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Pierre Boucher wrote:

> 12h 06m is the meridian passage time at the ZM (Zone Meridian) i.e. at 135d
> 00.0'E
>
> Difference of longitude between the ZM and the observer's meridian is 2d
> 20.0' west of the ZM
> Converted to time this gives 9m 20s (later because west of ZM)
>
> So  12h06m +  9m20s = 12h15m20s
> Bear in mind that the meridian passage times tabulated in the almanac are
> rounded to the nearest minute.

True, but the equation of time is tabulated in seconds right next to it. That
renders the time of meridian passage completely redundant. It seems to me that
its only purpose is to eliminate the mental circus act of figuring out whether to
add or to subtract EoT from 12:00.

Indeed, EoT, nowadays and hereabouts being the correction to be added to mean
time to get apparent time, is negative on Jul 20 - as indicated by the shaded
background. Since we want to go from apparent to mean we have to subtract it from
12:00 But subtracting a negative value means adding its magnitude. Thanks
goodness, we can forget all this. First we look to the right side and see
meridian passage is at ca. 12:06 UT. Then we look to the left and find that it
actually happens at 12:06:19. At other times, when we see a meridian passage
around 11:xx UT, we know that we have to subtract EoT from 12:00.

I find this still cumbersome. Would it not be much easier if the nautical almanac
would dispense with EoT altogether and just give the sun's upper and lower
meridian passage through Greenwich in UT to the second?

Herbert Prinz

   

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