Earthshine blog

"Earthshine blog"

A blog about a telescopic system at the Mauna Loa Observatory on Hawaii to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

B minus V albedos

Post-Obs scattered-light rem. Posted on Jul 16, 2013 13:01

After some deliberations we now return to the ‘edge fitting method’ and its results. There are 19 nights on which more than 17 B and V pairs of closely spaced images exist. We consider those B and V image pairs with negative lunar phases, and look at how the B minus V (albedos, not colours!) values are distributed against lunar phase. We see this:

pdf here:

The error bars are estimated on the basis of error propagation in the B and V albedos from the edge-fitting uncertainty procedure. We have added small offsets in phase to get separation of points. The above actually contains data from 19 nights. To better see things we calculate the nightly median value Of B and V albedo, and now plot these:

pdf here:

Here, error bars are given by the standard deviation of the nightly values. Each point is labelled with the JD number. We see three night-after-night sequences: 2456073-76, 2456016-17, and 2456045-47. Given the error bars, these sequnces follow the same pattern – a rise in B minus V (albedo) as you go from new moon (left) to half moon (-90 degrees). Since the three sequences painmt the same pattern we are confindet in saying that ‘Something Is Going On Here!’.

At the moment we do not know if this is geophysics in the shape of Earth albedo changing with phase – or some aspect of the halo (which increases from left to right above) being harder to model as phase grows.

Next it may be appropriate to study what the Earth actually looked like in the above three sequences. The negative phases selected for the above plots all correspond to the Sun illuminating Earth from Western Pacific/Australia/East ASia and westwards. As time passes on a given day the Moon sinks further in the West (as seen from Hawaii) and more of Asia contributes to the earthshine. On days in a sequence, at the same local hour, less of Earth is illuminated as seen from the Moon so the contribution to earthshine drops but is more and more ‘sicle-like’. SUmmarizing:

For the negative phases selcted we expect:

a) during a single day – contribution to earthshine by continental areas increases, ocea contributions decrease,
b) same time of day but consequitive days – earthshine contribution comes from areas closer and closer to the edge of Earths disc.

I think this means that – if clouds are evenly distributed – we should see reddening of the albedos during a sequence taken on one day, and a blueing in a series of days.

We see b)! Have we seen a)? Below is a plot of the B albedo minus V albedo values for each of the days in question plotted against JD day fraction.

sharper pdf here:

We see little evidence of any up- or down-turns in this. Perhaps 2456104 shows an upturn – i.e. the opposite of what we thought we’d see. The remaining narrow sequences above seem to show a slight downward trend as the day passes – i.e. reddening. Potentially, clouds dominated these days so that little of the surface was visible?

Force method vs Edge Fitting method: The Fight

Post-Obs scattered-light rem. Posted on Jul 16, 2013 09:28

In understanding this this post, we now compare the results to those from the edge fitting method. We remind thereader that the Force method gives us the difference in B-V [magnitudes] between the BS and the DS, whil ethe dge-fitting method gives us albedos fro Earth in e.g. B and V bands. We look at the same range of phases in the two methods and repeat the relevant ploits here. First the edge-fitting method results:

Upper panel shows us B-band albedo minus V-band albedo against lunar phase (upper panel) and against the average alfa (B-alfa and V-alfa averaged).

We repeat the plot from the Force method here:

Here are the pdfs:

In the upper panels we see a slight dependence on lunar phase – B albedo minus V albedo as well as B-V [mags] do tend to rise towards Full Moon. Note that this implies opposite colours! The rise in B albedo relative to V albedo means that the Earth appears bluer as we approach Full Moon in the edge-fitting method, while larger B-V [mags] in the Force method implies a redder Earth as we approach Full Moon.

In the lower panels we see that the alfas used in the edge fitting method and the derived albedo differences have little relationship with each other, while the ‘incremental alfa’ needed in the Forcing method bears a strong relationship with B-V, as discussed before.

This seems to rule out the worst of the possibilities given in the Force posting – namely that if B and V albedos depended as strongly on alfa as B-V does then we would have a serious problem. In the present situation we are not in that position – but where are we then?

For both methods we seem to have outliers for phases less than -120 degrees. Let us take a closer look at these points. They are from night JD 2456073. The upper plot shows that the B and V albedos we could derive from edge fitting’ differs by .1 between the upper group and the lower group. From the Force method we see that B-V [mags] differs by 0.5 between these two groups.

The images involved have these names :

2456073.7452091MOON_B_AIR_DCR.fits
2456073.7472223MOON_V_AIR_DCR.fits
2456073.7555269MOON_B_AIR_DCR.fits
2456073.7658635MOON_B_AIR_DCR.fits
2456073.7758928MOON_B_AIR_DCR.fits
2456073.7781942MOON_V_AIR_DCR.fits
2456073.7862409MOON_B_AIR_DCR.fits
2456073.7882301MOON_V_AIR_DCR.fits
2456073.7964398MOON_B_AIR_DCR.fits
2456073.7983881MOON_V_AIR_DCR.fits

There are more data points for the edge-fitting method, in the plots above, than there are for the Forcing method. This is because more B and V combinations were picked tested for the former. The images picked were observed withing half an hour of each other. These images should be visually inspected.

[later:] Tried that – it gets messy: differences in the level-differences of the DSs in B and V can be spotted but do seem to depend on knowing the exposure times, and this is one of the things we do not think worked – better to trust the edge-fitting method, since it is a ‘common mode rejecting method’.