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Vernal Equinox March 20, 2017, 3:29 AM, PDT — Part I

March 19, 2017

This year we report on that other large object in the sky, known as the sun.

Our Sun (Alan Friedman ~ 4/22/14, on NASA site)

Not a bad peach, but our Sun – 860,000 miles in diameter, 8 light-minutes away (Alan Friedman ~ 4/22/14, on NASA site)

The first event is the Vernal Equinox, scheduled in Los Angeles for March 20, 2017 at 3:29 AM PDT.  Sunrise: 6:56 am at 89° East, 1 degree north of due east.
Sunset: 7:04 pm at 271 ° West, 1° north of due west
The sum will pass the meridian (north-south line) at 1 pm, at an angle of 56° above due south. Daylight will last 12 hours, 8 minutes and 43 seconds (12:08:43); nighttime is 11:51:17 long.  You will note that these periods of day and night are not equal. Day and night were nearly equal on March 16, with 12:00:11 of daylight.

Definition of the term
Vernal: Of or pertaining to Spring [Latin vernal(is)]
Equinox: When the sun crosses the plane of the earth’s equator [from Latin aequinoctium, the time of equal days and nights].

Equinoctial daytime exceeds nighttime for two reasons
First: Sunrise occurs when the leading (upper) edge of the rising sun first becomes visible above the horizon.  Sunset is when the trailing (not the lower) edge drops below the horizon.  The width of the sun adds about six minutes of daylight.
Second: Refraction of the sun’s rays by the earth’s atmosphere permits us to see the sun both before it has actually risen and after it has actually set, adding several minutes each to sunrise and sunset.  In total, day exceeds night on March 20, 2017 by 17 minutes , 26 seconds.

Spring is Arriving Earlier and Earlier
This is not due to climate change, but to fluctuations in earth’s elliptical orbit, the gravitational pull of the other planets, and the precession of the equinoxes (google that). Although we traditionally expect spring to start on March 21, the last time that happened in the entire United States was in 1980. From 1981 to 2102, the vernal equinox will occur no later than March 20. In 2020, it will start on March 19 for the entire United States. The length of the seasons are changing as well. Spring is currently losing one minute per year to Summer, and Winter is losing 1/2 minute to Autumn. Winter is currently the shortest season at 88.99 days, and is expected to reach its minimum length of 88.71 days around the year 3500. (From Joe Rao)

Seasonal Fluctuation
Because the two equinoxes (vernal and autumnal) mark when the sun crosses the plane of the earth’s equator, these are also the only days of the year when the sun rises exactly in the east and sets exactly in the west.  The earth’s axis (and equatorial plane) is tilted 23.4° with respect to the plane of the earth’s orbit around

Northern Summer (famous artist - name withheld by request)

Northern Summer (famous artist – name withheld by request)

the sun. In the northern summer the earth’s north axial pole tilts towards the sun, the sun’s rays have less insulating atmosphere to filter them, and the northern hemisphere warms up. In the northern winter, the north pole tilts away from

Northern Winter (same famous artist)

Northern Winter (same famous artist)

the sun whose warming rays now must penetrate more atmosphere, and the northern hemisphere cools down.  Seasons are opposite south of the equator.  The closer you are to the equator, the more equal are day and night, summer and winter, warmth and cold.  The temperature extremes of winter and summer are replaced by rainy and dry seasons.

At equinox: right diagram shows earth in distance over top of sun

At equinox: right diagram shows view past top of sun towards earth.

Eastern Sunrise, Western Sunset
Throughout the northern winter and spring, the points of sunrise and sunset move farther and farther north.  The extremes are the Winter Solstice (around December 21), when the sun rises and sets farthest to the south, and the Summer Solstice (around June 21) when they are farthest to the north.  The equinoxes mark the halfway point, when sunrise and sunset are exactly east and west.  Well, not exactly.  On March 18, 2017, the sun rises at 90° – exactly east – at 6:58 AM, and  sets at 270° – exactly west – at 7:03 PM. It also rises and sets exactly east and west on March 19. Well…not exactly, perhaps on either day but a fraction of a degree off from exactly 90° and 270°. But still closer to exact than on March 20.

So make sure you run outside at 3:29 AM on March 20 to witness the vernal equinox, despite the fact that you won’t be able to see anything. Why? Because the sun will be currently eclipsed (by the earth). By the way – the sun doesn’t rise and set. The earth revolves on its axis. But you knew that. [Chuck Almdale]

Part II to follow: Vernal Equinox Festivals, Goddesses,  Sunspot Cycles and an Eclipse

Interesting Links
Space Weather Radio – Meteor echoes & other live sounds from space
TimeandDate.com – March Equinox
TimeandDate.com – Los Angeles sunrise, sunset & day length for March 2017
InfoPlease – A Tale of Two Easters
TimeandDate.com – Current Day and Night map
Heliophysics – A Universal Science
Los Angeles Equinoxes and solstices for 2010–2020

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2 Comments leave one →
  1. E. Koers permalink
    March 24, 2017 9:15 am

    My shrubs need pruning, but I do not want to disturb birds. When should I not prune? I have mostly humming birds here. I live in Santa Monica about 1/2 mile from the beach. Any advice?

    Like

    • March 24, 2017 10:47 am

      Liz – for advice on pruning in general try this: https://www.arborday.org/trees/tips/when-to-prune.cfm
      As for when to prune to protect birds, now is, generally speaking, a bad time, because nesting is going on. However, in my personal opinion and not the considered opinion of the chapter, if you have small bushes and inspect them carefully for nests (and finding hummingbird nests requires very close inspection) then light pruning might be OK. You need to watch the bushes you want to prune closely to see if any female hummingbirds are around. Watch them carefully to see if they are building nests and/or laying eggs. If they are, you and any gardeners should keep away for the month or so it takes to raise the chicks.

      Like

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