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 – Mercury and Venus
When a planet is aligned with the Earth and Sun in its orbit, this is called a conjunction. During a conjunction, the planet and the Sun would appear at nearly the same position in our sky. There are two conjunctions, an inferior conjunction when the planet is between the Earth and the Sun, and a superior conjunction when the planet is on the opposite side of the Sun away from the Earth.
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 greatest western elongation occurs when Mercury or Venus reach the point where they are farthest west from the Sun in our sky, which is the highest position the planet would be seen when looking at the sky just before sunrise. Conversely, when Venus or Mercury is at greatest eastern elongation, it has reached its highest position in our sky as seen just after sunset.
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. This occurs when the planet is on the
opposite side of the Earth away from 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
2. Set the view to Mercator. Go to View
à Projections à
Mercator
3. Zoom out by right clicking anywhere on the
screen. Select Zoom To à
Maximum
4. Display the ecliptic. Go to View à 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
7. Change the date to
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 except where the plot jumps from one side to the other. 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. Conjunctions will occur where two lines cross. Retrograde motion occurs when the right ascension of a planet decreases as you move up the graph.