# How do shadows prove that the Earth rotates?

The sun appears to travel across the sky from east to west during the day. The earth actually rotates from west to east. Shadows provide some evidence to back this up. The shadows shift as the sun moves across the sky in an arc. This indicates that the earth is rotating.

Shadows are formed when light from one point on an object falls on another point on that object. If you watch clouds move across the sky, you will see that sometimes they cover more than one mountain or hill. That means that the cloud moved away from both of those points. You can tell that each point on the cloud was once directly under the sun because it got hot and then it snowed or rain ed it. The shadow that it left showed that it had traveled away from that point.

If two objects have their centers of mass far apart, they will fall toward each other due to gravity. If the center of mass of one of these objects is below the other one, they will merge together before they reach each other. If the center of mass of one of these objects is above the other one, they will part company at a distance from each other equal to their height difference.

Gravity pulls all objects with mass away from all other objects with mass. If you jump off a building, you generate new energy which you can turn into useful work; such as lifting weights or powering vehicles.

## Why do shadows change during the day?

The sun is the star that Earth revolves around. Because the sun's location in the sky changes during the day as a result of the Earth's rotation on its axis, the forms of outdoor shadows also vary throughout the day. Shadows can vary in length, form, and location. The angle at which sunlight strikes an object determines how dark or light it appears. When the sun is high in the sky, it hits the objects directly, so any shadow it casts is long and thin. As the sun lowers in the sky, more of the surface becomes illuminated, so the length of the shadow decreases. Finally, when the sun is low on the horizon, all parts of the object are exposed to light, so there is no longer a shadow.

Shadows change shape because the sun moves across the sky each day. If the sun was always in the same place in the sky, shadows would never change. But the fact that it rises some days and sets others means that shadows will grow longer in the morning and shorter in the afternoon.

Because shadows change shape, they can be used to determine the time of day. For example, if you see a short, fat shadow early in the morning, it must be late in the summer, before sunrise. If you see a tall, skinny shadow late in the afternoon, it must be early in the fall, after sunset.

## Why do shadows change direction?

Students may mention that the sun is traveling across the sky while they do this task. The sun is not moving; rather, the earth is. When the sun is high in the sky near noon, it is said to be in the zenith. At midday, the Earth's rotational axis is parallel to the horizon, so all points on the surface of the planet are given equal exposure to the sun. But by 3:00 p.m., when the sun has dropped low on the horizon, any object that was in the sun at 12:00 will have moved away from it. Thus, objects that were in full sunlight at 12:00 will be in deep shadow by 3:00 p.m.

Shadows move because the Earth is rotating. If the Earth were standing still, shadows would stay put until something blocked the light from reaching them. This would be like how headlights on a moving car create a bright spot before them and then slowly disappear as the car moves farther away.

Sunlight travels through space until it hits an object, such as the Earth. At that point, it is stopped or reflected back towards space. Since objects closer to the source of light appear brighter, and objects further away appear darker, shadows reveal the shape of nearby objects.

## What causes the shadows to change length throughout the day?

The rotation of the Earth causes the shadows to shift throughout the day. Day and night are also caused by the rotation of the Earth. Because of its orbital position, the Earth's revolution causes modest fluctuations in the length of these items. For example, the shadow of the Earth on its surface is about 6% longer in January than in July because the Earth is moving away from the Sun more quickly then.

Shadows can be either long or short depending on the time of year and the location on Earth. Long shadows are seen in the winter when the Earth is far from the Sun; this is called "winter darkness." In summer, when the Earth is close to the Sun, there is no winter darkness because it is sunny all the time. Short shadows are seen in the spring and autumn, when the Earth is between the Sun and the stars because people see only part of the sunlit portion of the Earth at those times.

People have used shadows to mark time for thousands of years. The ancient Babylonians counted time by watching the movement of sunlight across the floor of their temples, which was why many buildings in that country were designed with a north-facing entrance to allow sunlight into the hallways.

In modern society, shadows are important factors in determining what time it is.

Because the angle at which the sun shines on stationary objects changes with the rotation of the Earth, shadows change length throughout the day. When the sun is directly overhead in the middle of the day, the shadows get shorter as the angle of the sun changes. As night approaches and the sun moves lower in the sky, the shadows become longer.

Shadows begin their daily journey toward shorter lengths around sunset when the angle of the sun is high in the sky. As night falls and the sun drops further down in the sky, the shadows grow longer until it becomes light again next morning around sunrise when the angle of the sun is once more high in the sky.

Thus shadows oscillate between long and short throughout the day and night, always keeping one constant degree away from the Sun.

This is why shadows move even when no one is moving, and why you sometimes see double shadows beneath a single object like a tree or building. The first thing that happens when the sun comes out after it has been covered by clouds is that all surrounding objects take on a gray color, because they are illuminated by both the sunlight that reaches them directly and the sunlight that reaches them through you and your shadow.

Then the shadow of someone standing near the object begins to fall towards it, while the rest of your shadow remains where it was.

##### Regina Rivera

Regina Rivera is an astrologer, spiritual coach and mindfulness teacher. She believes that each of us has the power to change our lives for the better by tapping into our inner wisdom. She loves teaching people how to connect with their intuition through meditation, journaling and other practices in order to create a more fulfilling life.

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