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johnhaigh
2004-Feb-16, 10:20 AM
Can anybody explain or help me to find out how to predict the directions of Moon rise and Moon set and the Moon's path through the sky as viewed from various places on Earth.

I know that almanacs give a bearing for moonrise for different places; and times of moon rise and set are pretty straight forward. But while the point on the horizon that the Sun rises from is simple enough to figure, if you know the date and latitude, I haven't been able to find information to get an understanding of how the path of the Moon through the sky varies with time and the viewer's location on Earth.

Thanks in advance if you can either explain it or give me a link to a site that has an explanation.

Wally
2004-Feb-16, 01:51 PM
the moon sticks pretty close to the eliptic (meaning it's roughly on the same plane as the planets). Any good star chart will show you the eliptic in relation to the celestrial equator (which is a projection of our equator in space). This should give you a good idea on where the moon will rise and set, and it's path across the sky.

Astronomy mag's monthly star chart actually shows you the moon's path across the sky along with the planetary eliptic. You can see they kinda match up.

milli360
2004-Feb-16, 04:14 PM
The moon's orbital plane is about five degrees from the ecliptic, so sometimes it's on the ecliptic, sometimes it's five degrees away. That's pretty much the main reason that we don't have solar eclipses every month.

Go to an online star chart, or download one of the freebies, or demo versions of commercial programs.

johnhaigh
2004-Feb-16, 06:18 PM
Thanks milli360 and Wally,

I should be able to put together a mental model to figure it out with that info.

Can I infer that the moons of other planets are also close to the ecliptic? i.e are the Sun, the planets and their moons all in roughly the same plane?

I guess not, because we do get a fair look at the rings of Saturn from Earth, not just edge on.

Thanks again.

milli360
2004-Feb-16, 06:25 PM
Can I infer that the moons of other planets are also close to the ecliptic? i.e are the Sun, the planets and their moons all in roughly the same plane?

I guess not, because we do get a fair look at the rings of Saturn from Earth, not just edge on.
Good example--and in a few years, they will be edge on! So, enjoy them while you can.

I believe even Galileo was taken in by this--he observed their image, seeing them as "ears" on Saturn I think, but years later when he looked again, they appeared to be gone.

JohnOwens
2004-Feb-17, 01:27 AM
Thanks milli360 and Wally,

I should be able to put together a mental model to figure it out with that info.

Can I infer that the moons of other planets are also close to the ecliptic? i.e are the Sun, the planets and their moons all in roughly the same plane?

I guess not, because we do get a fair look at the rings of Saturn from Earth, not just edge on.

Thanks again.

No, not at all. The reason we get a good view of the rings is because the plane of the rings lines up pretty well with Saturn's equator, which is tilted with respect to the ecliptic (26.73 degrees). The tilt of Saturn's orbital path is (largely) irrelevant to the ring viewing (an extreme tilt would let us see a little more as our own orbital path gives us more parallax on it). On the other hand, Pluto is far and away the example for a planet not close to the ecliptic, with an inclination of 17.14 degrees. But it's not great for visibility. The next greatest planetary orbital inclination is Mercury, at 7.004 degrees.

Edit: Oops, I missed where you said "the moons of other planets", not just "other planets". For moons, I think they tend more to line up with the equators of the planets they orbit, rather than either the ecliptic or the orbital plane of the planet. But don't quote me on that.

johnhaigh
2004-Feb-17, 07:02 AM
Thanks again people.

I guess I was just building a model of the formation of the solar system and its planets and moons which was based on the idea of a disk shaped coalescence of gases and dust which would have the planets and moons aligning on that disk and holding roughly to those positions. It surprised me that the Moon's orbit followed the path of the ecliptic. But remembering star charts I recall that the moon is always there.

By the way, John Walker's Fourmilab site has a great astronomy section which includes a very easy to use and modify star chart called "Your Sky".

This is his homepage
http://www.fourmilab.to/

and this goes straight to the "Your Sky" star chart
http://www.fourmilab.to/yoursky/

I was imagining that it was not coincidental that the Earth's moon's orbit was very close to the ecliptic, but a consequence of the manner in which the Solar system was formed. i.e planet moon systems were formed in a similar way to the solar system. {captured comets/space debris, Velikovsky etc. etc. NOT applying}

I've still got to figure out how to predict where the Moon will rise and set each day in my head without using a star chart.

Thanks again for all your enlightening help.

John.

I like the quotes that members use as a signature so here is one I hope you find amusing:
A corollary of Murphy's Law: "1.You can't win. 2.You can't break even. 3.You can't quit the game." Capitalists believe you can win, socialists believe you can break even and religion is based on believing you can eventually quit the game.

Diamond
2004-Feb-17, 08:45 AM
Thanks milli360 and Wally,

I should be able to put together a mental model to figure it out with that info.

Can I infer that the moons of other planets are also close to the ecliptic?


No. Earth/Moon is really a double planet system, and unlike other planets, the Moon is in the ecliptic. Uranus, for example has an orbital inclination of 98 degrees and its moons can be seen all around the orbit, since they (roughly) orbit the equator.


i.e are the Sun, the planets and their moons all in roughly the same plane?

No. The planets are roughly in the same plane (the most extreme variation is Pluto whose orbit is 17 degrees inclined to the ecliptic)


I guess not, because we do get a fair look at the rings of Saturn from Earth, not just edge on.

The reason for that is that Saturn's axis is not at 90 degrees to its orbit. Like the earth, its axis is inclined, producing seasons.

johnhaigh
2004-Feb-18, 08:52 PM
For those of you who wanted, like me, to be able to able to bore their friends by saying "Hey it's nearly time for the Moon to rise", grab a pocket compass and predict where it is about to come up in a few minutes; the data can be found on the site below.

Jurgen Giesen's great site at
http://www.jgiesen.de/welcomeEnglish.htm

The read-off graph is an applet that you can use in your browser or buy to keep in your computer at
http://www.jgiesen.de/SunMoonHorizon/index.html

A comprehensive discussion of the calculations involved is at
http://www.jgiesen.de/sunmoonpolar/index.html#mondwenden

It seems too complex to have a simple model in your head that will apply to more than your hometown though. Apologies to the ultra IQ people who can.

Thanks again to the folk who replied and sent me on the right track.

John

PS Sorry that I haven't looked up how to create an active link.