Repeating Galileo's work

Repeating Galileo's work

Watching the moons of Jupiter

Introduction

Observing the movement of the Jovian moons was one of the first things Galileo did with his telescope in 1609. It helped pave the way for the adoption of the heliocentric model of the solar system. Due to the short time-scales involved and the relative closeness of Jupiter, this is a relatively easy and satisfying one-night project for any amateur astronomer.

Equipment

1x Telescope (Sky-Watcher 76mm mini dobsionian)
1x Modified webcam (MS LifeCam Studio)
1x Laptop (Acer AOD270 netbook)

Software

SharpCap
iPython notebook

Method

Step one was to find out when Jupiter began rising with Stellarium. I then looked out the window in the stair-well in our block of flats and found the brightest star. Pointing my mini-dob at it I confirmed that it was indeed Jupiter. I then removed the eyepiece and put in the webcam. I used SharpCap to take several still frames. This process was repeated every 20 minutes until Jupiter passed out of view. In total I managed to get 8 usable frames.

Figure 1: A comparison of the first and last images of the observing run. Note the shift in alignment with respect to the telescope

The next day I wrote a small script in iPython notebook to find the pixel coordinates of the centre of the moons and crop the image around them. The main problem was that, due to the rotation of the earth, the alignment of the moons moved in each frame. Thankfully the python module scipy has a function ndimage.rotate() which allows the user to rotate an array by any amount and resample it onto a new grid. Thus I found the angles of each alignment and rotated the images accordingly.

The final stage was to add each of the cropped, rotated and centred frames into a GIF image. For this I used an free online GIF animator. The result is seen below.

Results

Figure 2: A GIF showing, from left to right, the movement of: Callisto, Io, (Jupiter), Ganymede. This sequence was taken over a timespan of 208 minutes (3.5 hours)

Unfortunately seeing wasn't very good during the observation run. Hence why the images wobble and the size of Jupiter appears to change from frame to frame. However regardless of the seeing, the change in position of the three moons is clearly visible. Io (second from the left) was at its farthest from Jupiter and therefore was moving more along the line of sight than perpendicular to it. Consequently Io only moved by about 10 pixels. Ganymede on the other hand was approaching its closest point to Jupiter and so moved 25 pixels in the 208 minute observation run.

While not particularly useful for a science, this small 3 hour data-set is useful for understanding how modern astronomy began. I know I can read about the moons of Jupiter in any textbook and see them in Hubble images, but I still get a kick out of seeing the change with my own eye (and webcam) in real-time.

Clear Skies!