On Sep 25 2015 in California, at 8pm the moon was observed between 11 o'clock and 12 o'clock position in the sky.

On Sep 26 at 8pm at the same location the moon appeared roughly between 11 o' clock and 12 o'clock position as night before.

On Sept 27 at 8pm same location the super moon was not between the 11 o'clock and 12 o'clock but seen at 9:30pm at the 9 o'clock way behind trajectory. What is the cause of this?

  • $\begingroup$ What do you mean by 11 o'clock, 12 o'clock etc position in the sky? Right Ascension is expressed in time units, but I don't think that is what you are referring to, or is it? $\endgroup$ Commented Oct 13, 2015 at 14:59
  • 1
    $\begingroup$ Possible duplicate of Why did the moon abruptly change positions in the sky? $\endgroup$
    – Joan.bdm
    Commented Oct 14, 2015 at 8:56

1 Answer 1


This is the animation of the Moon's position over Los Angeles, CA skies at 8 p.m. local (PDT) time from September 25 to September 27, 2015 (made with Sky Map Online):

  Sky map

As you can see, there isn't any way to describe its apparent movement with a single angular distance, whichever method you use. In astronomy, position of objects on the night skies is most commonly given by RA/Dec (Right Ascension and Declination), which places an object on the celestial sphere.

Unless we count the Supermoon Eclipse on September 27, 2015 (your time, September 28 UTC), one that won't occur again until October 8, 2033, or exactly one saros period since the last one, there wasn't anything else as extraordinarily dramatic about the Moon's movement on those nights as you describe. So what happened is most likely that measurements by your observations weren't taken precisely enough; Be it you didn't time your observations precisely, didn't measure Moon's position relative to some fixed direction from some fixed vantage point, didn't project its position onto your scale taking the curvature of the celestial sphere into account, or a bit of any or all of these.

The Moon, if observed at the same solar time from the same location, moves in apparent eastward direction (when it's visible and above the horizon, westwards when it's below the horizon, regardless if that's during day or night) roughly 13° per day, or, saying it otherwise, each day, at the same time, it will appear to lag for about an hour relative to its position a day before. Why? Simply because it completes one orbit around the Earth every 27.321582 days, so at exact same solar time the next day, it will be 360°/27.321582 or 13.18° more East. Its apparent movement during the same day is of course still East to West, because the Earth rotates on its axis towards East much faster than the Moon rotates around the Earth.


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