The planet Uranus orbits the Sun every 84 years and twice every orbit the planet's equatorial plane is pointed exactly toward's Earth.  When this occurs, the moons of Uranus can be seen to eclipse and occult each other.  A series of these mutual events occurred in 2007.  I was able to observe one of these events using the CVAS 16" telescope.  This page presents my CCD observations of the August 13, 2007 Mutual Event between Ariel and Umbriel.


Observations

A CCD time series was performed on the Uranian satellites Ariel and Umbriel from roughly 2:55 to 3:16 UT on August 13, 2007.  Clear filtered CCD exposures of 5.0s duration were acquired every 7.5s 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: C
  • Exposure: 5.0s
  • Time Source: WWV

Location

  • 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 (IOTA)


Below is a crop of one image from the time series that shows the relative placement of the moons with respect to Uranus.

Uranus and moons

Reduction

Of 160 CCD frames acquired, 9 frames were not used due to bad tracking.  The remaining 151 frames were calibrated using master bias, dark and flat frames.  Aperture photometry was performed using a commercial photometry program.  The blended image of Ariel and Umbriel was measured with an aperture of 8 pixels diameter.  An oversized sky annulus was used to avoid contamination from Uranus and get an accurate measure of the sky background around the moons.  Titania was used as the comparison "star".  The results are presented below.

Mutual Event plot

The plot above shows the differential magnitude of Ariel+Umbriel measured against Titania and plotted vs. UT.  The time series shows a definite 0.2 magnitude drop in the combined brightness of Ariel and Umbriel.  Click on the image for a larger version.


Discussion

Observations of mutual events such as the one presented here allow the relative positions of the Uranian 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 compares the results of several mutual events to current orbital models for the Uranian moons.  It can be seen here.