As the four Galilean moons orbit Jupiter, they are eclipsed by the planet's shadow.  Photometry of these eclipses allows the positions of the moons to be determined with high accuracy.  I was able to observe an eclipse of Io using the CVAS 16" telescope on August 13, 2007.  This page presents my CCD observations of this eclipse.


A CCD time series was performed on Io from roughly 1:46 to 1:56 UT on August 13, 2007.  V filtered CCD exposures of 0.5s duration were acquired every 3.0s during this time.  The time on the computer used for camera control was set to within +/- 1s of UTC using a WWV radio time signal.  Transparency and seeing were good during the time series.

Equipment Used

  • Telescope: CVAS 40.0cm f/7 Newtonian Reflector
  • Camera: SBIG ST-7XME
  • Filter: V
  • Exposure: 0.5s
  • Time Source: WWV


  • Indian Hill Observatory, Huntsburg, Ohio, USA
  • Latitude: 41d 32' 48.2" N
  • Longitude: 81d 04' 52.4" W
  • Elevation: 389m
  • Datum: WGS84

Observer: Robert J. Modic

Below is a movie made from the time series that shows Io emerging from Jupiter's shadow.  Io is to the left of Jupiter, Ganymede and Europa are to the right.

Io eclipse


200 CCD frames were acquired during the time series.  All frames were calibrated using master bias, dark and flat frames.  Aperture photometry was performed using Extract, a program created by Anthony Mallama specifically for the measurement of Galilean satellite eclipses.  Ganymede was used as the comparison "star".  The results are presented below.

Mutual Event plot

The plot above shows the ratio of the sky subtracted fluxes of Io/Ganymede plotted vs. UT.  An aperture of 11 pixels diameter was used for this plot.  Click on the image for a larger version.


Observations of eclipses such as the one presented here allow the relative positions of the Galilean moons to be determined with greater accuracy than methods using simple astrometry.  An improved knowledge of the moon's relative positions, in turn, allows current orbital models to be refined.

Anthony Mallama has published a paper using my data and that of other observers.  He analyzes the results of more than 500 eclipses over 20 years to study changes in the orbital models for the Galilean moons.  It can be seen here.