I was resetting my Time-and-Tide clocks (I have three of them) today after changing the batteries. (It helped that it was Full Moon only yesterday.) I looked up last night's upper transit time of the Moon for my meridian, set the clock to that time and its moon-pointer to the "12 o'clock" (high-tide) position, then adjusted the time (all four clock-hands) forward to the current time-of-day and inserted the battery. I got the upper-transit data from the Data Services section of the US Navy's Astronomical Applications website on
this page.
It then occurred to me to compare the same data for yesterday and today to see exactly how long a tidal day is right now -- i.e. the time between two successive upper transits of the Moon (sometimes erroneously called a lunar day). Its MEAN length is about 24h, 50m. To my surprise, subtracting yesterday's upper transit time from today's gave me a difference of 1:03 (i.e. the tidal day is currently 25h, 3m.)
Now I know that since the Moon's orbit is an ellipse, not a circle, the Moon's motion is not uniform, i.e. its orbital speed varies. It is at its slowest at apogee and at its fastest at perigee. So, I thought, if the tidal day is currently so much longer than its mean length, the Moon must be moving fairly slowly right now. It must be fairly close to apogee. I then decided to check on this and, to my surprise, I discovered from
this site that only yesterday (coincidentally at almost exactly the same time as Full Moon) the Moon was at perigee! If so, I would have expected the current length of a tidal day to be shorter than its mean, not longer. Hence my question: why is the Moon so slow today?
