"The tilt angle itself can also change over
long periods.
This is the case on Mars which does not have a large satellite like
the Moon to stabilize the
angle which its rotation axis is tilted. The tilt angle of the rotation
axis on Mars varies between
15 and 35 degrees with a period of approximately 125,000 years. When
the angle is large, the
poles receive more solar energy than they do now (when the tilt is
about 25 degrees) and when
the angle is smaller the poles receive less solar energy. "
http://www-star.stanford.edu/projects/mod/id-season.html
You then have a total of 50 (2 times 25 degrees)
degrees of change with the planetary tilt at 25 degrees though this would
have varied between 30 (2 times 15 degrees) and 70 (2 times 35 degrees)
degrees in the past. The photo on the left below estimates the shadow
from the Viking picture, at the same angle as the face shadow, but foreshortened
to approximate a time in the day, probably late afternoon, when the
pointing shadow would touch the marks. The photo on the right gives an
estimated shadow for the afternoon on the day the mgs imaged this area.
The morning shadow, when the photo was taken, can be seen faintly pointing
in the opposite direction. From this you can see that as the 2 photos were
taken in different seasons the shadow point has moved on the scale about
30 degrees. This demonstrates the "sundial" is working over part
of the scale even now.
"As indicated in the figure, the profile was generated
from data which were recorded on March
24, 1998. The location of the measurement was 49.3 degrees South latitude
and 72.5 degrees
East longitude. A glance at the Mars map on the Late Martian Weather!
page on this Web site
will show that this location is in the deep Hellas impact basin of
Mars. The local time of the
measurement on Mars was 07:31 p.m. and the Martian season at the time
and place of the
measurement was early summer." (Southern hemisphere)
http://nova.stanford.edu/projects/mgs/prof/prof9803240415.html
Therefore in Cydonia at this time in the Northern hemisphere it would be early winter.
"As indicated in the figure, the profile was generated
from data which were recorded on January
28, 1998. The location of the measurement was 25.3 degrees North latitude
and 127.1 degrees
East longitude. The local time of the measurement on Mars was 05:35
a.m. and the martian
season at the time and place of the measurement was late fall."
(ibid)
Early winter and late fall around January to March
1998.
"Winter clouds cover much of the northern hemisphere
of Mars above 40° N latitude at this time
of the martian year." (Malin Space Systems)
Again winter seems correct for the mgs photo of
April 1998, so early winter would correspond to the pointer in approximately
the center of the scale.
The Cydonia area was first imaged on July 26th 1976,
or approximately 7939 Earth days ago which makes it 11.57 Martian years
ago. 57% of a year before early winter should be around early summer or
spring. We then have a 30 degree movement on the "sun calendar" through
this time of the Martian year, early winter to spring.
Since the mgs photo is early winter and it points
further down the scale than the Viking spring, it follows that during winter
the pointer should go even lower on the scale. In summer the pointer should
move up the scale. Here one might hypothesize the pointer would be around
the ends of the scale at midsummer and midwinter, but this is a subject
for future calculation. Again, all these measurements are approximate but
I do not have the facilities to calculate this more accurately at this
stage. Any help would be appreciated.
In the spirit of these approximations one might calculate an equinox at about 40% down the scale. If ideally this should be in the center of the scale it is still close considering all that may have happened if it was constructed, and all the approximations. If there is a 30 degree spread between spring and autumn this may give a basis for further estimation. If one assumes that spring is say one third of the axis tilt then 3 times 15 degrees is 45 degrees, with a total spread between midsummer and midwinter of 90 degrees. This agrees well with the estimate of the total angle of the marks to the center of the "sun calendar" of 100 degrees, a good correlation considering the approximations. It may be worth pointing out that this is at least close to an isosceles triangle bisected if you draw a line between the hill under the "sun calendar" and a mark directly above. This is shown below:
We then have evidence that the pointer moves up
the scale in summer and down in winter, and probably around the center
in late fall, close to the equinox. The "sun calendar" then is calibrated
well even today for such a task, and may have been even more so in earlier
times with a different planetary tilt.
Copyright © 1998 by Malin Space Science Systems, Inc.
There may be a semicircle of marks above the "sun calendar" that the pointer shadow can touch in the mornings. If there is there may be different measuring options to compare the shadow falling on a straight line of marks and a semicircular one, one being perhaps being an arithmetic scale and one logarithmic. The wall of the "sun calendar" may act as a windbreak to protect the marks, depending on the wind direction.
Note the angular turn in the marks at A and B,
with the marks at BC perhaps for the time of day if extended to the center
or to AD:
