Rising of the Moon

[ripvanwinkle]

Hello all,
I have a question about the position of the moon upon rising. Years ago I always noticed, or thought I did, that the moon rises in the same position on the horizon during the time of the month that it is visible. Over this past month I could have sworn that it rose from two different positions a week or so apart. One night I noticed that the moon rose just to the south of the neighbors house east of mine. About a week or so after that the moon rose just to the north of the same house. I don't remember ever seeing this before or am I just imagining what I think I saw? Is this suppossed to be normal? Can someone with better knowledge than myself help me out on this? Thanks!

 

[RyanX]

ripvanwinkle,

The moon varies where it rises in both a 28 day and an 18.6 year cycle. The 28 day cycle is obviously due to the moons orbit about the Earth. Because the Moon's orbit is inclined by about 5 degrees in relation to the plane of the earth's orbit about the sun (plane of the elliptic), and this inclination of the moon's orbit also varies over an 18.6 year period (think of a tilted spinning top), this adds another variable to the equation. So if we consider the rotational tilt of the earth (~23 degrees), the moon's declination in the sky can vary from (+/-)18 degrees to (+/-)28 degrees depending on where we are in the the 28 day and 18.6 year standstill cycle. Of course, what we actually see depends on where we are on the earth's surface too. During a major standstill, the moon can appear to vary quite widely in its declination over its 28 day period.

For a more detailed explanation and diagrams, check out the Wikipedia entry on Lunar standstill:

http://en.wikipedia.org/wiki/Lunar_standstill

That said, the last major standstill took place back in 2006 and the minor standstill will take place around 2015. (http://www.skyscript.co.uk/lss.html) So we're right in the middle of the downswing of the cycle at the moment.

There's a handy moonrise calculator here if you're interested in comparing what you're seeing to what is predicted:

http://www.timeanddate.com/worldclock/moonrise.html

 

[ripvanwinkle]

Wow! I never knew that. A huge misconception on my part and I've been on this earth long enough to have noticed it. Thanks RyanX!