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HOW COME? Leaves change color

Why do leaves change colors in the fall? asks Philip Henderson, a student in Brookville, NY. Why does the Moon turn orange or red in the fall? asks Nigel George, also a student in Brookville.

Leaves changing color in autumn are a tree's way of preparing for a long winter.

In summer, leaves on many trees are green because they contain the green pigment chlorophyll. Trees need sunlight to make chlorophyll, which uses sunlight's energy to split water into hydrogen and oxygen. Leaves also absorb carbon dioxide from the air. This photosynthesis makes food (carbohydrates) for the tree and releases oxygen into the air.

Most leaves also contain yellow and orange ("carotenoid") pigments. Trees don't need sunlight to make them; they are mostly masked by the abundant green chlorophyll in summer.

But fall brings fewer daylight hours and colder weather. The average leafy tree is rushing to save all the nutrients it can for winter hibernation. Nitrogen and phosphorus are pulled from leaves, stored in branches. A layer of corklike cells grows between leaf stems and branches, reducing the leaves' supply of nutrients and water.

The result? The leaves' chlorophyll synthesis slows. Worn-out chlorophyll breaks down and disappears, and yellow and orange emerge from hiding. Meanwhile, red and purple pigments ("anthocyanins") are made in a leaf's sugary sap. The red pigments may act as a sun shield, protecting the leaves' fading photosynthesis factories from too much UV light. Presto: autumn leaves.

The moon seems more brightly colored in the fall, too, glowing deep orange as it rises. But no matter what the time of year, as the moon rises over the horizon, it may seem yellow, orange or red. As the Earth turns eastward and the moon rises higher, the color pales to white.

The color shifts because of how the atmosphere plays with moonlight. The bottom layer of air is thickest, full of gas molecules, dust, pollutants. So as the moon first rises, we glimpse it through a heavy blanket of air.

Moonlight is reflected (white) sunlight, made of a hidden red, orange, yellow, green, blue, indigo and violet. As moonlight travels through thick air near the ground, gas molecules and particles scatter bluer light out of the beam. What's left is mostly red, orange and yellow. So near the horizon the moon's face reddens. But as the moon climbs we see it through thinner air. As more of white light's spectrum, including blues, reaches our eyes, we see a whiter moon.

In fall the moon's path across the sky reaches its shallowest angle, nearer to the horizon. It may look yellow-orange for longer than usual. Meanwhile, farm dust may add more light-scattering particles to the air, turning the rising moon blood-red.


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