PORTLAND, Ore. (KOIN) — This year’s harvest moon, the full moon closest to the autumnal equinox, will occur on Sept. 10 at 2:59 a.m.

A large, orange, near-full moon will first be viewable on Friday above the eastern horizon at 7:44 p.m. The harvest moon will ultimately set on the western horizon a week later on Sept. 10 at 8:06 a.m., OMSI Director of Space Science Education Jim Todd announced.

“The orange color of a moon near the horizon is a true physical effect,” Todd said. “When looking toward the horizon, we are actually looking through a greater thickness of Earth’s atmosphere than when looking directly overhead. The atmosphere scatters blue light — the reason the sky looks blue. The thickness of the atmosphere in the direction of a horizon scatters blue light most effectively, but it lets red light pass through. Therefore, a moon near the horizon takes on a yellow, orange or reddish hue.”

The larger-than-normal moon seen on the horizon, Todd said, is an optical illusion known as “the moon illusion.”

“The illusion is a matter of perception, a trick of the brain, which perceives the moon when seen overhead as closer than the Moon seen at the horizon,” he said. “When an object is perceived to be nearer, the brain may compensate by making it look smaller to us. Likewise, an object thought to be farther away will be seen as larger.”

Popular harvest festivals are held around the world each year to celebrate the seasonal change.

“This moon festival dates back more than 3,000 years to moon worship in China’s Shang Dynasty,” Todd said. “Native Americans also called the full moon of September the “Corn Moon” or “barley moon,” as Corn and Barley were among their main crops. Sometimes, the September full moon in the Northern Hemisphere is also known as the “fruit moon.” In the Southern Hemisphere, where winter is about to turn to spring, the September full moon is known as the “worm moon,” “crow moon,” “sugar moon,” “chaste moon,” or “sap moon.”