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Day length

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  • 28-12-2016 9:19am
    #1
    [Deleted User]
    Posts: 0 ✭


    I was looking at the day length times now that we have passed the equinox.

    http://www.sunrise-and-sunset.com/en/sun/ireland/dublin

    I noticed that while the sunset time were already getting later by approx 1 minute a day, there was no change in the sunrise times. Indeed they seemed to be getting later until the end of the December.

    I would have thought that changes in both sunrise and sunset times would have been symmetrical but seems not. Why is this?


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    #2
    mickmackey1
    Banned (with Prison Access) Posts: 3,288 ✭✭✭mickmackey1


    There's a good explanation in the second part of this article -

    http://www.skyandtelescope.com/observing/sun-swings-low-at-solstice-time/


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    #3
    ps200306
    Registered Users Posts: 1,601 ✭✭✭ps200306


    I've never been much of a morning person, so don't give a rashers about when the sun rises. But I always mark the time around the 12th of December (more than a week before the solstice) when the sunset starts to get later again. I reckon there should be a name for when the "sunset stands still" ... my Latin isn't great but it would be something like the occasustice. :pac:


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    #4
    Jan_de_Bakker
    Closed Accounts Posts: 1,403 ✭✭✭Jan_de_Bakker


    This is something I discovered only this year too ... never realised that the earliest sunset is around December 12 ... interesting .


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    #5
    Capt'n Midnight
    Moderators, Recreation & Hobbies Moderators, Science, Health & Environment Moderators, Technology & Internet Moderators Posts: 91,400 Mod ✭✭✭✭Capt'n Midnight


    https://en.wikipedia.org/wiki/Equation_of_time
    The equation of time describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of reconcile a difference. The two times that differ are the apparent solar time, which directly tracks the motion of the sun, and mean solar time, which tracks a theoretical "mean" sun with noons 24 hours apart.

    465px-Equation_of_time.svg.png


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    #6
    ps200306
    Registered Users Posts: 1,601 ✭✭✭ps200306


    That steep slope of the equation of time in December and January is really the answer to the OP's question. The Earth is approaching perihelion in January, so according to Kepler's Laws the Earth is sweeping out the largest angle per day in its solar orbit. This means it takes more than 24 hours (the length of the mean solar day) to point back at the Sun, and as a result the Sun slips eastward compared to clock time. This tends to make both sunrise and sunset later. But the day is also shortening at both ends as the solstice approaches, and lengthening afterward. So the combination of effects shifts things such that the earliest sunset occurs more than a week before the solstice and the latest sunrise more than a week after.


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