Can someone explain to me the reasons for the following facts:
The moon’s declination varies from +25deg to -25deg over a period of around a month.
At the (northern) winter solstice, the full moon occurs when the declination is at a maximum, so that when you are at very high latitudes, the full moon tends to stay above the horizon, but the new moon is never visible.
At the (northern) summer solstice, the situation is reversed, and the full moon is never visible from above the Arctic Circle.
I just can’t visualise what’s going on here, and I know there are people here who have a good understanding of these things.
I believe this is just because the moon is very close to the ecliptic. For the sake of illustration, let’s say that the moon lies exactly in the plane of the Earth’s orbit around the sun.
From a point very far north on the Earth, the Sun is in the sky all day on or about the summer solstice, and it is absent from the sky all day on or about the winter solstice.
If you just look at the moon as following the sun’s “path”, the behaviour you mention makes sense. Where is a new moon? A new moon is directly between the Sun and Earth, so it will be in the sky at exactly the same time as the Sun. Hence, the new moon will be in the sky all day at the summer solstice, and absent from the sky all day at the winter solstice.
Where is a full moon? It is exactly opposite the Sun from the Earth. Since we’ve assumed the moon orbits in the ecliptic, the moon is exactly where the Sun will be in six months. Thus, at the winter solstice it follows the path the sun would follow at the summer solstice. So, at the winter solstice the full moon is in the sky all day, and at the summer solstice it is never in the sky.
The orbit of the moon is inclined from the ecliptic a little, but not near as much as the Earth’s axis which is inclined at 23.5 degrees - remarkably close to your “+25 -25” variation you’re asking about.
Thank you douglips. I still didn’t understand when I read your post, but I went home and thought about it and now it all makes sense.
Does anyone know whether the moon’s orbit is precessing, or is it always above the ecliptic for the same half of its orbit?
I’m glad you have grasped this, but I’m not satisfied with my first answer, so I’ve come up with a slightly different and shorter answer.
The reason the full moon is in the sky all day at the winter solstice above the arctic circle is that the moon orbits the Earth in a path that makes it trace out roughly where the Sun goes during the entire year, but the moon does it once each month. So, you get lunar “summer”, “spring”, “fall”, and “winter” once a month, instead of once a year. During the season of winter (i.e. December-March in the Northern Hemisphere) the “lunar summer” occurs at full moon, while during the season of summer the “lunar summer” occurs at new moon. Those near the arctic circle are already familiar with what the sun does during each season, and it corresponds exactly to what the moon does during each of these notional “lunar seasons”.
As for your second question, I believe that precession of the moons orbit is used in the prediction of tides. From Harmonic Analysis and Prediction of Tides:
If my math is right, that means the moons orbit precesses 360 degrees in about 18.6 years.
Perhaps we can convince The Bad Astronomer to drop by after his holidays.
How about a picture ?
The earths axis of rotation is inclined 23.5 degrees from being perpendicular to the ecliptic.
The axis always points in the direction of the north star.
This means that during part of the year, summer, the northern hemisphere is tilted towards the sun. During winter the northern hemisphere is tilted away from the sun:
North
x[sup]…[/sup]/[sup]…[/sup]O[sup]…[/sup] Summer
[sup]…[/sup]O[sup]…[/sup]/[sup]…[/sup]x Winter
South
The O represents the sun; the dotted lines the ecliptic, the slash (/) the orientation of the earth’s axis of rotation, and the x the position of the full moon. North is up.
The moon orbits nearly in the plane of the ecliptic, and is full during that part of its orbit when it is farther from the sun than earth. During the summer the northern hemisphere is pointed away from the position of the full moon, and during the winter it’s pointed more towards the position of the full moon.
The Moon’s orbit precesses, making a circuit once every 18.6 years or so. The primary factor, I believe, is the Sun’s gravity providing a torque on the Moon. I put the words “Moon orbit precession” into google and found lots of good sites; this one has a good explanation and some diagrams.
Another basic site is Astronomy Notes which has good diagrams too. Go to the “Astronomy without a telescope” section.
Anyway, douglips’ explanation looks right to me. The Moon’s orbit is slightly tilted (about 5 degrees) from the ecliptic, which itself is tilted by 23.5 degrees from the Earth’s equator. So the Moon ranges from 28.5 degrees north of the celestial equator to 28.5 degrees south. When it is full, it’s opposite the Sun in the sky, so does the opposite stuff: it’s high when the Sun is low, and vice-versa.
Thank you Bad Astronomer, and can I say off-topic that after watching “Mission to Mars” I went straight to your website to see if you had noticed some of the laughable stuff I did. You had, and more. I salute you.
It’s a simple math calc, dude!
90 degrees - YOUR latitude + dec of any object = max height an object will reach (altitude) above YOUR horizon. This is true for the moon, sun, stars, etc…!
Don’t get bogged down with the phase of the moon. But, if you’re curious about full moons, just realize it’s in the southern zodiacal constellations in the summer months and vice versa. (It lags behind the sun by 180 degrees.) As for your mention of “seeing” a new moon, forget it! A new moon is located very close in the sky with the sun, so you’ll never see it!
Also, the fact that moon’s declination (dec) changes from about +25 to -25 is just a trivial matter regarding your specific question. It doesn’t really matter what declination you use in my simple equation above. However, to answer your question, the Dopers are correct that the moon’s orbit lies in roughly the same plane as the earth’s orbit.
So, for example, at the north pole, if the moon’s declination (regardless of phase) dips below 0 degrees, it cannot be seen.
Simple! Play around with the equation above for various latitudes and/or various declinations…like the equator, tropics, etc.
Also, you might like to know that circumpolar stars (i.e.: stars which never dip below YOUR horizon) are found by simply 90 degrees - YOUR lat = min declination to be circumpolar. So, I challenge you: Under what conditions will the sun become a circumpolar star, at least for part of the year? (Assume the sun’s max dec = +26.5 degrees, min dec = -26.5 degrees)
Good luck,
- Jinx
Oops! In the above “extra credit” question about figuring out when the sun is a circumpolar star, I meant to say: Use +23.5 and -23.5 for sun’s max and min declinations!
Sorry!
- Jinx
Whoah! Hi Jinx. I know you didn’t mean to patronise me here, so thanks for the information. However, the question in my OP was a little more subtle, and has been wel answered. Plus, travelling in the Arctic is a hobby of mine. So you’ll understand if I pass on your challenge and leave it for someone who would find it, well, challenging:).
I’m really sorry, Hibernicus…I didn’t mean to sound patronizing. That’s not me! I just meant to share the trick with you - to say it’s simple math, for once, thank goodness! …and to imply nothing more by that statement.
That is to say, the math is surprisingly simple when most other astro calcs get quite involved. So, it may seem so daunting, on the surface, to try and make sense out of a star’s (or the moon’s) max altitude above the horizon (your local horizon) for various latitudes.
You see, I’ve only found two field guides (both by the same author) that explain this simple concept (and its math) well. And, these books are out-of-print and not commonly stocked in most libraries…so it’s a well-hidden secret!
…just trying to help you and others so all may learn in a practical, hands-on sense without having to read and grind through tons of theory… 
- Jinx
I truly apologise for the snotty tone of my previous post. If I’m offended by being told something I already know, that’s my problem, and I really need to get over myself.
Keep up the good work, Jinx. People like you are what make this message board great.