth's atmosphere. For our purposes it will be sufficient just to locate this "Messenger of the Gods" on two different neighboring apparitions. Once in the morning sky and once in the evening sky. It may appear as a pinkish star-like object. Finding this elusive planet is its own reward. Watch for charts published in your favorite observing periodical. A pair of binoculars can be most helpful for the twilight observations, but you must wait until the Sun has sunk fully below the horizon. Record the time and date of the observations and the approximate azimuth (270°, 300°, etc.) and altitude (20°, 15°, etc.).

Venus IconVENUS: Low Power Crescent

Earth's "sister planet" will show its crescent phase in a high quality binocular that is held perfectly still. You might try mounting it on a tripod. Consult the astronomy periodicals if you are unsure when or where to look. This observation will have to be accomplished when Venus is nearer the Earth and in its crescent phase. Galileo's observations of this the brightest of the planets provided crucial evidence for the triumph of the Copernican Sun-centered solar system. Since Venus exhibited phases it had to revolve around the Sun instead of the Earth. Can you repeat his observations? View before the sky gets too dark or Venus' brightness may obscure her phase.

Venus IconVENUS: Daytime Observation

With a polar aligned telescope equipped with setting circles and a low power eyepiece, Venus can be readily observed during the day. Observing during the day can be a decided advantage. The planet's brightness will be subdued enough to not dazzle the eye. The planet is also high in the sky away from the denser portion of Earth's atmosphere. CHOOSE THIS PROJECT ONLY IF YOU HAVE A TELESCOPE PROPERLY POLAR ALIGNED AND CAPABLE OF LOCATING THE PLANET WITHOUT ENDANGERING EITHER THE INSTRUMENT OR YOURSELF - USE EXTREME CAUTION - EYE DAMAGE COULD RESULT.

In your favorite astronomy periodical note the right ascension and declination of the Sun and Venus. Center the Sun in your telescope by projecting the image onto a screen or the ground. Set your setting circles to that of the Sun and turn on your drive. Now offset the appropriate amounts to arrive at the coordinates for Venus. (Make sure your focus is correct, an out-of-focus planet may be impossible to see.) You should be able to see Venus in your finder scope. An orange filter in your main eyepiece will help increase image contrast. Describe your experience.

Venus IconVENUS: Phases

Like the Moon, Venus goes through phases. At Venus' brightest, about magnitude -4, it will be a thin crescent in your telescope. At its faintest the entire disk will be lit. This seeming contradiction is due to the fact that the thin crescent phase happens when our sister world is nearest us. The full phase happens when she is farthest away beyond the Sun. Try to watch Venus over about a two month period, making sketches. This will give you size and phase changes over about one forth of its orbit of 224.7 days. Keep them all at the same scale and always use the same eyepiece so you can get a feel for the changes in Venus' apparent diameter. Try to make them about a week apart. Viewing while the sky is still light will help cut down glare from the planet's brilliance and also help to eliminate atmospheric distortion because the planet will be higher in the sky. If the sky is still very light an orange filter will increase the contrast between Venus and the blue background and will also cut down Venus' glare.

Note the day/date/time and seeing conditions under each sketch on an 8-1/2X11 sheet of paper.

Mars IconMARS: Albedo Features

Observing the planet Mars can be either exciting and rewarding or boring and disappointing. It all depends on where the red planet is in its orbit compared with the Earth. Every 26 months Earth catches up to and passes Mars in Earth's smaller, faster orbit, and it is during these times that Mars can best be seen. This point of "catch up" is called an opposition. This is the time when Earth and Mars is on the same side of the Sun, resulting in the Sun being on the "opposite" side of the sky from us as is Mars. During this time Mars rises as the Sun sets and sets as the Sun rises, and is at its highest point in our sky at midnight. All oppositions are not created equal, however. Mar's orbit is more elliptical than our own, and these variations in distance makes Mars appear as small as 13.5 arc-seconds in diameter, or as large as 25 arc-seconds.


A few months before or after these oppositions Mars can still be observed, depending on the objective size of your telescope. Consult your favorite observing periodical for favorable Mars observing times. Many helpful hints will be given and times suggested for successful observing.

Drawing the "god of war" can be literally an illuminating experience. Sketching can help train your eye to see more detail than you would have otherwise noticed. Examine the planet for several minutes. Try an orange filter to see if that helps image contrast. Use the first accompanying circle to sketch in the major features after first locating the polar cap or possible slight phase defect. Just outlining the major features will do. Try to place them as accurately as possible. Note to the nearest minute when you have completed these steps. The first sketch should give accurate positions.

A soft pencil can be used to make a more finished looking version on the second circle. The second can be completed away from the telescope if desired, although as soon as possible while the memory is still good. It can be more "artistic", shaded to give a B&W photo appearance. If done carefully a very satisfying rendition can be had, and you will not have to be an artist to have accomplished it.


1. The day/date/time.______________________________________________

2. The seeing conditions____________________________________________

3. The aperture of the telescope.______________________________________

4. The focal length of the telescope.____________________________________

5. The focal length of your telescopes eyepiece.___________________________

6. Your own observational comments and impressions.______________________




Mars Sketches (Include a copy of your Mars sketches with your report.)











To show the East-West direction of your sketches show with an arrow the direction of drift in your field-of-view without a drive running.

Mars IconMARS: Retrograde Motion

Early naked eye observers had a problem. The planet Mars, slowly drifting west to east from night to night, when seen against the background stars, would once a year act very strangely. As Mars approached opposition it would suddenly slow down, reverse itself, drift westward for a while (retrograde motion), before again reversing to assume its normal (prograde) eastward motion. We now know that this is an illusion caused by the motion of the Earth catching up to and passing the slower Red Planet, causing Mars to appear to be moving backward. You are to plot the apparent motion of Mars through this regrograde loop. Determine what constellation Mars will be in at the time of opposition. This can be done by consulting the astronomy periodicals. Make a copy of that area out of a star atlas. For example, Will Tirion's Star Atlas 2000.0. Then watch Mars beginning about a month before opposition until a month after opposition. Plot the planet's daily position on your copy by comparing its position to the fixed stars of the constellation. After these two months you should be able to trace out Mars' retrograde motion. Fortunately for us, the Copernican Revolution solved nicely the odd behavior of Mars, and also the behavior of Jupiter and Saturn, the other classical outer planets which exhibit a lessor amount of retrograde motion.

Include Copy of Your Map of Mars Retrograde Motion.

Asteroids IconASTEROIDS: Course Plotting

Finding and following one of the small rocky planetoids that accompany the major planets around the Sun can be a most satisfying project. The small size of asteroids can make them a challenge to find, however. Although the largest, Ceres, is about 1000 kilometers (620 miles) in diameter, most range from about 100 kilometers (62 miles) to 200 k (125 miles) across, down to one kilometer (0.6 mile) or less. This means they all remain starlike even in the largest of amateur scopes. The four largest can be found in binoculars under dark skies specially at opposition when they are the brightest. All four are magnitude 8.5 or brighter. Since they are stellar in appearance their true nature can only be discerned by their movement compared to the background stars from one night to the next. Each year the daily or weekly positions for these fascinating little worlds are published in the astronomical periodicals. Using the information thus obtained, find and track an asteroid over a period of 3-5 nights. As little as three nights may be acceptable if weather is a problem. Copy an appropriate section of a star chart, preferably one that has a fairly large scale such as Wil Tirion's Star Atlas 2000.0 or the Uranometria 2000.0. From your observations mark the asteroids position as close as you can comparing it to the position of the background stars. Observe it again the following night locating and marking it again on the same star chart. Do this for three to five nights, then connect the dots showing the direction of the asteroid's movement with an arrow. Note the time and date of each asteroidal position in your notes. SEE ALSO THE NEXT PROJECT.

Asteroid IconASTEROIDS: Measuring their Movement

Having plotted an asteroids pathway among the background stars for at least three evenings you can now figure out its approximate hourly movement. Using a finely graded ruler such as a millimeter rule, measure the distance from the dot representing your first observation to the dot representing your second observation. If these two observations were, for example, about 24 hours apart, divide that measurement by 24 (or whatever the time interval was in hours) to find out how far the asteroid traveled in one hour. Do the same thing for each subsequent observation. How far did the asteroid move in one hour? Using the same rule, measure the width of one degree on your star chart. If, for example, your asteroid moved 2mm in one hour and if a degree on your chart is 32mm wide, your asteroid was moving one degree in 16 hours. How long did it take your asteroid to move one degree? This determination is only a rough one, of course, but non-the-less it can be fun to do, and it will give you a sense of familiarity with YOUR asteroid.

Related Links

Read the Glossary and References
Read the Projects for the Sun and Moon
Read the Projects for the Outer Solar System
Find Your Planetary Observers Club Award
Planetary Observers Club Introduction

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