Earthshine blog

In V band, extinction is about 0.1 mag/airmass.

The Sun’s apparent brightness is -26.74.

0.1 mag of extinction means that about 10% of the light is taken from the sun and spread over the sky, about 21,000 square degrees. Half of this is scattered out of the atmosphere, half down to make the blue sky.

The integrated magnitude of the sky would then be of order -26.74 + 2.5*log10(10*2) = -23.5.. i.e. 3.2 magnitudes dimmer than the Sun.

Spreading this over half the sky (21000 square degrees or 2.7E11 square arcseconds) gives a reduction in surface brightness by 2.5*log10(2.7E11) = 28.6 magnitudes.

This yields a daytime sky brightness in V band of -24.24+28.6 = 4 mag square arcsecond.

If we have any daytime exposures with the ES telescope — we could check this and get numbers in other bands as well…

On JD2455945 conditions were such that the BS halo in the B and V images cancelled almost perfectly, giving us the possibility of seeing the DS colour itself. We have a link to material discussing scintillations here.

Is it possible to understand which meteorological conditions led to this unusual situation? At the MLO there is a meteorology tower and data are taken and stored for every minute. A link to the data is here.

A plot of selected parametrs for the night in question is here:

The plot shows 3 days on either side of the observation moment. A more legible pdf version is here

Note:

a) The rising pressure. The daily cycle is due to heating of the atmosphere,

b) that the observation occured towards the end of the night – temperature at ground level was dropping,

c) the vertical temperature gradient was positive (it was indeed almost the maximum seen that night) – the air was warmer higher up,

d) wind speed was relatively low,

e) relative humidity was relatively low.

We next plot ‘alfa’ (the parameter that sets the width of the PSF in our model images, found by fitting), against four of the above parameters:

There seems to be no strong pattern. Slightly, there may be an indication that low vertical T gradients allow for larger alfas, and that low relative_humidity allows for larger alfas. Ignoring the outliers at low alfa (a sign of a very poor fit or night) we look closely at the dependence on relative_humidity:

It would seem that for RH between 10 and 20% a large value of alfa is obtainable. Large lafa implies a narrow PSF. However, fo rthe nighjt JD2455945 (where the halos cancelled) we have unremarkable conditions: alfa is small (1.54), pressure is medium (680 hPa), relative humidity is medium (42%) – only wind speed is lower (3 m/s) than most other data points. Th ethree values of alfa always determined from the same image are very stable, however, differieng by 0.001 only.

Speculating wildly: Is it because alfa is small that we get halo cancellation? With a small alfa the halos widen and perhaps their ‘tails’ cancel better?

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Here are some papers that discuss meteorological conditions and ‘good seeing’ conditions:

http://adsabs.harvard.edu/abs/1974Obs….94…14M
http://www.aanda.org/articles/aa/pdf/2004/30/aa0215-04.pdf