Earthshine blog

What B-V colour should we expect for the earthshine?

We will here estimate it by using the change in colour of Sunlight that has struck the Moon once, and the colour of Earth as estimated from spacecraft.

The Sun’s B-V is +0.650 [Allen, 1973] [Holmberg et al, MNRAS, 367, 449, 2006]

The Full Moon’s B-V is +0.85 [Lane&Irvine, AJ 1976 78, p. 267]
[vdBergh has +0.876 for Mare Serenitatis;
Allen 4.ed. table 12.16 has ‘Moon’ B-V 0.92;
Gallouet (1963) has +0.94;
Wildey & Pohn (1964), AJ vol 69, p.619 have a range of values near +0.86 to +0.87 (their work seems good and a milestone).]

The Earth’s B-V is 0.2 [Allen 3. ed, but appears based on a 1961 work – so pre-spaceage?]

The Moon’s DS B-V is 0.64 on average given data in [Franklin (1967), JGR 72, p 2963]

If Sunlight is reddened by one reflection off the Moon by 0.85-0.65=+.2 mags, then we expect earthshine, bounced once off the Moon to redden by the same amount.

If the Earth has B-V=0.2 as seen from space then seen after one reflection it ought to be redder by +0.2 or appear to us observing it from Earth at B-V=0.4. This is not what Franklin measured.

Basically, we do not yet know Earth’s B-V colour! I am making inquiries, and we shall see.

Note that Danjon did lots of colour observations of earthshine – but in the Rougier system. Wildey [JGR vol 69, p.4661+] refers to a transformation from Colour Index (“C.I.”) in the Rougier system to B-V in the Johnson system – but without giving numerical details. The method is based on transformations using the Full Moon and the Sun colours.

The transformation should be made specific and the data from Danjon placed online. Another student project!

We compare the B-V values on the DS of images from JD2456034: we look at images only exposed to bias-removal (‘DCR’ images) and images cleaned with EFM and images cleaned with the two variants of the BBSO method: linear and log. We show a ‘slice’ across the disc at row 256:

Top left: black is the B-V slice from the DCR images; red is the B-V values from the BBSO-linear cleaned images. The second graph in the upper left panel is the difference between the B-V values (BBSO-linear images minus cleaned image). The vertical dashed lines show the edge of the lunar disc and the start of the BS in column units.
Top right: same, but for BBSO-log method.
Bottom left: same but for EFM images, as shown elsewhere on this blog.

The results are quite different – the BBSO-linear method has given us fairly constant B-V values across the disc – they are about .05 mag below the DCR values (i.e BBSO-linear are bluer than DCR values).

The BBSO-log values seem completely unrealistic.

The EFM-cleaned values also look a bit unrealistic in that there is a spatial dependence on the magnitude of the B-V relative to DCR – in a way that looks like a remnant effect of the halos. The Delta(B-V) value changes sign across the disc.

The night JD2456034 is very close to New Moon and we know from other results that this is when the BBSO-linear method is likely to work best (the halo being small). Since there is a phase-dependency in the overall results for BBSO methods over and above what EFM shows we know we cannot universally use BBSO results. On the DS, towards the edge, the BBSO-linear method should be very good – it is ‘anchored in the sky’, unlike the present EFM method, and therefore should be unbiased near the edge. Our EFM method, at the moment, only minimizes the square of the residuals on the sky, inside a mask.

Near the sky, the DS B-V values in the BBSO-linear and EFM-cleaned images differ by about 0.04 mags. We should look at EFM methods that also ‘anchor in the sky’ and see what we get then.

The present largest worry is not the B-V offset but rather the dependence on position on the disc that the EFM method shows.

What does the literature tell us we should expect B-V to be under earthshine?

At the moment I only know of Franklin’s 1967 paper. It gives B and V values of the earthshine.

http://adsabs.harvard.edu/abs/1967JGR….72.2963F

The average B-V seems to be +0.64. Can we find any other information on B and V?