NavList:

Antoine, you wrote:
"The Moon Limb / Procyon closure rate is slightly under 2.0' during a 10 minute elapsed time."

How did you calculate that? By the geocentric lunar distances, the rate is nearly double that -- about 3.6' in 10 minutes of time. You can see that from the table that I posted earlier. At 03:00 UT, the distance would be 31° 23.7'. At 04:00 UT, it's 31° 02.1'. This rate is lower than the typical "straight down the ecliptic" rate (in this case about 30.3') by a factor of about 0.71 because Procyon is about 45° off the ideal axis. The ideal direction is directly ahead of or directly behind the Moon's motion which is reasonably well defined by the line passing through the Moon perpendicular to the Moon's horns. In historical lunar distance tables, off-axis/off-ecliptic stars, like Altair and Fomalhaut, were included (sometimes, not every case) as long as they were no more than 45° off-axis.

The decreased rate of change is definitely an issue here. I'm not questioning that! I'm only saying that it's not as bad as it may first have looked to you. A tenth of a minute generally corresponds to 12 seconds of UT in lunars. In this case, with the rate of change reduced by a factor of about cos(45°) or 0.707, the resolution of UT would be around 17 seconds per tenth of a minute of arc instead of 12 seconds. As long as we're here, note, too how rapidly this improves with smaller off-axis angles. If the other stars is 22.5° off the ideal axis, then the UT resolution is 13 seconds instead of 12. Inconsequential.

Frank Reed