Tall black obelisk with shadow cast on radial blue and yellow checkerboard with times on it.
Note that the shape of your gnomon or stick doesn’t matter. The tip of this huge obelisk’s shadow at the Chinook Trails Elementary School in Colorado Springs, Colorado, traces a straight line from west to east, throughout the day of an equinox. Read below why equinox shadows follow a straight path from west to east. Image via John Carmichael/ Wikimedia (public domain).
The March equinox will fall at 9:01 UTC (4:01 a.m. CDT) on March 20, 2025. Read about the March equinox.
The 2025 EarthSky lunar calendar makes a great gift. Get yours today!
Equinox shadows are unique
Do you enjoy sundials and shadows? Did you know that on the day of an equinox – and only on the day of an equinox – the tip of an upright stick’s shadow follows a straight west-to-east path? If you track the shadow’s tip (aka its terminus) as it moves across the ground on the day of the equinox, you’ll see it tracing out that straight line, as shown – beginning around 00:20 – in the video below:
Grassy area, one tall vertical stick with shadow touching first of east-west line of vertical sticks.
If you track the tip of an upright stick’s shadow on the day of the equinox, you’ll find that it follows a straight line. In this image, the larger stick is serving as the gnomon, and the smaller sticks are recording the shadow’s passing. Image via Woodland Ways. Used with permission.
Tracking the equinox sun
By tracking the sun’s shadow in this way, you’re making a simple kind of sundial. Sundials are the earliest type of timekeeping device.
This shadow fact – that is, the tip of a shadow traces a straight west-to-east path only at the equinoxes – applies everywhere worldwide, except at the North and South Poles.
In 2025, the equinox comes on March 20. So that’s a good day to see the equinox shadow for yourself.
Given a sunny day and an open sky, you can see the line traced by the moving shadow. Find a level spot, and pound your shadow stick (aka your gnomon) upright into the ground.
Alternatively, you could use an existing flagpole or utility pole as a makeshift shadow pole. Just be sure to have enough flat terrain to accommodate the moving shadow.
Try using spikes, coins or small rocks to record the shadow’s passing throughout the day. Remember, you’ll be recording the points struck by the shadow’s tip, or terminus. On the day of an equinox, these points will make a straight line, on or near a line from due west to due east.
About that word ‘gnomon’
A shadow pole or shadow stick, when used to track the sun by its shadow, is called a gnomon. Apparently, “gnomon” is from an ancient Greek root meaning “to know.”
So the term seems appropriate because – after all – the gnomon’s shadow knows! The gnomon of a sundial, for example, knows both the hour of the day and the season of the year.
Diagram: 3 suns projecting curved and straight lines through a hole on the end of a stick onto a surface.
More sophisticated than an upright stick, the washer-shaped eyelet creates a “pinhole” gnomon. On the equinoxes, the opening projects the pinpoint of light upon the straight equinox line. On the summer/winter solstices, this point of light travels along the respective curved solstice paths. Image via François Blateyron. Used with permission.
Curved shadow paths at other times of year
Then at other times of the year, but most especially at the solstices, the shadow paths follow a curve, as shown on the graphic below. Technically speaking, these nonlinear curves are called hyperbolas.
Vertical plaque with elongated figure 8 on it and a stick at the top casting a shadow.
The Greenwich Royal Observatory presents a fine example of a pinhole sundial. In this instance, the vertical sundial faces south and records the meridian (noontime) passing of the sun in both sun time and clock time. Image via Wikimedia (CC BY-SA 3.0).
Worldwide similarities of shadow paths
In both the Northern and Southern Hemispheres, the morning’s longest shadow happens right after sunrise. Midway between sunrise and sunset, the shortest shadow of the day occurs at solar noon. Then, after noon, the shadow starts to elongate again, with the longest afternoon shadow happening just before sunset.
Once again, on the day of an equinox, the tip of an upright pole’s shadow travels in a straight line, from west to east.
Here are more experiments you can do yourself at home.
Northern and Southern Hemisphere differences
By the way, there are some major differences between the two hemispheres. On an equinox-day in the Northern Hemisphere, the straight shadow path passes to the north of the gnomon. In the Southern Hemisphere, the straight shadow path passes to the south of the gnomon.
And, at the equator, the shadow path goes neither north nor south of the gnomon. That’s because the noonday sun swings directly over the upright pole on the equinox, casting no midday shadow.
Equinox shadows: Complex diagram with sun in 3 positions and 3 radiating shadows aligned at ends.
On the day of an equinox – and only on the day of an equinox – the tip of a shadow from an upright stick or “gnomon” traces a straight west-to-east path along the ground. In fact, you can watch the equinox shadows yourself. Illustration via EarthSky.
Bottom line: On the day of an equinox, the tip of an upright stick’s shadow follows a straight west-to-east path all day long.
Read more: March equinox 2025: All you need to know
Bruce McClure
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About the Author:
Bruce McClure served as lead writer for EarthSky's popular Tonight pages from 2004 to 2021, when he opted for a much-deserved retirement. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also wrote and hosted public astronomy programs and planetarium programs in and around his home in upstate New York.