General astronomy discussion

Re: Total eclipse.

zachsdad — Fri, 26 Jun 2009 20:55:34 GMT

Centaur:
Excellent question, zachsdad, but none of those would result in total eclipses.

I used your formula and found that from Saturn even grand old Jove would only block roughly 22% of the sun's disc. But still, that would be a neat transit to see.

Re: Total eclipse.

Oliver Tunnah — Fri, 26 Jun 2009 20:15:02 GMT

Interesting, what about the 'outer' satellites? Would anything be close.
How about the other 3 gas giants?

Re: Total eclipse.

Centaur — Fri, 26 Jun 2009 15:42:03 GMT

Oliver Tunnah:

I think the the satellites around the gas planets needs another look.
Obviously the best place to stand is on one of them. Would another eclipse from the viewpoint of one of the satellites?

Oliver, in the cases of the four large Galilean satellites of Jupiter, their shadows can indeed pass over each other. In fact, this year that occurs quite frequently since this spring their orbital planes intersected the Sun (and Earth). As viewed from each other, these solar “eclipses” would be either total or partial, never annular. During those eclipses, the occulting satellite would appear much larger than the Sun to an observer on the satellite in shadow.

The next such event will happen today (June 26) at 22:41:10 CDT (UT-5), when as viewed from Earth 41% of Ganymede will appear covered by the shadow of Io. An observer on Ganymede within that shadow would see the Sun totally occulted (eclipsed) by the larger appearing Io. There will be several other similar events over the next few days.

Re: Total eclipse.

Oliver Tunnah — Fri, 26 Jun 2009 10:47:17 GMT

I think the the satellites around the gas planets needs another look.
Obviously the best place to stand is on one of them. Would another eclipse from the viewpoint of one of the satellites?

Re: Total eclipse.

Centaur — Thu, 25 Jun 2009 21:09:55 GMT

zachsdad:

But, Shady, you might not be done yet. When Mercury and Venus cross the Sun's disc from our perspective we call it a transit. I wonder what happens when the Earth transits from Mars? Or Jupiter from Neptune? I wonder if any of those events come close to an eclipse?

Excellent question, zachsdad, but none of those would result in total eclipses. More properly, a total solar eclipse is an occultation, while an annular solar eclipse is a transit. So your examples of transits might also be referred to as annular eclipses. It's less confusing if the term eclipse is used exclusively for the cases of movement within a shadow, such as with lunar eclipses.

Re: Total eclipse.

zachsdad — Thu, 25 Jun 2009 20:57:06 GMT

But, Shady, you might not be done yet. When Mercury and Venus cross the Sun's disc from our perspective we call it a transit. I wonder what happens when the Earth transits from Mars? Or Jupiter from Neptune? I wonder if any of those events come close to an eclipse?

Re: Total eclipse.

Shady — Thu, 25 Jun 2009 20:47:51 GMT

Good evening Centaur.

Thanks for the mathematical formula for working out the answer to my question.Well I have to admit that I tried another formula....surfing the net.And here is the answer to my question:

Earth is the only planet in the solar system with spectacular solar eclipses. Thanks to an apparently improbable coincidence, the Sun and the Moon are almost exactly the same size as seen from Earth. The Sun is 400 times larger than the Moon, but it is also 400 times farther away. The table below, adapted from The Sun in Eclipse by Maunder and Moore, shows that there is no other planet where the angular diameter of a satellite is so close to that of the Sun. The most interesting case may be Jupiter's moon Amalthea, which subtends an angle of 7' 24" as seen from the cloud tops of Jupiter, and where the sun is nearly the same size at 6' 09".

I didn't include the aforementioned table because it contains a bit too much information for me and my question has been answered.

Re: Total eclipse.

chuck81 — Thu, 25 Jun 2009 18:41:27 GMT

Centaur:

I assume Chuck was referring to the four large Galilean satellites of Jupiter. In the cases of their shadow transits, they would appear to be total eclipses (more correctly, occultations) from the viewpoint of Jupiter. But the satellites would appear much larger than the Sun. The cases for Jupiter’s many other satellites would still have to be examined.

Yes and Yes.

Re: Total eclipse.

Centaur — Thu, 25 Jun 2009 17:38:49 GMT

Centaur:

Shady:

...fortunately Chuck has ruled out Jupiter with his reply...

He ruled out Jupiter’s four largest satellites, with no mention of its many others.

I tried to delete my previous post, but permission was denied.

I assume Chuck was referring to the four large Galilean satellites of Jupiter. In the cases of their shadow transits, they would appear to be total eclipses (more correctly, occultations) from the viewpoint of Jupiter. But the satellites would appear much larger than the Sun. The cases for Jupiter’s many other satellites would still have to be examined.

Re: Total eclipse.

Centaur — Thu, 25 Jun 2009 17:29:21 GMT

Shady:

...fortunately Chuck has ruled out Jupiter with his reply...

He ruled out Jupiter’s four largest satellites, with no mention of its many others.

Re: Total eclipse.

Shady — Thu, 25 Jun 2009 17:17:49 GMT

Good evening Centaur.

LOL! My intial thought to your reply was,thanks but no thanks.And there are quite a few moons out there............but,I'll give it a spin. It should be interesting as mathematics is not a strong subject for me & fortunately Chuck has ruled out Jupiter with his reply.Thanks Chuck.

Interesting though isn't it Centaur? So far no one knows the answer to this question.

And I kind of feel that I'm being led up the garden path but I will give it a spin.

Re: Total eclipse.

chuck81 — Thu, 25 Jun 2009 16:36:45 GMT

Shady:

Going back to eclipses,do other moons in our solar system appear to be the same size as the Sun on their host planet during an eclipse? For instance,does Titan appear the same size as the Sun from saturn?

Don't bother looking into Jupiters moons. I've watched the shadow transits. No total eclipses there.

Re: Total eclipse.

Centaur — Thu, 25 Jun 2009 16:19:06 GMT

Shady:

Going back to eclipses,do other moons in our solar system appear to be the same size as the Sun on their host planet during an eclipse? For instance,does Titan appear the same size as the Sun from saturn?

Apparently no one here immediately has a definite answer, Shady. Getting it will require a modest amount of effort. Since you are the one who dearly wants to know, I suggest you be the one to perform the work. It’s won’t be difficult, but a little tedious. Once you’ve obtained your results, then you can report back to the rest of us as payback for information others have already shared with you.

Go to http://en.wikipedia.org/wiki/Main_Page or another reference source. Learn the diameter of the Sun and call that A. Learn the distance to the Sun for a planet of interest and call that B. Learn the diameter of a planetary satellite of interest and call that C. Learn the distance between the planet and satellite and call that D. Make sure the units are all the same, e.g. kilometers. Here is the formula for determining the apparent angular size of the satellite relative to the Sun as viewed from the planet, which will be designated R.

R = (C x B) / (A x D)

If the result is close to 1, then from the planet the satellite would appear about the same size as the Sun. If greater than 1, then the satellite would appear larger than the Sun, and smaller if less than 1.

Re: Total eclipse.

Oliver Tunnah — Thu, 25 Jun 2009 16:11:55 GMT

Shady:
Going back to eclipses,do other moons in our solar system appear to be the same size as the Sun on their host planet during an eclipse? For instance,does Titan appear the same size as the Sun from saturn?

I thought I answered this one. As for Titan, It most likely doesn't. That's if you could stand on Saturn and look out.

Re: Total eclipse.

Shady — Thu, 25 Jun 2009 15:36:14 GMT

Good afternoon Centaur.

Yes,I forgot that the moon is moving away from us & that has led me onto another question which I'll put to you shortly.

Going back to eclipses,do other moons in our solar system appear to be the same size as the Sun on their host planet during an eclipse? For instance,does Titan appear the same size as the Sun from saturn?

And my other question about the moon is this.In it's present state how close has the moon ever been to Earth?