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A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Lunar apparent magnitude with phase

Data reduction issuesPosted by Chris Flynn Sep 11, 2012 05:22AM
The plot shows the apparent magnitude of the moon in V and B as a function of lunar phase (phase=0 is new moon).

We measured the flux in images in which the filter was reliably V or B and used the transformations determined from NGC6633 (i.e. to get the apparent magnitude.

These data have been obtained for a large range of airmass (z) -- from z = 1 to 10, with most of the data in the range z = 1 to 3. We derived extinctions of 0.10 mag/airmass for V and 0.17 mag/airmass for B, by comparing to the apparent V magnitude from the JPL ephemeris for the Moon ( (more about this below). The solid black line shows the V apparent magnitude as a function of phase after extinction correction, and adjusting the zeropoint by 0.2 mag in V to fit.

Note that B is ~ 1.0 mag fainter (i.e. B-V ~ 1.0, as we'd expect).

The airmass fits are shown above : the plot is the difference between the apparent magnitude from the JPF ephermeris and our transformed instrumental V band (or B band) magnitudes, shown as a function of airmass. The two lines show 0.10 mag/airmass (V band) and 0.17 mag/airmass (B band) -- they are not fits. There are some bad outliers, especially in the V band, which are probably due to the incorrect filter being in the beam.

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Posted by Chris Flynn Sep 13, 2012 02:10PM

Peter e-mailed JPL and we got back that the source of the JPL magnitudes for the moon is Eqn 8 of

Posted by Chris Flynn Sep 12, 2012 06:27PM

I checked their web based documentation already but they don't seem to cover this for the moon specificially - but you may find sonething I missed. I will produce a more accurate comparison with JPL than the plotted one there could still be considerable structure in there.

Posted by Peter Sep 12, 2012 03:55PM

Argh! JPL knows more than us then! I will check what the absolute mag formula is based on.

Posted by Chris Flynn Sep 11, 2012 09:11AM

Yes - the black line is from JPL - apparent Lunar magnitude directly from their ephemeris. It has only been shifted by 0.2 mag to fit, no other adjustments. I don't know if it is empirical or theoretical (ie for some assumed reflectance model) but we are certainly close to it!

Posted by Peter Sep 11, 2012 08:43AM

Where is the smooth black line from? JPL? In that case you seem to have found a function for V with better built in reflectance than we have access to - or is it an empirical function?Anyway - looks great!