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Re: Why is the Moon so slow today?
by Brillig
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You are probably forgetting that the moon goes from west to east, not
east to west. So if it goes faster, it will take longer to get to the
same place since the earth rotates the same way. The moon effectively
is catching up a little bit to the earth, so the earth takes longer to
lap it.
Victor
On 5/7/12, Moongazer <motti.yarchinai@...> wrote:
> 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 http://aa.usno.navy.mil/data/docs/RS_OneDay.php 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
> http://www.fourmilab.ch/earthview/pacalc.html 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?
>
> --
> View this message in context:
> http://old.nabble.com/Why-is-the-Moon-so-slow-today--tp33763520p33763520.html
> Sent from the Calndr-L mailing list archive at Nabble.com.
>
--
Victor Engel
http://victorspictures.com/blog
east to west. So if it goes faster, it will take longer to get to the
same place since the earth rotates the same way. The moon effectively
is catching up a little bit to the earth, so the earth takes longer to
lap it.
Victor
On 5/7/12, Moongazer <motti.yarchinai@...> wrote:
> 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 http://aa.usno.navy.mil/data/docs/RS_OneDay.php 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
> http://www.fourmilab.ch/earthview/pacalc.html 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?
>
> --
> View this message in context:
> http://old.nabble.com/Why-is-the-Moon-so-slow-today--tp33763520p33763520.html
> Sent from the Calndr-L mailing list archive at Nabble.com.
>
--
Victor Engel
http://victorspictures.com/blog
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