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Thread: Total Solar Eclipse - August 21, 2017

  1. #211
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    By leaving the solar filter on during totality I was able to magically produce a black hole through the Sun. *wink*

    Black hole eclipse.jpg
    We know time flies, we just can't see its wings.

  2. #212
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    Last edited by George; 2017-Aug-24 at 06:03 PM.
    We know time flies, we just can't see its wings.

  3. #213
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    Quote Originally Posted by Hornblower View Post
    Yes, but if the impactor was not exactly in the ecliptic plane it could strike well north or south on the proto-Earth, and all bets would be off on the trajectories of the ejecta that eventually reforms into the Moon.
    Fair enough.

  4. #214
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    Quote Originally Posted by Amber Robot View Post
    No doubt, but it is a crazy coincidence. Not just that the size to distance ratios are the same, but that the orbit of the Moon is near the ecliptic.
    That is likely because the Moon had spiraled out from near the Earth. When it originated, it was orbiting relatively close to the Earth, and if it was like other inner moons, its orbit plane would have been close to the Earth's equator. As it spiraled out, its orbit plane would have moved from close to the Earth's equator to close to the Earth's orbit plane.

    This is because both the Earth's equatorial bulge and the Sun perturb the Moon's motion, and their combined perturbations produce a sort of average reference plane, the Laplace plane. This plane goes from close to the equator to close to the ecliptic as one goes outward.

    For the Earth, a "surface satellite" in an equatorial orbit would have a nodal precession period of about 0.1 years. The Moon's nodal precession period is 18.63 years. The equatorial-bulge period varies as a7/2, and the Sun period varies as a-3/2. The two meet at about 210,000 km, a little more than half the Moon's present distance. Each period is about 110 years there, and they combine for a period of around 55 years. The average orbit plane is about halfway between the equator and the ecliptic.

    This can be understood with vectors. A precession rate w with a north-pole vector n combine to make W = w*n. Different sources of precession will then combine: W = W1 + W2.

  5. #215
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    Looking elsewhere in the Solar System, these moons' orbits are all close to their planets' equators:
    • Mars: both of them
    • Jupiter: out to Callisto
    • Saturn: out to Hyperion
    • Uranus: out to Oberon
    • Pluto: all of them


    Iapetus is an in-between moon of Saturn. Its orbit inclinations are currently
    17.28 (to the ecliptic)
    15.47 (to Saturn's equator)
    8.13 (to Laplace plane)

    Saturn's axial tilt is 26.73 (to orbit)

  6. #216
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    For all the astrophysics I studies, orbital mechanics wasn't a subject I really got into. I'm happy to learn more about it though.

  7. #217
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    Quote Originally Posted by Amber Robot View Post
    For all the astrophysics I studies, orbital mechanics wasn't a subject I really got into. I'm happy to learn more about it though.
    And I'm going to need your cell phone number.

  8. #218
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    Quote Originally Posted by Noisy Rhysling View Post
    And I'm going to need your cell phone number.
    I don't answer my cell phone.

  9. #219
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    Quote Originally Posted by Amber Robot View Post
    I don't answer my cell phone.
    And so it goes.

  10. #220
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    Quote Originally Posted by lpetrich View Post
    For the Earth, a "surface satellite" in an equatorial orbit would have a nodal precession period of about 0.1 years. The Moon's nodal precession period is 18.63 years. The equatorial-bulge period varies as a7/2, and the Sun period varies as a-3/2. The two meet at about 210,000 km, a little more than half the Moon's present distance. Each period is about 110 years there, and they combine for a period of around 55 years. The average orbit plane is about halfway between the equator and the ecliptic.
    Ouch! I recalculated that switchover point, and it's at
    62,000 km / 9.7 Earth radii / 1/6.2 present Moon distances

    The bulge and Sun precessions there are have periods of about 290 years, combining to make a period of a 145 years.

  11. #221
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    What I remember most came from watching the NASA TV feed on C-SPAN. There was one kid I overheard in the background talking about how this was the coolest thing he ever saw.
    The child was likely born after 9/11--and has probably seen lots of CGI summer blockbusters. And yet...this simple but profound moment awed him. Hope for the future.

  12. #222
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    Quote Originally Posted by publiusr View Post
    What I remember most came from watching the NASA TV feed on C-SPAN. There was one kid I overheard in the background talking about how this was the coolest thing he ever saw.
    The child was likely born after 9/11--and has probably seen lots of CGI summer blockbusters. And yet...this simple but profound moment awed him. Hope for the future.
    At the event we went to, one boy looking through a pinhole projector shouted "Oh my Jesus, that's amazing!" It was both heartwarming and funny.

  13. #223
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    I was at an Eclipse party a bit South of Rexburg ID. Here is a 4 minute video showing both the total eclipse, and the environment of the party.
    Forming opinions as we speak

  14. #224
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    I just learned that fog suddenly formed on Puget Sound just prior to the eclipse and immediately thereafter. The person I visited with said he stopped his car and watched fog form where he was on the Snohomish river but he could see the same thing happening on Puget Sound. About 10 minutes before maximum, he said it formed quickly, perhaps forming in less than one minute. Not long after maximum, it dissipated more slowly than it had formed.

    He said he talked to a friend in Oregon and she told him that the same thing happened to her at her lake.
    Last edited by George; 2017-Sep-11 at 06:08 PM.
    We know time flies, we just can't see its wings.

  15. #225
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    Quote Originally Posted by George View Post
    I just learned that fog suddenly formed on Puget Sound just prior to the eclipse and immediately thereafter. The person I visited with said he stopped his car and watched fog formed where he was on the Snohomish river but he could see the same thing happening on Puget Sound. About 10 minutes before maximum, he said it formed quickly, perhaps forming in less than one minute. Not long after maximum, it dissipated more slowly than it had formed.

    He said he talked to a friend in Oregon and she told him that the same thing happened to her at her lake.
    Seems believable. There's definitely a pronounced drop in temperature during the eclipse, so if the air is close to saturated with water, you'd expect mist to form.
    Conserve energy. Commute with the Hamiltonian.

  16. #226
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    Quote Originally Posted by Grey View Post
    Seems believable. There's definitely a pronounced drop in temperature during the eclipse, so if the air is close to saturated with water, you'd expect mist to form.
    Yes. It is logical, but I don't recall seeing this mentioned elsewhere and I looked at all of the first two pages from a Google search, as opposed to my normal first page only search.

    He noted that he watched the temperature reading in his car drop lower and lower, but gave me no specific values. I said I had known of about a 4 or 5 degree drop within totality from another person I know, but he was sure it was much more than that for him, though his area was in the lower 90% of totality range. I too noted significant cooling about 15 minutes before totality, but I had been in the Sun for quite a while taking pictures. [I was promised a pop-up canopy but the other 16 members of our group looked so comfortable under the set of three combined, I didn't want to raise unwanted darkness to the wanted darkness, so to speak.]

    As for the amount of temperature drop, I thought he may have just felt much cooler since cool moisture cools the body quicker, but that's when he commented on the temperature gage in his car. When I asked, he said that the temperature of the day was normal except what happened during the key part of the eclipse period.
    Last edited by George; 2017-Sep-11 at 06:09 PM.
    We know time flies, we just can't see its wings.

  17. #227
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    We saw a fog/cloud layer form on the north bank of the Columbia during the 1979 eclipse, only maybe 5 minutes before totality. Did I mention being on the southern bank?

  18. #228
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    Quote Originally Posted by ngc3314 View Post
    We saw a fog/cloud layer form on the north bank of the Columbia during the 1979 eclipse, only maybe 5 minutes before totality. Did I mention being on the southern bank?
    I will take a wild guess that the cooler waters flow into the Columbia from the north allowing the fortunate fog formation fancifying your ecliptic moment.

    Or, like me, do you favor southern banks more than some of those northern banks?
    We know time flies, we just can't see its wings.

  19. #229
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    It was just the opposite in western Kentucky, where puffy fair weather cumulus clouds dissipated as totality approached. My educated guess is that the decrease in solar heating of the ground reduced the amount of convection in the overlying air.

  20. #230
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    Quote Originally Posted by Hornblower View Post
    It was just the opposite in western Kentucky, where puffy fair weather cumulus clouds dissipated as totality approached. My educated guess is that the decrease in solar heating of the ground reduced the amount of convection in the overlying air.
    That's a helpful side effect! We experienced a similar reduction as well, now that you mention it. There were fewer clouds by the time C1 came, though there was one very small cloud that managed to block the first minute or so of C1. Thereafter we had no cloud problems.
    We know time flies, we just can't see its wings.

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