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The lunar terminator illusion

Light travels in a straight line so why does the lit side of the Moon appear to point to a spot well above the setting Sun?

Posted 2023-10-23T01:52:59+00:00 - Updated 2023-10-23T01:52:59+00:00

Tony Rice

, NASA Ambassador

The Moon rises around 3-4 p.m. this early this week and is high in the sky by sunset. It reached first quarter, sometimes called half-moon, phase on Saturday with the right half of the Moon, at least from our perspective here on Earth.

You would expect that lit side to point directly to the Sun since that's where the light originates from and light travels in a straight line. You would also expect that this straight line during the first and last quarter phases would be perpendicular to the Sun. But it appears to point to somewhere above the Sun. Why?

This is the lunar terminator illusion at work.

Sunlight on the lit side of the Moon appears to be coming from a direction other than the Sun because of an optical illusion created by the horizon.

The answer lies in distance and in the visual cues (or lack of) provided in the sky. Our brains want to take what appears to be the shortest path to the Sun. But that's based on the only visual cue we have, the horizon.

In reality, the path is more curved on the dome of the sky, or what astronomers call the celestial sphere.

Researchers Brian Rogers and Olga Naumenko projected two dots representing the Sun and Moon onto a dome, not unlike the one at Morehead Planetarium and Science Center, and then asked volunteers to place a dot on the line connecting them. Their findings published in the journal Attention, Perception, & Psychophysics found that people consistently placed the third dot too low, trying to stay parallel to the horizon, because it was the only visual queue available.

We see this in aviation as well, for the same spherical reasons.

Consider a flight from New York to Madrid. Both cities lie at about 40 degrees north latitude, so the shortest path would be a straight line along the 40th parallel. But when viewed on a flat map such as the Mercator projection we are used to seeing, the shortest path looks curved. But how can that be the shortest?

great circle route vs "straight path" between points on Earth. Image: Google Earth/Rice

Like the celestial sphere, the Earth's surface is curved. The straight line our brains want to apply because of experience, and lack of visual queues, is actually longer. We also see this in the ground track, or the path satellites like the International Space Station take over the Earth, When displayed on a 2D map, the ISS's arrow straight path orbiting the Earth appears to follow a very curved line.

The ISS, like every other satellite, follows what appears to be a very curved path around the Earth when viewed on a flat map. (Image: NASA/European Space Agency)

Distance also plays a role in the optical illusion.

A more down-to-earth is a straight stretch of railroad tracks. Our brains know how trains work and that the rails stay a constant distance apart, but they appear to converge a the horizon. The further away the horizon, the stronger the illusion is.

We also see this convergence in crepuscular rays, beams of light streaming from gaps in the clouds. When observed from the ground, the rays appear to radiate outwards.

But when viewed from above, you can see that the light rays are parallel. Our distance from the cloud and the Sun's distance, more than 92 million miles away, creates the visual effect here on the ground.

his astronaut photograph from the International Space Station provides an unusual viewing perspective from above the rays and a clear illustration of their parallel nature. The sun was setting to the west (image left) on the Indian subcontinent, and cumulonimbus cloud towers provided the shadowing obstructions. The rays are being projected onto a layer of haze below the clouds. This image was captured by the ISS Expedition 29 crew above the Indian Ocean on October 18, 2011

Take note of the Moon and its terminator line over the coming days. It will continue a march westward until the full Moon on October 28, when there will be a (brief) lunar eclipse. If you were standing on the Moon, you could actually outrun the terminator as it moves less than 10 mph across the lunar landscape.