An equinox occurs when the Sun crosses the equator, and day and night are of approximately equal length.

"We get the seasons because this is the Earth, but it never rotates like this, it's actually tilted on its axis 23.5 degrees," said Dr. Art Alt, Professor of General Science at the University of Great Falls.

At the autumnal equinox, the Sun appears to cross the equator, from north to south; this marks the beginning of autumn in the Northern Hemisphere.

"So, what happens is you get a bigger thickness of air to go through. And, so the sunlight gets down here and the heat gets through here and gets absorbed," said Dr. Alt.

As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter.

"Chlorophyll is what allows the leaves and the plant to gather in sunlight and use that plus carbon dioxide and water to make the sugars that the plants going to live on at that point. But there's also other chemicals present in the leaves. There's carotenes, there's xanophylls, there's a whole bunch of them. And they're all different colors, but chlorophyll is the one that gets expressed the most. So, during the summer months the leaves are a nice green. The other colors are there, you just don't get to see them because there's so much chlorophyll," said Dr. Alt.

During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during the summer. They begin to shut down their food-making factories.

"As the green vanishes all the reds and the golds and the oranges and the carotenes and the xanophylls become expressed," said Dr. Alt.