CLEA - Astrometry of Asteroids
How do we find solar system objects such as asteroids and KBOs, how do we measure the distance to the object, and how do we determine their orbits?
In this experiment, you will determine the angular speed of an asteroid and the distance to the asteroid.
The short answer is that we take pictures (perhaps minutes or hours apart) of a region of the night sky and look for any objects that move relative to the background stars.
Using the background stars as a reference, we measure the RA and DEC of the moving object. By calculating the change in the angular position of the object per second, we can determine how fast it moves across our sky (in degrees per second).
By measuring the position of the object relative to background stars (that do not move significantly) from different vantage points, the object will appear to be at different positions.
We can then use its parallax (this difference in the apparent position of the object when viewed from different vantage points) to determine the distance to the object.
Parallax is the apparent shift in the position of an object as a result of looking at it from two different vantage points. It's fairly easy to understand by doing the following activity.
- With both eyes open, hold your index finger about 6-9 inches in front of your eyes, between you and the 0 on the scale below.
- Now, while holding your finger steady, close your right eye and use the scale to measure the position of your finger.
- Now, open your right eye and close your left eye and use the scale to measure the position of your finger.
When I did this experiment, I found that my finger was at the 3.0 mark with my left eye open and at the -3.0 mark with my right eye open. Your numbers will depend on how far away your finger is from your eyes. Thus, the difference in the positions as measured by each eye is 6.0.
Now, repeat this experiment with your index finger about 15-18 inches in front of your eyes. Again record the position of your finger measured with your left eye closed and the position of your finger measured with your right eye closed.
You should have noticed a difference in measurements that is less than what you measured before. In my case, with my finger about 18 inches in front of my eyes, I measure my finger at 1.0 and -1.0 for a difference of 2.
The greater the distance of my finger from my eyes, the less the apparent shift in the position of my finger as measured using the scale. The amount of apparent shift can be used to calculate the distance from my eyes to my finger. This method is called trigonometrix parallax, or just parallax for short.
We will use the CLEA program called Astrometry of Asteroids. You should have already installed it.
1. Download two files: (1) the handout that describes what to do for this experiment; and (2) the spreadsheet used to do the calculations. Read the handout briefly before continuing.
2. Watch the following instructional videos that show you how to use the Astrometry of Asteroids simulation.
CLEA: Astrometry of Asteroids -- Part 1. This video describes how to load images and blink the images to see the asteroid.
CLEA: Astrometry of Asteroids -- Part 2. This video describes how to measure the RA and DEC for the asteroid and how to calculate the angular speed of the asteroid.
CLEA: Astrometry of Asteroids -- Part 3. This video describes how to measure to measure the parallax angle for the asteroid and how to use this to calculate distance to the asteroid.
3. Make the measurements described in the handout and enter your data in the spreadsheet.