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Sunrise and Sunset

  • 09-12-2013 1:51pm
    #1
    josip
    Registered Users Posts: 13,624 ✭✭✭✭


    Looking at the sunrise and sunset times for the week ahead, I can see accoriding to this website that the sunset time will change very little while most of the shortening happens in the morning.

    Date - Sunrise - Sunset
    9th - 08:28 - 16:07
    10th - 08:29 - 16:06
    11th - 08:30 - 16:06
    12th - 08:31 - 16:06
    13th - 08:32 - 16:06
    14th - 08:33 - 16:06
    15th - 08:34 - 16:06

    Assuming that's correct, what is the reason for the unbalanced shortening?
    Tagged:


Comments

  • #2
    animaal
    Closed Accounts Posts: 815 ✭✭✭animaal


    Remember the new levy on daylight that the government brought in

    Seriously, I wondered the same, until I came across this:
    http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980116c.html


  • #3
    Zubeneschamali
    Registered Users Posts: 16,686 ✭✭✭✭Zubeneschamali


    More explanation here:

    http://h2g2.com/approved_entry/A3239796


  • #4
    Popoutman
    Registered Users Posts: 1,363 ✭✭✭Popoutman


    Another general factor in odd sunrise and sunset times is how much farther west you are from 0 degrees longitude. Each fifteen degrees difference is an hour's difference in time. 7.5 degrees west is a half hour later for most of us. That would mean that if the sunrise and sunset times were at exactly 06.00 and 18.00 in London, they'd be 06.40 and 18.40 in Kerry.

    <rant> As a matter of fact a good portion of Ireland should be in the GMT-1 timezone because when farther west than about Tullamore or Clonmel, you are closer to the centre of the GMT-1 zone than the centre of the GMT zone. One more reason why it's really dumb to do Summer Time in this country. If the kids need more light for school, change the school opening hours.. </rant>


  • #5
    ps200306
    Registered Users Posts: 1,589 ✭✭✭ps200306


    josip wrote: »
    Looking at the sunrise and sunset times for the week ahead, I can see accoriding to this website that the sunset time will change very little while most of the shortening happens in the morning.

    Date - Sunrise - Sunset
    9th - 08:28 - 16:07
    10th - 08:29 - 16:06
    11th - 08:30 - 16:06
    12th - 08:31 - 16:06
    13th - 08:32 - 16:06
    14th - 08:33 - 16:06
    15th - 08:34 - 16:06

    Assuming that's correct, what is the reason for the unbalanced shortening?
    I've been celebrating the 10th as my personal winter solstice for some years. I don't get up early enough to care about the time of sunrise, so the 10th is when my days start getting longer as far as I'm concerned. :p

    But, as with your table above, I only know the time of sunset to a resolution of one minute. Does anyone have a link to a table with second resolution, so we can figure out which day is really the earliest sunset?

    I also have a question about the solstice sunrise, even though I'll never be up to see it. The entrance passage at Newgrange is supposed to let in the light of sunrise on the solstice. That means the roof box is oriented towards the most southerly azimuth of sunrise -- the point on the horizon at which "the sun stands still" (from which we get the word solstice). After the solstice the azimuth of sunrise starts travelling north again.

    Now, if the date of latest sunrise is not actually the conventional solstice, does that mean the sun keeps travelling south so that it is spotted from Newgrange and then once again a few days later on its way back? I'm pretty sure it doesn't mean that, and the solstice is the most southerly azimuth of sunrise regardless of the equation of time, but can anyone explain why?


  • #6
    Zubeneschamali
    Registered Users Posts: 16,686 ✭✭✭✭Zubeneschamali


    ps200306 wrote: »
    Now, if the date of latest sunrise is not actually the conventional solstice

    Solstice isn't the date of latest sunrise, it's the date the sunrise is at it's extreme Northerly or Southerly point. This movement is caused by the tilt of the Earths axis.

    There is another effect which changes sunrise and sunset: the elliptical shape of the Earth's orbit. The Earth continues to get closer to the Sun until January 3rd or so, so even as the Sun starts to move North at sunrise, it doesn't really start to rise earlier until we pass aphelion.


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  • #7
    ps200306
    Registered Users Posts: 1,589 ✭✭✭ps200306


    Zubeneschamali wrote: »
    Solstice isn't the date of latest sunrise, it's the date the sunrise is at it's extreme Northerly or Southerly point. This movement is caused by the tilt of the Earths axis.

    There is another effect which changes sunrise and sunset: the elliptical shape of the Earth's orbit. The Earth continues to get closer to the Sun until January 3rd or so, so even as the Sun starts to move North at sunrise, it doesn't really start to rise earlier until we pass aphelion.

    If I was to be picky I would say it's not the elliptical shape of the earth's orbit per se, but the fact that the inverse square law of gravity moves us along that ellipse with a particular acceleration. If Kepler's second law said that the radii to the planets sweep out equal angles in equal time intervals, instead of equal areas, then solar time would be the same as mean time.


  • #8
    Barry Garlow 55
    Site Banned Posts: 7 ✭Barry Garlow 55


    ps200306 wrote: »
    I've been celebrating the 10th as my personal winter solstice for some years. I don't get up early enough to care about the time of sunrise, so the 10th is when my days start getting longer as far as I'm concerned. :p

    But, as with your table above, I only know the time of sunset to a resolution of one minute. Does anyone have a link to a table with second resolution, so we can figure out which day is really the earliest sunset?

    I'd like to know this also.
    But from the website linked, we should really start to notice a stretch in the evenings from tomorrow...


  • #9
    ps200306
    Registered Users Posts: 1,589 ✭✭✭ps200306


    Here's an article on why getting sunrise/sunset down to second resolution may not be practical:

    http://mintaka.sdsu.edu/GF/explain/sunset_time.html


  • #10
    coylemj
    Registered Users Posts: 25,313 ✭✭✭✭coylemj


    ps200306 wrote: »
    If I was to be picky I would say it's not the elliptical shape of the earth's orbit per se, but the fact that the inverse square law of gravity moves us along that ellipse with a particular acceleration.

    You are being picky, Kepler's Law applies whether the orbit is circular or elliptical.
    ps200306 wrote: »
    If Kepler's second law said that the radii to the planets sweep out equal angles in equal time intervals, instead of equal areas, then solar time would be the same as mean time.

    With or without Kepler, equal angles swept out in equal time can only happen with a perfectly circular orbit in which case as you say solar time and mean time would be the same all year round.


  • #11
    ps200306
    Registered Users Posts: 1,589 ✭✭✭ps200306


    coylemj wrote: »
    You are being picky, Kepler's Law applies whether the orbit is circular or elliptical.

    With or without Kepler, equal angles swept out in equal time can only happen with a perfectly circular orbit in which case as you say solar time and mean time would be the same all year round.
    Totally not true! ;)
    I can easily make an elliptical orbit that sweeps out equal angles in equal time intervals, and obeys none of Kepler's laws. Here's one I made earlier:
    KAoDrHC.gif
    Of course, such an orbit is strictly impossible under an inverse square law of gravitation. As I said, it's the inverse square law, not the ellipticity of the orbit per se, that causes the deviation of solar time from mean time. In my elliptical orbit above there would be no deviation. Kepler's laws are just a more complicated restatement of the inverse square law.


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  • #12
    Zubeneschamali
    Registered Users Posts: 16,686 ✭✭✭✭Zubeneschamali


    ps200306 wrote: »
    Of course, such an orbit is strictly impossible under an inverse square law of gravitation. As I said, it's the inverse square law, not the ellipticity of the orbit per se, that causes the deviation of solar time from mean time.

    In our Universe, with our laws of physics, it is the elliptical shape of our orbit that causes the effect.

    If our orbit was perfectly circular, there would be no such effect. If our orbit was much more eccentric, this effect would be much greater than it is.

    Meanwhile, in your imaginary Universe with different laws of physics, why is the orbit elliptical at all? In ours, the elliptical shape is a consequence of... the inverse square law.


  • #13
    ps200306
    Registered Users Posts: 1,589 ✭✭✭ps200306


    Zubeneschamali wrote: »
    Meanwhile, in your imaginary Universe with different laws of physics, why is the orbit elliptical at all?
    Same laws of physics, but the planet's not in freefall ... there's a big rocket engine strapped onto it that maintains the equal-angles-in-equal-times orbit. (Could also be done by angels beating their wings -- doesn't really matter where the force comes from).
    Zubeneschamali wrote: »
    In our Universe, with our laws of physics, it is the elliptical shape of our orbit that causes the effect...
    Nope. We could have an elliptical orbit without the effect if forces other than gravity operated.
    Zubeneschamali wrote: »
    In ours, the elliptical shape is a consequence of... the inverse square law.
    Only under the assumption of freefall. I did say I was being picky. ;)


  • #14
    Zubeneschamali
    Registered Users Posts: 16,686 ✭✭✭✭Zubeneschamali


    ps200306 wrote: »
    Only under the assumption of freefall. I did say I was being picky. ;)

    If it's not in free fall, it's not in orbit.

    You're not being picky, you're being very, very silly :P


  • #15
    ps200306
    Registered Users Posts: 1,589 ✭✭✭ps200306


    Zubeneschamali wrote: »
    If it's not in free fall, it's not in orbit.
    I had a nasty feeling you were going to say that. :D
    But what you're referring to is a free orbit or Keplerian orbit. You'll have to take it up with NASA who use powered transfer orbits all the time (at least every other month, in fact, to keep the space station aloft). And with Wikipedia which says: "Kepler's laws of planetary motion may be derived from Newton's laws, when it is assumed that the orbiting body is subject only to the gravitational force of the central attractor. When an engine thrust or propulsive force is present, Newton's laws still apply, but Kepler's laws are invalidated."


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