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[Chicago Astronomer - Astro Joe]

Rare Occultation by Pluto's Moon

In a feat of astronomical and terrestrial alignment, a group of scientists from MIT (Cambridge, Mass.) and Williams College (Williamstown, Mass.) recently succeeded in observing distant Pluto's tiny moon, Charon, hide a star.

Such an event had been seen only once before, by a single telescope 25 years ago, and then not nearly as well. The MIT-Williams consortium spotted it with four telescopes in Chile on the night of July 10-11.

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You know....It's rather remarkable that with the countless amount of stars in the heavens, that these events don't occur more frequently.

[Maddad]

There is SO much distance between stars though.

[tim]

I used to think like that too. At the risk of seeming silly, I don't recall the object, but somebody on the astronomy site was talking about why we could'nt see something in cygnus and I said probably because of the millions of stars in between us and it ( this was a couple years ago when I first started in astronomy ) actually there are probably very few. If this makes sense to anybody let me know what I'm talking about.

tim

[Chicago Astronomer - Astro Joe]

Why arn't there more Occulations

Ok...I'm going to push discussion here.

I cannot believe that the swath the planets and Moon make in their orbits against the ecliptic, that more occultations don't regularly occur. They pass thru the major constellations, and although many stars fainter than Mag 4 are there, cameras still could document the event.

Hard to say that all the planets, (all 8 of them & their moon's), skim, pass, dodge, fly-by or near-miss plenty of stars...regardless of the Mag of the star. Is this path that they take completely devoid of stars? I doubt it, but I'll buy into not occulting the major brighter stars, easy enough to understand, even for me.

Hmmm...

[tim]

Joe
Thats what I was thinking. Maybe with the brightness of the moon it washs all the fainter stars out.

[Maddad]

Consider this Joe. Why don't we see an eclipse of the Moon every month?

[Chicago Astronomer - Astro Joe]

Only three bodies to consider in the dance. Odds are against a regular event.

Stars...countless.

Still with mind open for an explanation....

[Maddad]

So why don't we see an eclipse ever month?

[Centaur]

maddad said:

So why don't we see an eclipse ever month?

And Joe wrote, "I cannot believe that the swath the planets and Moon make in their orbits against the ecliptic, that more occultations don't regularly occur. "

We don't see an eclipse of the Moon or the Sun every month because the plane of the Moon's orbit is inclined to the plane of the Earth's orbit (ecliptic) by 5.15°. The luminaries each appear to be about 0.5° wide. When the Moon is near one of its two nodes (points of intersection of the two orbital planes), and in conjunction or opposition with the Sun, an eclipse is possible. During most months the Moon passes at too great an angular distace from the Sun or anti-Sun to create an eclipse. In fact, that's how the ecliptic got its name: the Moon has to be nearly on the ecliptic for an eclipse to occur.

Properly speaking, a true eclipse occurs only when one body moves into the shadow of another. What we call a solar eclipse is actually not an eclipse but an occultation of the Sun, if it is total, or a transit by the Moon, if it is annular. Lunar eclipses are good examples of actual eclipses.

Regarding "eclipses", occultations and transits generally: the stars are spread further apart on the celestial sphere than most people believe intuitively. There's a lot of black out there. The angular widths of the Moon, planets, asteroids and planetary satellites are less than most people realize. Thus those juxtapositional events are less likely than most people might suppose.

[Maddad]

Well, that's a whole carload of words for saying that the Moon's just a squak tilted in its orbit and only cirectly lines up wiht us and the Sun once in a while.

The reason though that we don't see more occulations with other bodies is that they're very much further away from us than our Moon is. If the distance is ten times as great, then an object of the same intrinsic size will have a visual diameter of one tenth as much. Those occulations will occur one tenth as often.

Planetary distances though are much more than ten times as far away from us as the Moon is. Jupiter at its closest is about 600,000,000 kilometers, and the Moon is 380,000. That makes it 15,000 times as far away.

Now what about the object you want to occult? Does it have the same cross section as the Moon, 30 arc minutes? No, taking Jupiter at its closest again, it's just 50 arc seconds, making the Moon about 36 times as wide. Multiply 15 ,000 by 36 and you get half a million. The Moon will eclipse 500,000 times as often as one of these other objects.

[Centaur]

maddad said:

Well, that's a whole carload of words for saying that the Moon's just a squak tilted in its orbit and only cirectly lines up wiht us and the Sun once in a while.

The reason though that we don't see more occulations with other bodies is that they're very much further away from us than our Moon is. If the distance is ten times as great, then an object of the same intrinsic size will have a visual diameter of one tenth as much. Those occulations will occur one tenth as often.

Planetary distances though are much more than ten times as far away from us as the Moon is. Jupiter at its closest is about 600,000,000 kilometers, and the Moon is 380,000. That makes it 15,000 times as far away.

Now what about the object you want to occult? Does it have the same cross section as the Moon, 30 arc minutes? No, taking Jupiter at its closest again, it's just 50 arc seconds, making the Moon about 36 times as wide. Multiply 15 ,000 by 36 and you get half a million. The Moon will eclipse 500,000 times as often as one of these other objects.

Sorry, but your math is a little fuzzy. You already reduced giant Jupiter to a mere 50 arcseconds - no need to try to reduce it a second time, especially not by 15, 000 when 1500 is nearer the actual distance ratio. Nevertheless, that point is moot since it was already taken into account when the 50 arcseconds was determined. However, the unmentioned factor is the relative speeds at which the Moon and Jupiter appear to traverse the starry background. From our vantage point the Moon appears to be moving about 150 faster, meaning it will sweep by certain stars 150 times more frequently. That is the figure that must be multiplied by the 36 number in your example. It should also be noted that due to the previously discussed orbital inclination, the Moon can sweep a wider section of the sky.

[Maddad]

It does need reduction a second time because we're talking about Jupiter occulting another object which I arbitrarly decided was about the size of Jupiter.

As for the 15,000 factor, you're right. 1,500 is the correct number.

The speed of the moon though is wiggle room that I deliberately left out to cancel another objection, not the inaccurate multiplication factor though. I thought of it as the Moon orbiting our Earth once every month; twelve times a year. Jupiter orbits the sun once every twelve years, which means the Moon as compared to Jupiter has 140 times as many opportunities to occult something in our field of view.

[Centaur]

maddad said:

It does need reduction a second time because we're talking about Jupiter occulting another object which I arbitrarly decided was about the size of Jupiter.

As for the 15,000 factor, you're right. 1,500 is the correct number.

The speed of the moon though is wiggle room that I deliberately left out to cancel another objection, not the inaccurate multiplication factor though. I thought of it as the Moon orbiting our Earth once every month; twelve times a year. Jupiter orbits the sun once every twelve years, which means the Moon as compared to Jupiter has 140 times as many opportunities to occult something in our field of view.

Now if you're talking about another object matching the size of Jupiter within the solar neighborhood; sorry there is no such thing. Even if there were, then we would have to distinguish among partial/total occultations/transits. The likelihood of each permutation varies greatly when the objects' distances from Earth are similar. But such similarities of distance and size would rarely be the case regarding objects Jupiter might occult.

Saturn is the planet that comes nearest to Jupiter's size, but it is significantly further away. You would be correct in assuming that Jupiter rarely occults Saturn. I performed a search with self-written software that found no such event between 1001 BC and AD 2800. A computer search by Jean Meeus found no such event between 1001 BC and AD 4600.

In the cases of occulted stars, they are so far away that their sizes are not a consideration.

In any event, Jupiter's polar diameter is 38.42 times that of the Moon, which is the ratio that matters in this discussion. It's the polar diameter that sweeps across the sky. Whenever Jupiter is exactly 1500 times further away than the Moon, the Moon will appear to be 39.04 (1500 / 38.42) times wider between poles. No other size or distance factors are nearly as relevant. Certainly there is no justifiction for squaring that factor of 1500, which is the effect of applying it twice.

Regarding the Moon's greater apparent angular speed: there are some objects it can occult which Jupiter cannot because Jupiter does not deviate as far from the ecliptic plane. However, Jupiter's relative occultation frequency is enhanced for those objects within its range since the Moon is so often outside of that range.

We are currently in an unsually long drought for the mutual occultations/transits of planets. The last one occurred in 1818 when Venus totally transited Jupiter. The next event will be a partial transit involving the same pair in 2065. Interestingly, there were two such events in 1477 and another two in 1708. There is only one recorded observation of such an event since the invention of the telescope. That was by an English amateur astronomer who in 1737 watched Venus totally occult Mercury.

The brightest star that will be occulted by a major planet between now and 2040 is magnitude 3.0 Pi Sagittarii. Venus will occult it on 2035 MAR 12 for observers in a portion of Antarctica. Make your travel arrangements now before the rush.

[Maddad]

Correct, there ain't no such thing as another object that's the size of Jupiter. Which makes the case for an occulation even more difficult. I picked Jupiter as a worst case scenario.

As for stars being so small that their size is not a consideration, of course it is. That's sorta the point. Your occulting object, say Jupiter, has to cross a particular point in your field of view. There's no partial eclipse.

Earlier today I was considering looking up the angular size of Saturn and working out how often Jupiter ought to occult it, but your numbers are good enough. Very rarely works fine. Part of the reason is that the angle of tilt will not match exactly, so each time Jupiter catches up to Saturn in orbit about the Sun, which should work out to something more than Jupiter's year, the two have to be in the same point in their orbital tilt to produce an occulation. Anyway, I think I've bashed this thread enough. Time for me to move on.