Vol 49, No 4, April 2012 - BAA Lunar Section

Sunrise on Plato
Nigel Longshaw
In the last circular I discussed sunrise on Plato and the possibility of the duality of the ‘ashen light’ previously
observed by Phil Morgan. As an example of early sunrise on Plato a drawing by Gerald North which appeared
in the JBAA 1986, 96, 3, was reproduced. I was pleased to subsequently receive a note from Gerald regarding
his drawing and further discussion in relation to early sunrise on Plato. Gerald forwarded me a copy of his
original drawing from his observing notebook, the copy I used from the original journal paper was not
reproduced very well. It is clear from the original drawing (Figure 1) that Gerald recorded two sunlight
‘streaks’ on the crater floor. Gerald notes the following:
As you can see it (Figure 1) shows
the main spire of light together
with a shorter and dimmer one,
which was missed out of the LSC
reproduction. This is vaguely
reminiscent of the CCD image in
Tony’s piece, coincidentally about
Plato shadows, on p19 of the LSC
(March issue). I don’t suppose
readers of the LSC will realise that
mine is not a topographic
drawing, merely a quick sketch
showing an effect that I deemed of
note... perhaps it would be helpful
to supply a little of my original
notes from my observing log:
Figure 1.
Telescope: 18.25-inch (464 mm) Newtonian, 2.59 m focal length.
Date: 1981 February 12
Time: 18h 54m - 20h 56m UT
Magnifications: 86×, 144×, 207×, 288×, 414×, 432×, 576×
Transparency: Poorish due to haze (limiting magnitude approx. 3.0 - 3.5)
Turbulence: Poorish at first but becoming better during the observation period (AIII-II)
Then I make notes on the appearances of features of the Moon against times. I will leave most of these out
but one of relevance is:
19h 06m UT, Change to 144×: Interior of Plato a featureless inky blackness. I carry on commenting on other
details in this entry and then I go on to make other entries, including occasional mentions of Plato but with
no sign of the sunrise spires until:
20h 40m UT, 414×. Plato—two misty streaks are seen on the floor of the crater—local dawn
20h 44m UT, 207×. Impression confirmed. Used outline chart and marked in pencil the general appearance
on the drawing. No attempt was made to fill in other fine details. 20h 56m Markings on outline chart
completed and after a final check observations were terminated.
Over the drawing which is now stapled in my observing log I have written:
Plato: 1981 February 12d, 20h 53m UT—so that is the time at which I fixed the appearance of the spires.
(…) notice that I show the tip of the brighter spire is brighter than the rest of it—indicative of higher ground
As to the differing historical appearances of the morning spires (or spire plus broad lighter patch), it does
occur to me that libration in latitude could make a significant difference. Accepting that the differing angle
of sunlight incident on the western rim and skating the interior isn’t going to vary by very much, the lighting
situation could be critical enough to produce the variations. Indeed, from an observer positioned near the
centre of Plato the rising point of the Sun will appear several degrees different over a large span of lunations
thanks to libration in latitude.
In my article I queried why Molesworth did not record the sunlight spires on his drawing of the shadowed
interior of Plato when colongitude values suggested he should have recorded these features. Gerald responded
with the following further comments:
BAA Lunar Section Circular Vol. 49 No. 4 April 2012 5