# Will time that moon crosses meridian always be periodic?

If I observe the moon pass my celestial meridian at a specific north polar clock 'time' will I observe every subsequent passage at ( 'current time' + 13.2* X 'days passed' mod 360* ) ? Does the moon pass celestial meridian at exactly the same time if observed from every point on this longitude?

• No. The Moon's speed varies from day to day. Can you think why that's the case? – PM 2Ring Jan 9 '18 at 0:26
• astronomy.stackexchange.com/a/715/20197 moon follows ecliptic +-5* inclination. How big is the difference? Since its a metter of periodicity errors do not accumulate over time. what is max difference from formula up there? – ran8 Jan 12 '18 at 1:04
• The Moon's orbit is fairly eccentric (about 0.0549), so its daily speed varies quite a bit. The Babylonians discovered that the mean daily motion is approximately 13° 10' 35", but that it can range from 11° 6' 35" to 15° 14' 35". The Moon's motion is rather complex, the Wikipedia article on lunar theory is a good introduction, with a rundown of the history, and quite a few relevant formulae. – PM 2Ring Jan 12 '18 at 1:28
• thank you. if you put that as an answer I would ?accept it. – ran8 Jan 13 '18 at 2:43