Midwestern Wanderlog

2016-03-20 – Equal night dawns

_MG_7663Ah, the day of equal night!  That’s what ‘equinox’ means, you know.  But do you know how it’s calculated?

Sunrise Stats!
First light: 6:44 AM
Sunrise: 7:13 AM
Daylight: 13 hrs, 8 min

Ne, neither.  Not until the past couple of days, anyway.  So it turns out that there’s an imaginary line called a celestial equator in the sky directly above the planetary equator.  Because of the twenty-three-and-a-half degrees of axial tilt, as the earth orbits, the sun’s light hits at a slightly different angle each day.  The vernal (spring) equinox is the day, right down to the minute, when the light crosses that imaginary line and enters the northern hemisphere.  In that moment of balance, the Earth’s terminator – the line, seen from space, where day and night meet – is perfectly vertical.

Which explains why it was that my sunrise app was calling the 19th, yesterday, the equinox, when the calendar had it labeled as the 20th; the moment of crossover arrived at 4:30 AM UTC (Coordinated Universal Time) March the 20th, which corresponds to 11:30 PM Central Daylight Time on March the 19th – a fine detail that the app I use is clever enough to recognize.  And I’m sure that printers use UTC to label the celestial events in their calendars, like full and new moon, equinoxes and solstices.

Cool, huh?

_MG_7698Although, ‘equal night’ is actually almost an urban legend; there’s no place on earth that I’ve found where day and night are exactly twelve hours each.  Even in cities on the equator, it’s not quite equal at any time of year – at least not according to my sunrise and astrolabe apps.  There’s about a four- to seven-minute difference in the amount of daylight on any given day, even on the equinox.

It is the day when the sun rises as due east as he’s going to get, though!  Happily, today there was just enough of a gap between clouds and horizon to see exactly where east is from the Bluff.  And it means that winter is finally loosening its grip on the world; soon there will be green grass and flowers and gardening and li’l baby birdies!

It’s going to be such a wonderful spring!

Panoramic 080

Oh – and if you’d like to read a bit more about the equinox, check out this page on earth sky.org!

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