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.
You will make observations of the planets using The Sky program. With this program you can make observations in a few minutes that took ancient astronomers years and even centuries to make. Or in other words, without the aid of the computer you would have to go out into the night sky every few days and record what you saw and then continue to do this for years in order to see the patterns of the planets’ motions. And even then you would have to deal with hindrances such as cloudy weather, changing perspective on a rotating Earth, and the intrusion of daylight.
Start the program and open the file called Wanderers. Using these settings we see the entire celestial sphere at once and without the blockage of the Earth’s horizon or the effect of daylight. The simulation is still based on an observer at a certain point on Earth (in this case Knoxville) but it is as if the Earth and its atmosphere were invisible.
Complete the checklist below in order to learn how to use the program.
q As before you can Go Forward in time or Step Forward in time by various time skip intervals. Try going forward with a 1 day interval.
q 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.
q You may wish to “turn off” the stars. There is a push button that does this. Find this button and try it. It may be easier to make the observations without the stars showing.
q You may wish to “turn on” the equatorial grid which is found under reference lines or the push button with the blue grid.
q Push the button with the little tag for Common Names and you should see labels for the planets and Sun. You may want to turn this off while the planets are in motion.
q The push button with the red dot on it records trails that show the path of the planets or the Sun. Try running the simulation while recording trails. In order to erase the trails turn off this button.
q Next to the Record Trails button is the Tracking Setup button. With this you can turn on or off each of the planets and the Sun. This is useful when you want to study the motion of just one of the planets or if you want to compare the motion of one planet to the Sun or to another planet.
q You may measure the angular separation between any two objects. This is how far apart the two objects appear in the sky measured in degrees. To do this just click on one object and then the next and then scroll down in the information box until you find angular separation. You also can use the grid to estimate the same angle (remember 1 hour of right ascension is 15 degrees).
Complete the data sheet by working through the following observations.
Turn on all of the planets and the Sun and put them into motion with a 1 to 5 day time skip interval.
1. The planets are always located near what reference line on the celestial sphere?
2. What planet is the greatest exception to this?
3. 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.)
4. 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.)
Watch individual planets one at a time, turning every thing else off and recording trails. Set time skip to about 5 days.
5. Describe the shape or shapes of the paths that planets travel during retrograde motion.
6. Describe the rate at which planets cross the celestial sphere – is it a constant rate? Does the rate increase and/or decrease? Note: you can judge the speed by the spacing of the trail – the farther apart the trail markers the faster the planet is moving across the celestial sphere.
7. Rank the eight 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.
Turn the trails off and turn the Sun on. Watch each planet, one at a time, in motion along with the Sun. Compare the motion of the planet to the Sun. As you answer the following questions fill in the table. Each question corresponds with one column in the table.
1. Indicate which planets “pass” 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 attain a right ascension that differs by 12h 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: for planets that reach a point of opposition you do not need to measure this – just write it down!
4. For each planet determine when retrograde motion occurs – around the time of conjunction or 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.
Data Sheet
1. The planets are always located ___________________________________________.
2. The greatest exception to this is __________________________________________.
3.
The planets spend the majority of the time
__________________________________.
This is called _________________________________________________________.
4.
The planets all occasionally _____________________________________________.
This is called _________________________________________________________.
5.
The shape(s) of a planet’s path during retrograde is
___________________________
____________________________________________________________________.
6.
The rate at which a planet crosses the celestial sphere
_________________________
____________________________________________________________________.
|
Planets (fastest to slowest) |
Conjunction occurs? |
Opposition occurs? (yes or no) |
Maximum elongation (in degrees) |
Retrograde occurs around |
Synodic Period (days) |
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