A Cerulean Moonset over CTIO

Another night of observing begins at the U.S. National Science Foundation Cerro Tololo International Observatory (CTIO) in the Chilean Andes, a Program of NSF NOIRLab, as the Moon sets below the horizon. The Moon appears massive and with a mysterious yellowish glow as it retreats behind the SMARTS–GSU 0.9-meter Telescope on the left, the SMARTS–GSU 1.5-meter Telescope in the center, and the Víctor M. Blanco 4-meter Telescope on the right. Both the SMARTS-GSU telescopes are operated by the Georgia State University Research Foundation. 

Since ancient times the Moon has been recorded to appear larger when it rests near the horizon. Although some camera lens magic is at work in this Image of the Week, this is a real phenomenon that humans experience, known as the Moon illusion. It is entirely an optical illusion produced by the human brain! To prove the Moon is not actually bigger when it’s near the horizon, extend your thumb towards the Moon when it is high in the sky. The Moon will be around the size of your thumbnail. When the Moon sets or rises, making it appear larger, extend your thumb again towards it and you will find it is still the size of your nail. While there is no definitive explanation yet for why the brain interprets the Moon as larger when it is closer to the horizon, there are many factors that contribute to the illusion. For example, when the Moon is lower in the sky and peeking behind mountains or buildings the brain can perceive the Moon to be closer because of our familiarity with these structures.

You can see a close-up of this viewpoint before the Moon reached the horizon here. This photo was taken by Hernán Stockebrand, a NOIRLab Audiovisual Ambassador.