We all know the apparent path of the sun across the sky each day and how this varies throughout the year. But what about the moon?
How does the apparent path of the moon vary through the seasons? Why? What about the side that the shadow is on - how does that vary?
I had no luck googling.
I wish you much luck with the answer to this question – it brought back vividly the memory of an energetic discussion I had with the professor of an ‘intro to physical astronomy’ course I took in my third year of university over an exam question:
“When is the full moon highest in the night sky?”
I can’t remember all the details, but I got so frustrated with the prof’s seeming inability to either understand my line of reasoning or express his own in any way that made more sense to me than ‘simplistic’ that I eventually dropped the course just around the drop deadline after which it would have shown up as an incomplete on my transcript.
Didn’t trust that guy with the final exam.
(This was after I’d taken the slightly less intensive gen-ed astronomy course two years before and done quite well, except for flaking on the ‘photograph a constellation’ assignment because I was too lazy to find someone with a camera I could borrow and go outside on a clear night. :D)
Sorry for the mostly-OT reply in GQ
Here’s a site to get you started 'til we find you a better one. It’s commercial, but good, simple information. By the way, has anyone seen Te Bad Astronomer lately?
Solar Analemma
Solar Analemma on Mars (simulated)
Lunar Analemma (1 month, simulated)
Measuring Moon Shadows
Variation in the size of the Moon at apogee and perigee.
Animation of the Moon’s path relative to the ecliptic.
I don’t know but the things that determine the moon’s orbit are the initial conditions and the gravitation from other objects. The earth and sun are the main determinants followed by Jupiter and Venus. I think the answer is somewhere in the initial conditions and/or the interactions of the moon and those members of the Solar system.
The “path across the sky” on a given day is close to due east-west, modified by a slight drift north or south, caused by the angle of the moon’s orbit to the equator and it’s position in its orbit-- i.e. how far north or south the moon appears to move during the course of a given day. This will always be a very small fraction of a degree.
However, I suspect you may be more interested in the analemma formed by its precession at “high moon” (by analogy to “high noon”) during the month
IIRC, the Earth orbits at a angle of 25° 35’ to the solar ecliptic (or from a geocentric view, the sun revolves around the earth at that angle to the Earth’s equator). This orbital tilt determines the “height” of the analemma (“figure 8”) progression of the position of the sun at solar noon over the course of the year. The width is determined by the variation in orbital velocity as the the earth moves from apogee (furthest) to perigee (closest approach) in its slightly eccentric elliptical orbit.
The latitude of the observer is the other major factor determining the shape of this figure-8. Essentially, the further you are from the equator, the more lopsided the “8” becomes (one loop becomes much larger than the other)
To a naive observer on Earth, the “apparent” (geocentric) physics is much the same for the Moon orbits at a 5° 8’ angle to the equator, creating a much shorter figure 8, and the eccentricity of the lunar orbit (and hence its variation in orbital velocity) is greater, resulting in a “fatter” figure-8. Latitude plays the same role as it does withthe sun. The fact that the Moon is much closer than the sun also makes these (and other) variations more apparent.
There is a complicating factor, however. The Earth’s solar orbit takes 365.24 days so sun does not return to quite the same spot every year, but the leftover .24 day drift (by far the largest of many complicating factors) is small compared to a year, and can be ignored in casual use. The moon’s orbit (seen from the perspective of an Earth observer, aka the sidereal month) is 27.32 days, and the leftover .32 days is a much larger fraction of the orbital period. The “drift” from orbit to orbit is therefore much larger.
The combined effects of greater orbital eccentricity and drift also explain why nearly 60% of the moon’s surface is visible from the Earth at various times, even though the Moon is tidally locked to Earth.
That’s a general description of the moon’s precession at “high moon” across the sky. I sense you are looking for something more precise, but I hope this is enough to help you at least plot a reasonable first approximation, and that flaws in my recollections don’t embarrass me to much.
Theis site says that the precession of the plane of the moon’s orbit around the earth is the cause of the nutation. Unfortunately is doesn’t say what causes the precession of the moon’s orbit so we haven’t advanced much.
You’d think Bad Astronomer would knock off that teaching and astronomical investigations to be available to give us the straight dope on questions like this. First things first and all that, you know.
I’m not an expert, but I believe the answer is : half way between sunset and sunrise.
The moon follows a similar path (the ecliptic), but it moves through the ecliptic much quicker than the sun. While sun changes position subtly, day by day, the moon’s change in position is very, very obvious.
Since the full moon gets the most attention, here is a general rule of thumb for this moon phase - which I presume you are most concerned: The full moon will take the highest path across the Northern night sky around the Winter Solstice, and the lowest path around the Summer Solstice. For those living below the Equator, this rule of thumb will be the opposite.
Now, just how high will the moon peak above the horizon (i.e.: altitude)? We take your latitiude and add the moon’s “latitude” (declination) to that. So, if I’m at +40 latitude, and it’s a full moon around the Winter Solstice, I know the moon’s declination is about 23.5 degrees. Therefore, the total angle above the horizon (altitude) will be approximately 63.5 degrees above the horizon.
This is also the sun’s approximate position above the horizon around noon on the Summer Solstice.
I hope this helps…
Oops, one small correction! It is not your latitude, but the complementary angle, or in other words 90 - your lat + moon’s dec = altitude. So, for this example:
90- 40 = 50 +23.5 = about 73.5 degrees altitude at its peak. For your purposes, this is close enough to the answer.
Nutation is negligible, it is not even factored into star catalogue calcs for determining star coordinates in star atlases for say, Epoch 2000.0, for one. (Ref:
Peterson Field Guide “Stars and Planets”, Menzel and Pasachoff, Mifflin, Boston, MA, 1983, pg. 415) One reason for this is because the telescopes used by amatuers cannot be set such a high precision as to account for this very small factor.
So, the neophyte stargazer need not worry about (and be scared-off by) such details. Also, you don’t need to worry about the analemma and such just to know, instinctively, when the full moon reaches the max/min (extrema) in its course during the solar year. - Jinx
I’ll swear there was a question somewhere about the small nutation of the earth on an approximately 19 year cycle but it’s gone now. In any case, my answer was to that question - wherever the hell it got to.
Gloriosky, Zero! The question I was trying to answer was in Jinx’ thread on the ultimate cause of the 19 year cycle of nutation of the earth’s axis.
So what was the crux of the disagreement? I can think of two proper answers: a) at midnight, and b) in mid-late December. Was one of you arguing for a, and the other was arguing for b?
Think about how the sun’s daily path across the sky varies over the course of a year (northerly in summer, southerly in winter). The moon completes that same north-to-south-and-back cycle every month.
It doesn’t. Again, the moon goes through the north-to-south-and-back cycle every month. What varies is which phase the moon shows at which point in the cycle.
In June, the moon is new when it is northerly (so you don’t see it) and full when it is southerly. In December the moon is new when it is southerly and full when it is northerly (so it shines all through the long winter nights). At the equinoxes, of course, it’s in between.
The lit side always faces the sun.
As a bonus, since I’m an egotist, I’ll provide a link to my guide to the movements of the sun and moon at the North Pole.
Correction on that North Pole link.
Thanks all. A lot of info there to help me conceptualise the movement of the moon across the sky. Will look in detail tonight.
Thanks freddy the pig. Good explanation. About the shadow - I know it is on the side facing the sun - just wanted to know how it varies periodically. It switches from left to right doesnt it?
btw, the bad astronomer will be in Sydney in 10 days - I hope to see him then - maybe he is already here.
ok … I know the lit side faces the sun. sheesh :hitting head smiley:
Taking the lead of a brilliant writer and teacher educator, Eleanor Duckworth, I have several times actually made a daily observational journal of the moon, for a month at a time. I have done this at different times of the year. And, I have had students, from 6th grade through graduate school, do the same, and have read them. Nothing I have ever seen in print, on the net, or on TV has done for my understanding what those moon journals have done for me. I recommend you combine the helpful responses here with your own observations throughout the year (good science often takes time and patience). For your journal, be sure you make at least one attempt and hopefullly several attempts to see the moon every day. Remember that except for two or three days every month, the moon is visible at some time of the 24 hour day, every day - assuming no cloud cover. Be willing to look before sunrise, at 3 am, and times of the day that the sun is out, too. Record as accurately as you can the height, direction, and appearance of the moon. Try, also, to keep in mind where the sun might be in relationship to you, the earth, and the moon. That will help you understand the phases. Also keep subjective notes - nature is wondrous and should occupy that type of space in your notes as well. I guarantee a most wonderful experience. xo C.
Indeed it does. As the moon “waxes” from new to full, the lit side is on the right (as you look at the moon facing south)–facing the recently-set sun. As the moon “wanes” from full back to new, the lit side is on the left–facing the soon-to-rise sun.