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Rovers roll on, horizon moons loom  (or do they?)

Q: Are the Mars’ Rovers still active or did they finally say goodbye? (George)

Mars exploration rovers quest for watery clues [NASA]

A: Opportunity and Spirit roll on. NASA extended the original 3-month mission to September (if they last that long).

The early lander, Spirit, tootling in northern climes half a planet from Opportunity, has racked up a record 1.9 miles (3 km) — 25 times farther than the last (1997) mission rover and 5 times farther than planned this time.

Meanwhile, down south, Opportunity finished the rovers’ primary mission in March when it found evidence of an ancient shallow salt lake. Water one time flowed on Mars!!!

But, who knows what else they’ll find... More geological clues to Mars’ watery past? Divining the seasons on Mars from atmospheric hints? Evidence of life!?

By the way, on May 25, mission controllers told Opportunity to creep 4 inches (10 cm) closer to the edge of Endurance Crater. Opportunity "saw" the crater’s edge in its navigational camera and stopped ("Good grief! No!"). Since then we have sent commands to bypass such "over conservatism." On June 10, she tentatively dipped over the edge and then returned successfully to brim safety. A few days later she ventured about 13 feet (3.9 m) deep into the crater, got comfortable, and began drilling rocks.

Further Reading:

Mars Exploration rover mission: latest news

NinePlanets.org: Mars by Bill Arnett

Horizon moons loom  (or do they?)

Q: Recently in the early West Texas morning the moon was setting through a dusty horizon. Why did the moon seem so incredibly large? I am guessing it is the same reason the sun can look so large. (Scott, Tucson, AZ)

Moonrise in Chatham Strait, Alaska [John Bortniak, NOAA]

A: It is the same reason that the setting Sun looks huge. Also constellations near the horizon look large for that same reason. The reason, though, is a mystery. We can rule out some hypotheses. Others make some sense but don’t explain the whole phenomenon.

The illusion: Some people think the Moon looks twice as large near the horizon as up in the sky. Most think it’s about 50% to 75% larger. A few don’t think it looks larger at all. For most people, however, the effect is large.

The Moon doesn’t change size as it rises. The atmosphere doesn’t make it look bigger. We’ve checked this out by comparing photographs. The Full Moon’s apparent size is constant 0.5° as it moves from horizon to horizon across the sky — about the size of a pea held at arm’s length.

The bending of the Sun’s rays through the atmosphere does distort the low Moon: a slight 1.7%. That’s too small to notice with the naked eye and the effect is in the wrong direction: refraction makes the horizon Moon appear flatter (smaller), not larger.

We can refute a common explanation (reference cues) simply by looking at the horizon Moon between our legs or standing on our heads. The Moon looks normal size then (for most people). Also airline pilots above the clouds see a "big" Moon when it rises or sets. The horizon Moon looks big to sailors on a featureless ocean or to farmers on a flat plain.

So, the explanation can’t be that the presence of nearby objects (for example, trees, mountains, or roads) makes the Moon look larger than it does when high in the sky where we have no reference objects.

It’s something about how our brains process information — filling in details from memories or allocating the brain’s resources.

"Many visual illusions result from the brain’s attempt to reconcile conflicting information into a sensible picture," says Donald Simanek, emeritus physics professor at Lock Haven University of Pennsylvania. "Multiple depth and distance cues provide consistent, unambiguous information for everyday nearby scenes that we can observe from various vantage points and even touch. But the perception mechanisms that work so well in ordinary situations are insufficient to deal unambiguously with the unusual setting of the horizon moon in comparison with the moon seen high in the sky. For many people, this distortion looms large."

But, how? As early as 7th century BC, Chinese and Greeks wondered about the illusion. Aristotle pondered the matter in 350 BC. And the reason still escapes us. Good question.

Further Reading:

Lock Haven University: The Moon illusion, an unsolved mystery by Donald E. Simanek

(Answered June 25, 2004)