AstronomyEquinox shadows trace a straight line from west to...

Equinox shadows trace a straight line from west to east

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On the day of an equinox – and solely on the day of an equinox – the tip of a shadow from an upright stick or “gnomon” traces a straight west-to-east path alongside the bottom. Actually, you’ll be able to watch the equinox shadows your self. Illustration through John Jardine Goss/ EarthSky.

The equinox occurs at 21:24 UTC (4:24 p.m. CDT) on March 20, 2023. Read about this equinox.

Equinox shadows are distinctive

Do you get pleasure from sundials, and shadows? Do you know that – on the day of an equinox, and solely on the day of an equinox – the tip of an upright stick’s shadow follows a straight west-to-east path? In case you observe the shadow’s tip (aka its terminus) because it strikes throughout the bottom on the day of the equinox, you’ll see it tracing out that straight line, as proven – starting round 00:20 – within the video beneath:

Word that, on this video, what he labels as equinoctial line is the road from due east to due west.

Monitoring the equinox sun

By monitoring the sun’s shadow on this method, you’re making a easy form of sundial. Sundials are the earliest kind of timekeeping system.

This shadow truth – that’s, the tip of a shadow traces a straight west-to-east path solely on the equinoxes – applies in every single place worldwide, besides on the North and South Poles.

In 2023, the equinox comes on March 20. In order that’s a superb day to see the equinox shadow for your self.

Given a sunny day and an open sky, you’ll be able to see the road traced by the shifting shadow. Discover a stage spot, and pound your shadow stick (aka your gnomon) upright into the bottom.

Alternatively, you can use an present flagpole or utility pole as a makeshift shadow pole. Simply you should definitely have sufficient flat terrain to accommodate the shifting shadow.

Attempt utilizing spikes, cash or small rocks to file the shadow’s passing all through the day. Bear in mind, you’ll be recording the factors struck by the shadow’s tip, or terminus. On the day of an equinox, these factors will make a straight line, on or close to a line from due west to due east.

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Grassy area, one tall vertical stick with shadow touching first of east-west line of vertical sticks.
In case you observe the tip of an upright stick’s shadow on the day of the equinox, you’ll discover that it follows a straight line. On this picture, the bigger stick is serving because the gnomon, and the smaller sticks are recording the shadow’s passing. Picture through Woodland Ways. Used with permission.

About that phrase ‘gnomon’

A shadow pole or shadow stick, when used to trace the sun by its shadow, is named a gnomon. Apparently, “gnomon” is an historic Greek time period that means “to know”.

So the time period appears applicable as a result of – in spite of everything – the gnomon’s shadow is aware of! The gnomon of a sundial, for instance, is aware of each the hour of the day and the season of the 12 months.

Large, tall black obelisk with shadow cast on radial blue and yellow checkerboard.
Word that the form of your “gnomon” doesn’t matter. The tip of this enormous obelisk’s shadow on the Chinook Trails Elementary Faculty in Colorado Springs, Colorado, traces a straight line from west to east, all through the day of an equinox. Picture through Wikimedia/ John Carmichael.

Curved shadow paths at different occasions of 12 months

Then at different occasions of the 12 months, however most particularly on the solstices, the shadow paths comply with a curve, as proven on the graphic beneath. Technically talking, these nonlinear curves are known as hyperbolas.

3 suns projecting curved and straight lines through a hole on the end of a stick onto a surface.
Extra subtle than an upright stick, the washer-shaped eyelet creates a “pinhole” gnomon. On the equinoxes, the opening tasks the pinpoint of sunshine upon the straight equinox line. On the summer season/winter solstices, this level of sunshine travels alongside the respective curved solstice paths. Picture through François Blateyron. Used with permission.
Vertical plaque with elongated figure 8 on it and a stick at the top casting a shadow.
The Greenwich Royal Observatory presents a advantageous instance of a pinhole sundial. On this occasion, the vertical sundial faces south and information the meridian (noontime) passing of the sun in each sun time and clock time. Picture through Wikimedia (CC BY-SA 3.0).

Worldwide similarities of the paths from the equinox shadows

In each the Northern and Southern Hemispheres, the morning’s longest shadow occurs proper after dawn. Halfway between dawn and sundown, the shortest shadow of the day happens at solar midday. Then, after midday, the shadow begins to elongate once more, with the longest afternoon shadow occurring simply earlier than sundown.

As soon as once more, on the day of an equinox, the tip of an upright pole’s shadow travels in a straight line, from west to east.

Listed here are extra experiments you are able to do your self at house.

Northern and Southern Hemisphere variations

By the way in which, there are some main variations between the 2 hemispheres. On an equinox-day within the Northern Hemisphere, the straight shadow path passes to the north of the gnomon. Within the Southern Hemisphere, the straight shadow path passes to the south of the gnomon.

And, on the equator, the shadow path goes neither north nor south of the gnomon. That’s as a result of the noonday sun swings straight over the upright pole on the equinox, casting no noon shadow.

Backside line: On the day of an equinox, the tip of an upright stick’s shadow follows a straight west-to-east path all day lengthy.

Read more: March equinox 2023: All you need to know



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