Astronomy Lab – the Wanderers

Goal

The purpose of this activity is for you to learn how the planets appear in the sky and how their appearance changes over time.

Procedure

You will make observations of the planets using the Stellarium program.  With this program you can make observations in a few minutes that took ancient astronomers years and even centuries to make.  Lucky you!

Complete the checklist below in order to set things up and learn a few tricks using Stellarium.

❑ From the bottom pop-up menu, (or using shortcut keys) turn off the Ground, Atmosphere, and Cardinal points.  Turn on the Equatorial grid and switch to the equatorial mount.  

❑ From the left pop-up menu choose the Sky and viewing option window and go to the Sky tab.  Turn off (uncheck) the Milky Way and Zodiacal light.  Change the Projection to Hammer-Aitoff.  This will allow you to see the entire celestial sphere in one view.

❑ Still in the Sky and viewing option window move to the Markings tab.  Turn on the Equator (of date)  and the Ecliptic (of date).

❑ Now zoom out until you can see the entire celestial sphere on the screen.  Center your view on celestial coordinates RA = 12^h, and declination 0° (near the “head” of Virgo). At this point you should be able to see the equator as a straight horizontal line and the ecliptic as a “wavy” line, and the poles should be at the top and bottom. 

❑ Now fast forward in time and notice the planets’ changing positions relative to the stars.  They seem to wander about in the sky and this is where the word planet comes from – it means wanderer.

❑ A different way to “fast forward” is to pause time and then press the “=” key, which is a shortcut to skip forward one day.  If you hold down this key you go fast forward through the days.

❑ You may wish to “turn off” the stars.  From the left pop-up menu choose the Sky and viewing option window and go to the Sky tab.  Turn off (uncheck) Stars.  It may be easier to make the observations of the planets without the stars showing.  This is up to you.

❑ Select a planet you by clicking it – this may make it easier to follow as it moves.  Once a planet is selected, you can track it by pressing the “T” key or the spacebar.  Use a “right click” of the mouse to “deselect” an object.

❑ From the left pop-up menu choose the Sky and viewing option window and go to the SSO tab.  Click the check boxes to Show planet trails and Show trail only for selected planet and Show planet markers.  Try going fast forward in time with a planet selected and you will see a path showing how it moved through the stars and across the celestial sphere.  This too is a matter of preference so feel free to turn this feature on or off as you desire. 

Observations  Use the following instructions to help you fill in the data sheet.

Run the simulation fast forward in time or use the “=” key to skip ahead by days.  Go forward fast enough to see the planets moving across the celestial sphere.

1. The planets are always located near what reference line on the celestial sphere?

2. Most of the time each planet will move mainly in what direction across the stars – East to West or West to East?  (This is called direct or prograde motion.)

3. For a relatively brief amount of time each planet will move mainly in what direction across the stars – East to West or West to East? (This is called retrograde motion.)

4. Describe the shape or shapes of the paths that planets travel during retrograde motion.

5. For any particular planet the rate of motion decreases whenever what happens?

6. Rank the five “naked eye” planets in order of the average speed with which they move across the celestial sphere.  Fastest being the planet whose position relative to the stars changes most rapidly.  Write the result in the first column of the table.  (For now, this will leave three blank rows in the table.)

Watch each planet, one at a time, in motion along with the Sun.  Compare the motion of the planet to that of the Sun.  It may be helpful to use the “trails” feature.  Each item below corresponds with one column in the table.

1. Indicate which planets “pass” by the Sun or get “passed” by the Sun at some point in time.  When a planet passes the same point on the ecliptic as the Sun (ignoring declination) it is said to be in conjunction.  From Earth, the planet and Sun would appear to be at nearly the same point or in the same direction in the sky – the angular separation would be minimal.

2. Indicate which planets reach a position on the opposite side of the sky from the Sun (attain a right ascension that differs by 12^h from the Sun at some point in time).  When a planet reaches a point on the ecliptic opposite the Sun (ignoring declination) it is said to be in opposition.  From Earth, the planet and Sun would appear to be at very nearly opposite points in the sky – the angular separation would be around 180°.

3. Determine the maximum elongation that occurs for each planet.  The elongation of a planet is the angular separation between the Sun and that planet.  Elongation is always a value from 0° to 180°.  Hint:  You only need to do this for the two planets that never reach opposition.  (For the other planets that do reach a point of opposition you do not need to measure this – if a planet is opposite the Sun how many degrees apart is it?!)  You can find elongation in the information shown when a planet is selected.

4. For each planet determine when retrograde motion is maximum (moving most rapidly “backward”) – this will be either at the time of conjunction or at the time of opposition.

5. For each planet determine the approximate number of days between “like” conjunctions (passing the Sun and moving in the same direction).  This is the synodic period for a planet.  This is easiest to do by opening the Date/Time window and using the Julian Day tab.  The numbers there show a direct count of days - subtract two values to find the number of days that have passed from one conjunction to the next “like” conjunction.

Optional: 
Time permitting, complete the table for planets Uranus, Neptune, and dwarf planet Pluto.

Data Sheet

1. The planets are always located ___________________________________________.

2. The planets spend the majority of the time __________________________________.

This is called _________________________________________________________.

3. The planets all occasionally _____________________________________________.

This is called _________________________________________________________.

4. The shape(s) of a planet’s path during retrograde is ___________________________

____________________________________________________________________.

5. The rate at which a planet crosses the celestial sphere decreases whenever _________

____________________________________________________________________.

Planets (fastest to slowest)

Conjunction occurs? (yes or no)

Opposition occurs? (yes or no)

Maximum elongation (in degrees)

Retrograde max during conj. or opp.?

Synodic Period (days)