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Earlier in March, we explored Earth’s seasons as they relate to the sun’s position in the sky across the globe. This week, we’re delving into the vernal equinox, during which the sun’s rays strike the planet exactly perpendicular to the Earth’s surface at the equator.
At both the autumnal and vernal equinoxes, all areas of the Earth experience nearly equal periods of day and night. The spring, or vernal, equinox for the Northern Hemisphere takes place around March 20, the same day as the autumnal equinox in the Southern Hemisphere, and marks the first day of astronomical spring. Astronomical seasons are based on the position of the Earth with respect to the sun as the planet makes its annual revolution around it.
The word “equinox” derives from the Latin aequi, meaning “equal,” and nox, meaning “night”; however, in reality, we actually have a few more minutes of sunlight at the spring equinox—an additional eight minutes or so at the mid-temperate latitudes.
There are two reasons why we have more than 12 hours of daylight on this day of supposedly equal day and night: the sun is a disk, not a point, and atmospheric refraction. Refraction in the atmosphere bends the sun’s rays, causing the sun to appear above the horizon when it’s actually below, resulting in slightly more daylight than darkness.
The spring equinox sometimes falls on different dates because the Earth takes a little over 365 days to complete a single orbit around the sun. It takes precisely 365.25 days for the Earth to orbit around the sun. In the Gregorian calendar, this is accounted for by adding one extra day every four years, which is why we have leap years.
This means that the March equinox occurs with approximately a six-hour difference from the previous year. The spring equinox 2022 in the northern hemisphere is on Sunday, March 20, at 3:33 p.m. GMT; next year, in 2023, it will be around six hours later at 9:24 p.m. GMT.
During the course of the year, we all notice that the sun appears at different places during the same time of the day. At 6 p.m. in July it's still sunny outside, while at 6 p.m. in January the sun has already set. It's easy to notice the difference over the course of months, but what about the difference over weeks or even days?
With this activity you can verify that the sun appears in a different location at a specific time every day of the year with one exception. On March 20, the vernal equinox, and September 21, the autumnal equinox, you will find the sun in exactly the same position in the sky.
Supplies: 2 x 2-ft (60 x 60 cm) wooden board or cardboard square, 10–12-in (25–30 cm) wooden stick, ¼–½ in (6–12 mm) in diameter, tube of glue, marker
Glue the wooden stick to the cardboard square so that it stands upright. To assure that the full shadow fits on the cardboard, you may want to glue the stick closer to one of the edges.
Once the glue is dried and the stick can stand by itself, place the cardboard square on a flat surface where it will be exposed to the sun. Take note of the time of day. Mark the point on the board where the tip of shadow is located and write the date.
It is very important that the board be oriented in the same direction each time you lay it on the ground to mark the board. You might mark one of the edges of the cardboard square as a point of orientation.
Repeat this daily or weekly at the exact same time each day.
Discuss your observations. Are there any days where the shadow will appear in the same place at the same exact time? Do you think that there are places on the planet where the shadow would appear at the same place at the same time every day?
The cause of the change in the shadow's location (i.e., the location of the sun) is the tilt of the Earth's axis which causes the Earth to face the sun at an angle of 23 degrees. Where the Earth is located in its orbit around the sun determines the length of the day. Since the Earth's location around the sun is changing continuously, so are the length of the days.
If you continued this experiment the whole year round and connected the dots of your markings, how might the resulting picture look? Would there be any intersections of dots?