The Motion of the Planets in the Sky

 

 

Objective:

 

In this laboratory the inferior planets and superior planets will be discussed.  Three of the superior planets: Mars, Jupiter and Saturn will be the focus of this lab.  The concepts of opposition, conjunction and retrograde motion will be introduced.

 

Background: 

 

        All the planets orbit around the Sun in the same direction in nearly circular orbits. They also orbit in nearly the same plane. This makes it relatively easy to identify planets in the sky as they will always be somewhere close to the ecliptic. The path a planet takes in the sky is determined by how far it is from the Sun. Planets closer to the Sun than Earth, the so called inferior planets, exhibit motion quite different from planets that lie further form the Sun, the so called superior planets.

 

The Inferior Planets Ð Mercury and Venus

 

       Both Mercury and Venus can pass between the Earth and the Sun, when this happens it is called a conjunction.  When not in conjunction there is an angle between a planet and the Sun called elongation. The elongation for Mercury ranges from 0¡ to 28¡ and for Venus from 0¡ to 47¡.

 

The Superior Planets - Mars, Jupiter, Saturn, Uranus, Neptune, (Pluto)

 

            Planets orbiting the Sun beyond the Earth are called Superior planets. Superior planets can have elongations ranging from 0¡ to 180¡.  With a 0¡ elongation a Superior planet is said to be in conjunction with the Sun and will have the same right ascension as the Sun.  When a planet has an elongation of 180¡ it is at opposition with the Sun.  During opposition a planet has a 12 hour difference in RA from the Sun. 

 

 

Planetary Motion across the Sky

 

Typically the superior planets move against the background stars from west to east, this is called prograde motion.  However at times they undergo retrograde motion as viewed against the background stars, moving east to west.  This is a result of the faster Earth passing the slower planet at opposition.  It is quite noticeable for the planet Mars.  An easy way to determine if a planet is in retrograde motion is to look at the right ascension over time.  If it decreases, then the planet is in retrograde motion until the right ascension begins to increase.  An increase in right ascension represents prograde motion.

 

 

 

 

          

Procedure:

 

Part 1: Position of Sun and Planets

 

1.     Open the Sky6 Software and set the date to Sept. 1, 2007.

 

2.     Set the view to Mercator.  Go to View ˆ Projections ˆ Mercator

3.     Zoom out by right clicking anywhere on the screen.  Select Zoom To ˆ Maximum

4.     Display the ecliptic.  Go to View ˆ Reference Lines and check Ecliptic under General Lines

 

5.     Display the names of the planets.

 

6.     Record the RA for the Sun, Mars, Jupiter and Saturn on the Right Ascension Table for September 1, 2007.

 

7.     Change the date to October 1, 2007 and record the RA for each celestial object. Continue recording the RAÕs for each celestial object.

 

 

Part 2: Plotting

 

1.  For each month plot the positions of the Sun, Mars, Jupiter and Saturn on the Right Ascension Chart.  Use the corresponding symbol, (*), (+), (-), and (.), for each celestial         object as defined on the Right Ascension Table.

 

2.  Connect the points for each object with a smooth curve. You will notice that the line that   represents the Sun jumps to the right side of the plot after March.

 

3.     Answer the questions on your Data Sheet.