Happy Equinox

Discussion in 'Horological Misc' started by Tom McIntyre, Sep 25, 2019.

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  1. Tom McIntyre

    Tom McIntyre Technical Admin
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    I installed solar panels on my roof a few years ago and since then I look at the weather report and the time of sunrise and sunset pretty much every morning.

    I learned long a go in school that the Vernal and Autumnal equinoxes come around the 21st of March and September. I also thought that I learned that the day and night were the same length on those days.

    This year when the Autumnal Equinox arrived, I looked at the weather report and noticed that the day was several minutes longer than the night. The two have been approaching each other by about 5 minutes a day since then. It is now September 25th and the difference is still 2 minutes longer for daylight. My expectation is that tomorrow the night will be longer than the day.

    I understand that this phenomenon is related to the latitude and the Tropic of Cancer and Tropic of Capricorn delineate the furthest northward and southward excursion of the sun in the sky.

    In the southern hemisphere, the day has been shorter than the night but the difference has been declining since the "Summer" solstice last June when their day was its shortest.

    On the equinoxes, the day and night should be 12 hours everywhere. So why did that not happen on the 22nd of September and is still on the way on the 25th?

    I decided to use my weather app on my phone to look at other locations. I expected the phenomenon to be related to latitude but that does not appear to be the case. I am wondering if the problem may actually be bad sunrise and sunset data from the weather channel that drives my phone app.

    Do we have an astronomer who could explain all of this?
    sunrise.png
     
  2. MartinM

    MartinM Registered User

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    #2 MartinM, Sep 25, 2019
    Last edited: Sep 25, 2019
    I learned in school that the equinox simply means the equatorial plane is directly in line with the center of the sun. Equal day and night would only be possible along the equator on that one day. The closer you get to a pole, the more offset there is. At the June solstice, the North pole is in daylight, all day while the south pole is in darkness all day. In the December solstice, it is just the opposite. Every location on earth will have 2 days a year where the day and night are of equal length, but the latitude dictates what days those will be. Of course, I was also taught that the crescent moon was due to the shadow of the earth on the moon, so I could be way out there.

    EDIT:
    Yeah. I had it wrong. On the equinoxes, latitude is irrelevant to a 12 hour day.
    Equinox - Wikipedia
     
  3. Tom McIntyre

    Tom McIntyre Technical Admin
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    The other interesting thing about the Equinox is that if you look at the time of sunrise and sunset for locations in the same timezone, you can see that the longitude affects that time. You could compute the longitude by adding the time zone offset to the sunrise or sunset times, presuming you have those times for Greenwich. I think the sunrise at Greenwich was 8:52 as was sunset. :)

    Harvard MA is pretty close to the east edge of the Eastern Time Zone, but Dearborn Michigan is also in the Eastern Time Zone and is almost an hour later than here. Most people from New England are surprised at how far west Atlanta and Daytona Beach are from Boston.
     
  4. Jerry Treiman

    Jerry Treiman Registered User
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  5. zedric

    zedric Registered User

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    Tom - you can also see the effects of latitude through the equinox. When I grew up my teachers would tell me that the vernal equinox is in March and the Autumnal one in September. Nowadays, they have swapped, and the spring equinox is in September and the Autumn one in March. Something to do with moving house (and moving continents), but it still confuses me even after 25+ years, when someone talks of March I think of Spring, not Autumn.
     
  6. Tom McIntyre

    Tom McIntyre Technical Admin
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    When you go down under all the rules change. I am surprised that they switch the terms but I suppose the names are seasonal descriptions. Your days have been getting longer and will now continue to produce more daylight while ours in the northern hemisphere are now in the decline and will get shorter until the solstice. I presume you celebrate the Summer solstice and dance around a Maypole in December. :)
     
  7. zedric

    zedric Registered User

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    Closest I get to a maypole here is when my three dogs head in different directions and wrap their leads around me! We are more likely to celebrate December with an icy pole.. {{metaController.metaData.title}}
     

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