Wishing you a Happy Spring Equinox tomorrow! We’re celebrating the increasing amount of daylight, the awakening tree buds, and the growing chorus of birdsong.
At the spring and fall equinoxes, day and night are nearly equal in length. This is because the Earth’s axis is tilted about 23.5 degrees from vertical, and this tilt stays the same as we travel around the sun. When the northern hemisphere is tilted towards the sun, we experience summer because the direct (strongest) rays of the sun are north of the equator, centered over the Tropic of Cancer. When the northern hemisphere is tilted away from the sun, we experience winter because the direct rays are south of the equator, over the Tropic of Capricorn. In spring and fall, the tilt is parallel to the sun, the direct rays centered on the equator.
One way to understand the movement of the sun throughout the day is by observing our own shadows! If we shine a light on an opaque object that is smaller than the light source, we see a shadow behind it. This is because light waves travel in straight lines and do not curve or spread out.
With just some pavement and sidewalk chalk, you can draw your child(ren)’s shadows. On a sunny day, have your child stand along an approximately north-south line. Trace the outline of their shadow, including a circle around each foot, so they can stand in the same position later. Make predictions about whether the shadows will get longer or shorter and what direction they will move in.
After at least twenty minutes, return to stand in the shadow outlines. Did the length or direction change? How did the change compare to your predictions?
What do you notice?
Shadows change over the course of the day, being longest at dawn and dusk and shortest at midday when the sun is overhead. In the morning, with sunlight coming from the east, shadows are on the west side, and in the afternoon, they gradually stretch out to the east.
Only on equinoxes, the tip of a shadow traces a straight west-to-east path – try tracking shadows for yourself tomorrow!