Planet Four Talk

A Tent, the Moon and Mars

  • wassock by wassock moderator

    This won't be winning any Sky at Night Astronomy picture of the year prizes but I thought I'd put it up anyway, taken last night just after sunset.

    enter image description here

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  • Kitharode by Kitharode moderator

    It's better than some I've seen. 😃 Nice image and I'm glad you posted it. (Nice tent too)!

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  • wassock by wassock moderator

    Hand held as well which shows how bright Mars was

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  • angi60 by angi60 in response to wassock's comment.

    We'd give you a prize, Wassock 😉 Yes I went out and had a good gaze at them too. Absolutely beautiful. We've been lucky to have had such clear skies (for a change!!) 😃

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  • JellyMonster by JellyMonster

    I get a really great view of the moon through my velux window (while watching the New Twilight Zone), then just before retiring, it appears in the other one (while I'm at my computer).

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  • Kitharode by Kitharode moderator in response to JellyMonster's comment.

    So you don't get out much then, eh? 😉

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  • EA2014 by EA2014

    NICE photo, wassock! - there's nothing like camping out under clear skies. What a view the ancient Egytptains must have had, sleeping on their roofs in the summertime. Everynight I go out have a look at Mars - even with the late snows we've had ... pretty cool to see brilliant Mars and Jupiter both in the sky ... wonder what Jupiter looks like from Mars???.

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  • Kitharode by Kitharode moderator in response to EA2014's comment.

    Hi Elizabeth. 'Astronomy From Another Planet'; that's what I call it. Questions like your 'wonder what Jupiter looks like from Mars?' have always fascinated me. Not just because they're interesting, but also because they're often workout-able and a great deal of fun. Most times I also ending up learning something.

    Watching an eclipse one year I started thinking about shadows in the solar system because all planets and moons cast shadows when in sunlight. Bigger the object, bigger the shadow. More distant from the Sun, longer the shadow. So how many moons can cast a shadow on their parent planet? Can Jupiter cast a shadow on Saturn? If the Moon is gradually moving away from the Earth (which it is) how long will it be before the Moon's shadow no longer touches the Earth's surface and we see an end to the spectacle of a total solar eclipse? Hours of fun, with some rather interesting answers. 😉

    With your 'Jupiter from Mars' question, rather than spend a bit of time working out some sort of answer to post here, I thought it might be more fun to give you my initial 'hunch' and see if you wanted to dig into it and get us to find a better answer. Up to you of course, but here's my first offering.

    A view of Jupiter from Mars would be different from a view of Jupiter form Earth - But not much!! 😃

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  • EA2014 by EA2014 in response to Kitharode's comment.

    "Astronomy From Another Planet" - I like it! Your Shadow Puzzle is intriguing too (after all this time, I still have to remind myself that the light from the planets and moons is 'just' reflected light). .... I have a hunch about Jupiter's newest moon: Peggy & if it casts a shadow on Jupiter ( but my cat could probably get that one right 😃 )

    Okay, about the Jupiter-Mars question ... now I HAVE to find out 😃 ... even tho I think you're absolutely correct. I'll be getting back to you, Kith!

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  • EA2014 by EA2014 in response to EA2014's comment.

    CORRECTION.... after writing my previous post I signed out & went to bed (it's 2:30 am). My eyes were closed all of 20 seconds when I realized what I wrote: "that Peggy was a moon of Jupiter - ahem - I meant it's a 'moon' of Saturn"! ............. sheeze .....

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  • wassock by wassock moderator

    Surely the inner planets, given appropriate alignment, will cast shadows on the outer ones in the same way that Venus casts a , very small, shadow on the Earth when it transits the sun

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  • mschwamb by mschwamb scientist, translator in response to wassock's comment.

    I thought you all would find this interesting given the discussion. Simulations of Dec 2012 Transit of Venus from Saturn though they tried to observe it, but I'm not sure if they actually caught it with Cassini's instruments.

    Cheers,

    ~Meg

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  • p.titchin by p.titchin in response to Kitharode's comment.

    I guess that when we see a transit, there is an 'earth light 'shadow' , but further guess it's very faint. ~Pete.

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  • Kitharode by Kitharode moderator in response to wassock's comment.

    The distances between the planets are much greater than the lengths of their shadows so, far as I know, none of the planets can cast a shadow on any other planet. Of course, I'm referring to the 'umbral' part of the shadow (the cone of darkness extending from the planet to a point in space) rather than the 'penumbral' part of the shadow.

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  • wassock by wassock moderator

    So if you're stood on Saturn which has the larger apparent size, the sun (larger but further away) or the closer but smaller Jupiter?

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  • wassock by wassock moderator

    If you'd like to try a really tricky sum try this one. Fossil coral from the Devonian (400 Ma) shows 400 days to a year, which, apparently, gives us the moon half the distance from earth than now. This being so the apparent size of the moon would be bigger and thus total eclipses would be more total and last longer. So how long would an eclips last with the moon half as far away? (and would it last long enough to show up in tge corals record?)

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  • Kitharode by Kitharode moderator in response to wassock's comment.

    Two excellent questions, neither of which I can answer right now. The first one about apparent size shouldn't be too difficult to answer, but the 'closer moon' question will take a bit more working out.

    I'll get back to you - Unless you come up with the answers first 😃

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  • Kitharode by Kitharode moderator

    Angular diameter of Jupiter as seen from Saturn = 0.378 degrees.

    Angular diameter of the Sun as seen from Saturn = 0.057 degrees.

    Still working on the 'half-way' moon problem. 😃

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  • Kitharode by Kitharode moderator

    The Suns angular diameter from Earth = 0.518 degrees (then as now).

    When the Moon was half its present distance from Earth, its angular diameter = 1.036 degrees (currently ~0.520 deg).

    At the closer distance the orbital period of the Moon = 244.10 hours. Therefore Moon travels 1 degree in 0.678 hours. From first contact (leading edge of Moon touches edge of Sun) to last contact (trailing edge of Moon leaves the Sun) would be 1.554 degrees. Therefore the time for the whole eclipse episode would be 1.053 hours (currently a liitle less than 2 hrs).

    The amount of time that the Sun would be totally eclipsed would be approximately 21 minutes.

    We (my astrofriends and I) feel fairly confident that we've got it right. However, we always welcome second opinions 😃

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  • wassock by wassock moderator

    Top stuff Kith, one anomoly though, the time for the total duration has halved but the period of total eclipse has increased? Currently around 7 minutes?

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  • Kitharode by Kitharode moderator

    Yes. There does seem to be a contradiction in there and we are a little uncomfortable about that. Hence the call for second opinions. We're gonna run through it again next week and try to build a clearer picture of what's going on.

    It strikes me that we weren't clear enough amongst ourselves whether we were calculating the eclipse from a single point on Earth (the shadow passes over us) or whether we have calculated the eclipse path (the total time of the shadow across the Earth).

    An interesting problem.

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  • wassock by wassock moderator

    presumably it's going faster closer in as well?

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  • Kitharode by Kitharode moderator

    Yes indeed. The Moon would travel faster in its orbit if it was closer to Earth.

    Finding this new velocity is the key to the problem, but doing that wasn't quite as straightforward as we first thought. There's a possibility that we've gone wrong / misplaced a decimal point / mixed our units here, because we got some weird answers with our first attempts.

    It's a lot of fun though and we'll crack it eventually. We know what we need to do, just need to do it right. 😃

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  • wassock by wassock moderator

    Surely all you need is a correctly calibrated dumb bell, Christopher Dean and a taco' strobe 😃

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  • Kitharode by Kitharode moderator

    Have to take your word for that. Don't know what a taco' strobe is and I don't know Mr Dean. 😦

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  • wassock by wassock moderator

    Tachometer strobe, Torvil and DEAN, get him to spin with the dumbell at forearm length, measure thd rotationalspeed with the tacho the get him to put it out to arms length znd measure again, simples

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  • Kitharode by Kitharode moderator

    I'm pleased to say that we got it right first time round. We recalculated using two different approaches and our answers come out the same. Although the situation feels contradictory we find that a total eclipse using a 'halfway' moon is just about 21 minutes, which is about three times longer than an eclipse today.

    So there you have it ... 😉

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  • wassock by wassock moderator

    Tah-dah, well done y'all

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