Sunrise on Clavius

My 14-year-old son and I saw some spectacle last night. I pulled out
the trusty NexStar 5 for him to do a lunar drawing as part of the
Boy Scout merit badge project. The sun was just rising on the
floor of Clavius, so that the crater floor had a roughly centered
pool of grazing sunlight interrupted by Clavius C and D. Each crater
had a bright rampart and cast a long shadow, so the illuminated
floor was outlined like two overlapping comet heads. The fact that
the sunlight yet touched only the center of the crater floor was
a graphic illustration of how strongly curved the luanr surface is.
The whole of the inner walls of Clavius was still shadowed; part of
the wall of Rutherfurd stuck into the darkness like a scimitar.
Over 30-45 minutes, some new pieces of the wall of Clavius N and some
smaller craterlets caught the sunlight. This overall appearance
must not last more than a couple of hours - I've never seen it before.
This is the phase at which I've convinced myself in twilight, before
glare becomes a problem, that I could just catch a naked-eye hint
of the notch in the terminator when Clavius' floor is in shadow.

We also probed the limits of the NexStar's tracking in alt-az mode,
since at this phase of the Saros cycle the Moon passes within 4
degrees of our zenith. Had to help out with the handset buttons
for about 15 minutes. I grabbed a quick digital snapshot through
the eyepiece as a memento, amd to accompany my quick sketch,
but the NexStar short-term tracking wanders by arcseconds and
has always limited the quality of images with a camera physically
attached.

Bill Keel

On 26 Feb, 13:28, "William C. Keel" wrote:
My 14-year-old son and I saw some spectacle last night. The fact that
the sunlight yet touched only the center of the crater floor was
a graphic illustration of how strongly curved the luanr surface is.

Well observed Bill, I had a similar demonstration at our last clear
public night. What I initially took to be a star about to be occulted
by the moon was in fact a central peak standing a hair off the
northern limb ;-) I watched fascinated as it slowly dipped from view.
jc

On Feb 26, 9:10 am, "John Carruthers"
wrote:
On 26 Feb, 13:28, "William C. Keel" wrote:

My 14-year-old son and I saw some spectacle last night. The fact that
the sunlight yet touched only the center of the crater floor was
a graphic illustration of how strongly curved the luanr surface is.

Well done. Your son will never forget this observation and you
won't either.
Tonite's a good night to count the craters on the floor of Clavius.

Well observed Bill, I had a similar demonstration at our last clear
public night. What I initially took to be a star about to be occulted
by the moon was in fact a central peak standing a hair off the
northern limb ;-) I watched fascinated as it slowly dipped from view.
jc
I call those crater tops "floaters". They are more fun to
observe
than the craters themselves - really wierd looking.
They can also be very useful in determining altitudes if one
wants
to do some hardball Trigonometry.

Ben, 90.126 n 35.539