GEOL

Stellarium – Installation and Tutorial

Stellarium is a useful free virtual planetarium software that can be used to understand the night sky. It gives precise position and location of astrophysical objects (stars, planets and moons, comets and many deep sky objects) in the sky at a particular time of the day. It can be used to understand the position of astronomical object not only in the past but also in the future. It is extremely user friendly and available over every OS.

Installation:-

To download Stellarium on Windows/Mac/Linux OS follow the link

http://www.stellarium.org/

Once package is downloaded follow the instructions based on your OS

Windows

1. Follow the on-screen instructions

Mac

1. Drag Stellarium to the application folder before running it.

Open Stellarium on your computer.

1. If your laptop is connected to the internet your location and time must be automatically set. Your time and location can be seen at the bottom of the screen. Location can be set by dragging your mouse cursor to the mid left corner of the screen to open the left panel. Click on ‘location window’ and type “San Antonio, United States”. You can also use function key ‘F6’ to bring up the location window.

1. Date and Time can also be set in a similar way. Drag your mouse cursor to the mid left corner of the screen to open the left panel. Click on ‘Date/Time window’ and set the clock to today’s date and time to 22:00 hrs. You have now set the sky as it would appear tonight at 22:00 hrs.

1. Use the arrows keys on your keyboard to move around and see the virtual sky

Short key: – The most common and useful short keys are below

1. Locate an object: – To locate an object (say ‘Betelgeuse’ star), press ‘Ctrl F’ to open a search window and type ‘Betelgeuse’ and press enter. Hit ‘spacebar’ on your keyboard to center the object. This is extremely useful as it will automatically track the object over time without dragging the screen. The details of the object is mentioned on the left of your screen. This includes ‘Type’, ‘magnitude’, ‘color index’ to name a few

1. Forward and Backward in time: – To move forward/backward in time press ‘L/J’ key respectively on your keyboard. If you press more than once the time would move faster, so be careful. To stop the time press ‘K’ (you might have to press it couple of times to stop completely stop the time).

1. Now, as an exercise locate the brightest star in the night sky ‘Sirius’ and track its position on the sky by moving back and forth in time. Stop the time when the object goes below the horizon and can no longer be tracked.

1. Zoom in and Zoom out: – Zoom in and zoom out can be done using the ‘page up’ and ‘page down’ key on your keyboard.

Retrograde Motion

All planets (except Venus and Uranus) within our solar system rotates from west through east. Due to this counter clockwise rotation, on a ‘single night’ all object like the stars (except for North Star- Polaris), planets and deep sky objects rises in the east and sets in the west. They have east to west motion across the sky. Whereas, over ‘several nights’ planets move from west to east. Such a motion of planets is called ‘prograde motion or proper motion’. However, careful observation for planets shows that they sometimes move from east to west over ‘several nights’. This phenomenon is not surprising and was observed since ancient times. This change in direction of motion or backward motion of planets over ‘several nights’ is called ‘retrograde motion’.

	Prograde/Proper Motion	Retrograde Motion
Single Night	East West	East West
Several Night	West East	East West

Graphically, this can be shown in the figure below. In the figure, the path of a planet over ‘several nights’ (as long as 12 months) is shown (). Path to and to is called prograde motion as the planet moves from west to east over several nights. Path to is called retrograde motion as the same planet has now ‘appears’ to have changed direction and moves from east to west. Please remember that this is observed not on a single night but over several nights.

East West

Retrograde motion can be explained both by geocentric (earth center) model and heliocentric (sun center) model of our solar system. However, in geocentric model we require complex ‘epicycles -cycle within cycle’ to explain this motion and hence disregarded. Whereas, using heliocentric model and Kepler’s law this motion can be easily explained as follows.

According to Kepler’s first law (there are three Kepler’s law in total) all planets move in an elliptical orbit around the sun. The closer a planet is to the sun the faster it moves (Kepler’s third law). For example, Earth takes roughly 365 days to make one orbit around the sun. Mercury being closer to the sun takes roughly 88 earth days to do the same. Similarly, Mars takes 687 earth days to make one orbit around the sun. Jupiter roughly takes 12 earth years to so one orbit around the sun.

Based on these two principles retrograde motion is defined as an apparent backward motion of a planet in the background of other stars. This apparent backward motion of a planet appears when the inner faster planet overtakes the outer slower planet in its orbit. For example, earth overtaking mars. Retrograde motion is an illusion as planets rotate at different speed around the sun. Please see the below animation for a better understanding and visualization.

Please remember that retrograde motion is just an illusion as seen by an observer on a faster moving planet overtaking a slower moving outer planet (Earth and Mars). The opposite is true as well, i.e retrograde motion is just an illusion as seen by an observer on a slower moving outer planet being overtaken by a faster moving inner planet (Venus and Earth). The planet physically do not change direction and move east to west when in retrograde motion

Retrograde motion can be seen for all planets within our solar system. However, for planet mars it is easily visible and measured. In this lab we will track the motion of the planet mars for about 10 months (December 2013 to August 2014) and identify the retrograde motion. This will be done using Stellarium software.

The most common unit in astronomy to locate an object in the sky is right ascension (RA) and declination (Dec). The easiest way to understand these units is to assume our sky to be a two dimensional paper. RA represents left and right direction on the sky which is equivalent to the x axis on a graph. Dec is the up and down distance on the sky and represents y axis on a graph.

RA is measured in (hours, minutes, seconds) while Dec is measured in (degree, minutes, seconds). For example, an RA value of 23h35m45s reads as 23 hours 35 minutes 45 seconds. Similarly a Dec value of means 23 degrees, 43 minutes and 45 seconds

PROCEDURE:

1. Open Stellarium and set location to San Antonio (if connected to the internet it must set it automatically).

2. Set date to December 05th 2013 and time to 22:00 hrs from the left panel. It is convenient for the rest of the lab if you stop the time by pressing the ‘K’ key.

3. From the bottom panel uncheck ‘Ground’ and check ‘Constellation lines’ and ‘Constellation labels’.

4. Locate mars using ctrl F and type ‘mars’, press enter and hit spacebar to center your object. This is useful as the Stellarium will now automatically follow and track your object.

5. A list of values will appear on the left of your screen, which gives you details about mars.

6. Identify the values corresponding to ‘RA/Dec (J2013.9)’. Please do not use values corresponding to ‘RA/Dec (J2000.0)’.

7. Record the values for RA and Dec in the table below. For your convenience include only (hours, minutes) value for RA and only (degree, minutes) for Dec. A sample value is given in the table.

8. Now change the date as given in the table below (make sure to keep the time same as before) and fill the table completely.

9. Now copy the RA and Dec values to excel sheet to plot your graph. You are supposed to plot a graph of Dec (y-axis) vs RA (x-axis)

10. Use the following link for tutorial on plotting a graph in excel. Use ‘Scatter with smooth lines and marker’ type graph.

	RA	Dec		RA	Dec		RA	Dec
5-Dec-2013	11:59	2.05	23-Feb			18-May
9-Dec			2-Mar			25-May
15-Dec			9-Mar			1-Jun
22-Dec			16-Mar			8-Jun
29-Dec			23-Mar			15-Jun
5-Jan-2014			30-Mar			22-Jun
12-Jan			6-Apr			29-Jun
19-Jan			13-Apr			6-Jul
26-Jan			20-Apr			13-Jul
2-Feb			27-Apr			20-Jul
9-Feb			4-May			27-Jul
16-Feb			11-May			3-Aug	14:08	-14.08

11. After plotting, the values on the x-axis must be reversed to resemble your graph as below. This can be done by right click on the x-axis Format axis and check the box for ‘Values in reverse order’. Remember this is to be done only for the x-axis.

East West

12. Clearly label on the graph the prograde motion and retrograde motion path. Also identify on your graph the west and east direction.

Now, answer the following questions:

1. On what date was Mars located farthest to the west and east respectively

2. Identify the dates during which Mars was in prograde motion

3. Identify the dates during which Mars was in retrograde motion

4. During a prograde motion which direction does the planet move

5. During a retrograde motion which direction does the planet move

6. On a single night, which direction does the planet move when in retrograde motion

7. On a single night, which direction does the planet move when in prograde motion

8. Retrograde motion is a real effect. The planet physically move back and forth in its orbit. True or False, Explain

9. Suppose your instructor says that Mars is moving with retrograde motion tonight and will rise at midnight. Consider the following student statement:

Student: Since Mars is moving with retrograde motion that means that during the night it will be moving west-to-east rather than east-to-west. So at midnight it will rise in the west and move across the sky and then later set in the east.

Do you agree or disagree with this student? Why?

10. Describe a short conclusion of 300 words about the main objectives and understanding of the lab