Peter and Chris took images of the sky near the moon, going outwards from the Moon in 0.5 degree steps, in order to measure the powerlaw falloff of light using the MLO telescope.

Data taken on night of 2012-01-08.

We got good data for 4 positions, 1.0,1.5, 2.0 and 2.5 degrees from the moon,

WCS coordinates for the frames were found using This is a great site!

Stars in the fields were found by searching Simbad for objects brighter than 10th magnitude. A small program was written to locate these stars and do the photometry based on the magnitude equations determined previously for the CCD, and their J2000 coordinates. This is quite nicely automated now. Bias frames were subtracted from each of the images and the magnitudes and sky flux of the known stars in the field computed. The magnitudes were compared to the known magnitudes and came out very nicely.

The images were taken at
POS3 : 06:27:24.313 +22:22:27.07
POS4 : 06:27:25.994 +22:51:53.62
POS5 : 06:27:23.910 +23:22:22.01
POS6 : 06:27:26.336 +23:51:34.44

Some example images with stars of known magnitudes:

POS 3:

The Moon during all of this was at 06 27 36 +21 50 28.

Distances from the center of the moon were computed for each star, and the sky flux next to that star plotted as a function of Lunar distance on a log-log plot.

PDF version of this plot:

Upper plot shows the magnitudes of the stars found in all four fields versus their correct magnitude. Lines is the 1:1 relation.

Lower plot shows the brightess of the sky versus the distance from the Lunar center in log(arcsec).

Line is a powerlaw fit, and has a slope of -0.6.

This is very curious. We were expecting -1.5 to -2 or so. What is this extra light?

The surface brightness is about 16 mag/sqarcsec in the inner most position at 1 degree from the moon, dropping to about 17.5 mag/sqarcsec at 2.5 degrees from the moon. This is a very slow drop in the brightness. The Moon was in the Milky Way plane, but the luminosity of the Milky Way is about 20-21 mag/square arcsec. So it’s not the Milky Way we are seeing.

For the present this is a cosmic puzzle!


Improved coordinates of the Moon were computed from the times of the exposures by Peter and a better plot results. Here it is:

Slope of the halo is now -0.9. That’s a bit better. More work to be done on this at the next full moon.