Sun Time vs Clock Time

Okay
So on Sunday, September 17th, 2006
I attempted our homework, the assignment...to chart and measure the sun's shadow to fine solar noon. We were to conduct this experience to compare sun time to clock time. Clock time is a standardized and agreed upon measurment of time.

I began my investigation at 11:35 am CST clock time in Chicago Il in my yard. I used a 113cm garden stake as my gnomon. I ended the investigation at 1:56pm CST and was interupted by a couple of times when clouds prevented seeing the sun's shadow.
11:35 am the sun's shadow was 121 cm
11:45 113cm
overcast-no shadow
12:36 110cm
12:46 109cm
12:56 105cm
1:26 102cm
overcast-no shadow
1:56 116cm
The direction of the shadow changed starting left of the gnomon to right of the gnomon as time passed- the passage of time = the sun's transit
The longest shadow was at 11:35am = 121 cm and the shortest shadow was at 1:26pm = 102cm
If we conjecture that the shortest shadow would be caused from the sun being more directly over the gnomon causing the least amount of shadow due to the angle of the rays then 1:26 would be the solar noon.

The information from the US Naval Observatory on that date was :

Sun's Transit time:The following information is provided for Chicago, Cook County, Illinois (longitude W87.7, latitude N41.9): Sunday 17 September 2006 Central Daylight Time SUN Begin civil twilight 6:05 a.m. Sunrise 6:33 a.m. Sun transit 12:45 p.m. Sunset 6:57 p.m. End civil twilight 7:25 p.m. MOON Moonset 4:32 p.m. on preceding day Moonrise 1:28 a.m. Moon transit 9:23 a.m. Moonset 5:06 p.m. Moonrise 2:34 a.m. on following day


So, I noticed that some of my data, specifically 12:56 and 1:26 were shorter than they should have been, there must be some recorder error, because the expected trend of longer shadows showed up with the 1:56 measurements.

see the cool Sundial Bridge (gnomon)-