KEY POINTS

• This year's equinox will occur this week
• An equinox experiment can demonstrate the Earth's curvature
• Shadows would be identical if the Earth were flat

This year’s equinox, which marks the first day of spring, will happen this year. Aside from signaling the change in the season, the upcoming cosmic event can also demonstrate the curvature of Earth.

Depending on your location on Earth, this year’s equinox is scheduled to happen on March 19 or 20. During this time, Earth will pass across the center of the Sun. This means the light emitted by the Sun will reach the planet at a position that’s perfectly perpendicular to the planet’s axis.

Due to the orientations of the Sun and Earth in relation to one another, there’s a simple experiment that you can follow to see the curvature of Earth. According to Ethan Siegel, a senior contributor at Forbes, the experiment involves finding a vertical object and measuring its height.

As the Sun reaches its highest point in the sky during the equinox, its light will cause the vertical object to cast a shadow. This will allow you to measure the shadow of the vertical object.

Using the height of the object and the length of its shadow, you can then calculate the angle of the Sun as its rays hit the vertical object. Siegel noted that the resulting angle would be equivalent to your latitude, which only happens during an equinox. This is because the light from the Sun is hitting Earth at an angle that’s perpendicular to its axis.

A person who conducts the same experiment at a different location will have an identical longitude. Measuring the differences in the latitude and the angle will reveal the curvature of Earth.

As noted by Siegel, if the Earth were flat, the Sun would cast identical shadows no matter the time or location on Earth. However, since the Earth is round, the shadows created by the Sun’s rays vary at different locations and times.

“If the Earth were perfectly flat, then the Sun’s rays would cast identical shadows at the same time on all days everywhere on Earth, no matter where you were located,” Siegel stated. “But if the Earth’s surface were curved, shadows at different locations would cast different shadows on the same day, depending on the angle that the Sun’s rays struck the object in question.”