Does the Moon orbit the Earth on the plane of the Earth’s equator, on the plane of the Sun’s equator; on some other plane; or does it vary?
I know there are no such things as “up,” “down,” “left,” and “right” in space, but I don’t know how else to describe this. The way seasons are always explained is that the Earth is tilted towards the Sun in summer, and away from the Sun in Winter, leaving the distinct impression that the Earth is slowly wobbling. But look at page 89 of the January 2000 issue of Discover, or any other drawing of the Earth-Sun system. On the Winter Solstice, the Earth is to the far right of the sun. The North Pole is tilted away from the sun at 23.5 degrees. On the Summer Solstice, Earth is to the far left of the Sun, still tilted at 23.5 degrees, but now that means the North Pole is tilted towards the Sun. So, in other words, the Northern Hemisphere doesn’t tilt towards the Sun in Summer and away from the Sun in Winter. It is always tilted towards Polaris; rahter, it is the Earth changing its position relative to the Sun that causes the seasons. Is this right?
(Note: I am talking about the yearly cycle of the seasons, and not the longer Milankovitch cycles that do change the angle of the Earth’s tilt.)
Is it just me, or were all the recent astronomical events real duds? The meteor “storm” in August (the Orionids? the Leonids? I can never remember) netted exactly one shooting star in two hours of viewing. Granted, I was in Chicago. So for the next big storm in November, I visited a friend in rural Illinois. Two hours in the cold, and I doubt we saw more than half a dozen. And then this big Winter Solstice / Full Moon / Moon at it nearest approach / Earth near the Perihelion… all amounted to nothing. A somewhat larger, brighter moon, so the features were clearer. But “casting clear shadows” – no stronger than others I see every Full Moon. “Driving without headlights”? Not even close.
The Moon orbits in its own plane. It is very close to the plane of the Earth’s equator, but not exactly. The Earth’s equator is, of course, tilted at 23.5 degrees relative to the plane of its orbit.
You are correct. You can think of the Earth as always tilting towards Polaris. When the Earth is in the portion of its orbit closest to Polaris it is tilted “away” from the Sun and when it is furtherest from Polaris it tilts “towards” the Sun. The tilt doesn’t change, just the Earth’s relative position to the Sun does.
Didn’t see the Leonoids, it was cloudy here that night. I did see a really bright blue-green meteor a day or two before. It was very bright, left a very distinct trail and shot completely across the horizion. Really impressive, especially considering that I was inside the city and could see it over the street lights. There was even a brief blurb about it in the news the next day.
As for the Moon, it was bright last night but it was also a full moon on a cold clear night. It did cast clear shadows (but I have seen shadows cast by the Moon before). I suspect astronomers (amateur or professional) or people who are very famailiar with the night sky could probably tell the difference but the average person probably just went “yeah, it’s a bright moon”. I wouldn’t have thought much more than that myself if I hadn’t heard the hype in advance.
“Drink your coffee! Remember, there are people sleeping in China.”
The tilt of the Earth’s axis is what causes seasons. You are correct that the Earth does not “wobble” from -23.5 degrees to +23.5 degrees each year.
Take something round that has good top and bottom marks on it. Tilt it some [no need to be exact, tilt it about 45 degrees]. That axis is what it rotates around, causing night and day.
Grab it by its new top and bottom. That is the axis it revolves around [There are technical terms for all these.]
Now revolve it around a “sun”.
The tilt of its rotation axis does not change. But on one side of the sun, the north pole is close to the sun. As it revolves to the other side of the sun, the north pole is away from the sun.
You know this to be true if you have heard, that north of the artic circle, they have days with 24 hours of light in the summer, and 24 hours dark in the winter. This would not happen if seasons were just due to the Earth’s orbit being close or far from the sun. http://www.howstuffworks.com/question165.htm
Most of North America got shorted on the best part of the November Leonid show this go-round. The main part of the Tempel-Tuttle comet is in a pretty thin band, and our little planet passes entirely through the thick part of the debris stream in only a few hours. IIRC, the ’98 event was mostly in Asia, and the ’99 event was in extreme western Europe and, to a lesser extent, on the east coast of the US. We got a few good ones here, even through the city lights. Maybe we’ll do better 33 years from now.
While were on the subject of orbits, let’s extend the question a bit. Here’s question number 4:
Imagine traveling a few AU’s toward Polaris and looking back at the solar system. I believe that all the planets are orbiting the sun in the same direction (counter-clockwise?). Is that correct? And what about the moons of the planets? Do they all move in the same direction regardless of the planet they orbit, or in the same direction on a planet-by-planet basis, or is there no correlation?
Yeah - looking at the Solar System from “above”, all the planets orbit in the same direction. That’s the same as saying that their angular momentum vectors point in the same direction. Planets’ moons also orbit in this same direction (there may be a small captured asteroid somewhere that has a retrograde orbit, but the vast majority of them are prograde). As a matter of fact, all of the planets also spin in this same direction, except for Venus. I guess it just likes to be different.
They say I got the power, because I got the monkeys.
They are WRONG! I got the power because I am not afraid to let the monkeys loose.
I think there was some misinformation in the first couple of replies to the OP. The moon’s orbit is not particularly close to the earth’s equatorial plane. It is close to the earth’s orbital plane, which is, as mentioned above, tilted at 23+ degrees from the equatorial plane.
The moon’s orbital inclination (relative to the earth’s orbital plane) is 5.145 degrees.
I thought of a clever new sig line last night, but I forgot it when I woke up this morning.
If by that you mean did the events fall short of the media hype, then yes, they were duds. The hyperbolic news media gets people all excited about “The Biggest _______ of the Century”, then if the meteor shower is anything less than spectacular, or the moon in the sky isn’t a big pizza pie, then the Great Unwashed Masses feel ripped off.
I’m sure the Leonids were a bit more active than usual, and the Moon was a shade brighter than average; but IRL, nothing can live up to the predictions the news media uses to sell papers/commercials/banner space.
Anybody remember Comet Kahoutek? Whose fault was that?
TT
“Believe those who seek the truth.
Doubt those who find it.” --Andre Gide
Not so fast, Pluto. I was going to believe you, but I found this, which gives the Moons inclination as 5.14 degrees with the following definition of inclination:
Inclination
the inclination of a planet’s orbit is the angle between the plane of its orbit and the ecliptic; the inclination of a moon’s orbit is the angle between the plane of its orbit and the plane of its primary’s equator.
And you call yourself a planet! Your nothing but a runaway moon!
THe coolest thing to see is when a man-made satellite re-enters Earth’s atmosphere. I saw this happen in Feb/Mar of 1988. It looked like a huge fireball flying across the sky. THinking that the end was truly near, I hurried to the keg and filled my cup with more beer.
Re-checking my source, it gives the moon’s inclination with the ecliptic (i.e. the apparent path of the sun through the heavens, i.e. the earth’s orbital plane) as 5.14 degrees.
One of our sources is wrong. My source, with the lofty title Practical Astronomy With Your Calculator has been autographed by an actual Space Shuttle astronaut! (Janet Kavandi, who used to work for the same part of Boeing that I do, although I never met her till they announced her acceptance into the astronaut program and they had a little get-together for her. Where she signed my book!)
Conflicting sources notwithstanding, let us reason together. If the moon’s orbit were only inclined 5 degrees from the equator, its declination (it’s angle above the equatorial plane) would never be greater than 5 degrees. According to NASA’s ephemeris (there’re those darn space guys again!) it’s declination today is 20 degrees. If the moon’s declination were always +/-5 degrees it would always be low in the southern sky (from a northern hemisphere perspective) and would rise and set on a nearly constant 12-hour cycle. In fact it moves up and down in the sky through the course of a month and moonrise and moonset vary just like sunset and sunrise do.
Further, in order for an eclipse (lunar or solar) to occur the moon’s shadow has to lie in the ecliptic (eclipse, ecliptic, get it?). If the moon were orbiting in the equatorial plane this would be a rare occurrence. In fact there are at least two solar eclipses a year, sometimes as many as five, and two or three lunar eclipses each year.
I believe the confusion arises because of the special case of calculating the moon’s orbit vs. calculating the orbit of another planet’s moon. For the moon the inclination is given relative to the ecliptic, just as it is for planets or asteroids. For moons of other planets it is given according to the definition cited.
I thought of a clever new sig line last night, but I forgot it when I woke up this morning.
The 5.14 figure is relative to the ecliptic. Notice that any tilt relative to the ecliptic is going to make solar eclipses more rare–twice a year is about the going rate.
I just queried sci.astro.amateur about that SEDS page. Seems crazy to me, too.
I checked a couple of astronomy books and the ~5[sup]o[/sup] tilt is to the earths orbit, the ecliptic. They also said that total solar eclipises(sp?) occur every 1.6 years and lunar every 13.5 months.
They mentioned that the moon’s orbit is “like a wheel on a bent axle” and that it took 18 years 10 1/3 days (or 11 1/3 depending on the number of leap years.) to end up in the same place.
If you can’t trust a pocket guide to astronomy, what can you trust.
I found another URL that also says the 5 degrees is WRT the orbital plane. Given what JimB wrote, the footnote on the URL I showed makes sense: the tilt WRT the equator will vary that amount. JimB, does your source say why the moon’s orbital axis varies so quickly? I assume the cause is analogous to the precession of a gyroscope.
The page that ZenBeam referenced earlier has a link to a tutorial page, http://hotel04.ausys.se/pausch/comp/tutorial.html#7 , where it is explained. “The Moon’s ascending node is moving in a retrogade (‘backwards’) direction one revolution in about 18.6 years.”
Venus’ rotation can be kindof confusing - some sources will say that it’s rotating backwards with respect to the others, others will way that it’s rotating the right direction, but the axis is tilted somthing like 170 degrees. (Probably not the exact number, but you get the idea.)
In effect, the two perspectives amount to the same thing, but the second is more accurate if it’s true that Venus was knocked around early in it’s life during a massive collision.
And yeah, the whole Moon thing was way overblown. I understand that the Moon actually brightens much more shortly before a lunar eclipse, like we’ll have on January 20. If only the media will pay attention to THAT!
drewbert
Morehead Planetarium
(who talked to 5 or 6 newspapers, a radio station, a tv news crew and about 8742 other callers this past week about the friggin’ moon!)
