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Vernal Equinox March 20, 2015, 3:45 PM, PDT — Part I

March 17, 2015

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)

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, 2015 at 3:45 PM PDT.  On that day, daylight will last 12 hours, 7 minutes and 37 seconds (12:07:37); nighttime is 11:52:13 long.  You will note that these periods of day and night are not equal. Day and night were nearly equal on March 16, with 11:59:05 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, 2015 by 15 minutes , 14 seconds.

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 the sun. In the northern summer the earth’s

Northern Summer (famous artist - name withheld by request)

Northern Summer (famous artist – name withheld by request)

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

Northern Winter (same famous artist)

Northern Winter (same famous artist)

away from 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.  The sun sets at 270° – exactly west – on March 19 at 7:04 PM PDT, rises at 90° – exactly east – on March 20 at 6:27 AM, and sets March 20, 7:05 PM at 271°, slightly north of exactly west.

So make sure you run outside at 3:45 PM on March 20 to witness the vernal equinox, despite the fact that, unlike a full moon, there really isn’t much to look at. 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, Sunspot Cycles and an Eclipse

Interesting Links
Space Weather Radio – Meteor echoes & other live sounds from space – March Equinox – Los Angeles sunrise, sunset & day length for March 2015
InfoPlease – A Tale of Two Easters – Day and Night map for March Equinox 2015
Heliophysics – A Universal Science
Los Angeles Equinoxes and solstices from 2010–2020

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