Starry Night-The Motion of the Planets in the Sky Laboratory 9 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 from 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: The Starry Night program will be used to fill in the table below and answer the questions that follow. For each given date you will need to find the RA of each listed planet. Open Starry Night. Within the Find tab check the boxes to the left of the Sun,Venus, Mars, Jupiter, and Saturn. Within the Options tab open Guides. In the Celestial Guides field, check the box next to Equator. In the Ecliptic Guides field, check the box next to Equator. Still within the Options tab, open The Local View menu and UNCHECK Local Horizon. Set the Date to Feb 1, of the current year and the time to 10:00 pm with time stopped. Scroll around the screen until you have the Sun in view.
Your screen should look something like this Right click on the Sun to bring up a menu. Within the menu that appears, click on Centre. Place your cursor over the Sun and read off the RA (JNOW) listed in the info. Record this value to the nearest minute into the table. Click on the 1 in the displayed date of Feb 1, so that it is highlighted. You can now use the up and down arrow keys on your key board to change the day. Press on the up arrow key while watching the position of the Sun (on and relative to the equators). Stop when the date reads March 1 and again record the RA of the Sun. Continue until the RA of the Sun is recorded for each date in the table. Next, reset the date back to Feb 1 and then locate Mars on your screen. Centre the planet and repeat the procedure for obtaining the RA for the listed dates. Pay special attention to any days along the way that the planet changes its direction relative to the background or is conjunction with the Sun. Repeat this for the planet Jupiter and Saturn.
Right Ascension Table Date of Sun (*) Mars (+) Jupiter (-) Saturn (.) Venus (^) Current Year h m h m h m h m h m 1/01 2/01 3/01 4/01 5/01 6/01 7/01 8/01 9/01 10/01 11/01 12/01 1/01/+1 2/01/+1 3/01/+1 4/01/+1
1. When will each planet be in conjunction with the Sun? List them all. Date(s): mm/dd/yyyy Mars: Jupiter: Saturn: Venus: From your data, on the 1 st of what month/year is each planet closest to being at opposition? Mars Jupiter Saturn Why is Venus not considered here? 2. Write the date (mm/dd/yyyy) that the motion of each planet first changes from prograde motion to retrograde motion and the date it ends. Count the number of times this occurs. First Begins Ends # of Times Mars: Jupiter: Saturn: Venus:
April1 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. January 1 of the current is at the bottom. 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 at some point. Answer the questions that follow. Right Ascension Chart March 1 Feb 1 Jan 1 Dec 1 Nov 1 Oct 1 Sept 1 August 1 July 1 June 1 May 1 April 1 March 1 Feb 1 Jan 1 0h 2h 4h 6h 8h 10h 12h 14h 16h 18h
From your chart, approximate the date(s) that each planet is at conjunction. This is where the planet s curve crosses that of the Sun (they have the same RA). Date(s): mm/dd/yyyy Mars: Jupiter: Saturn: Venus: From your chart, approximate the date that each planet first changes from prograde motion to retrograde motion. This is where the planet s curve first changes from increasing to decreasing. Count the number of times this occurs on your chart. First Begins Ends # of Times Mars: Jupiter: Saturn: Venus: