Rovers roll on, horizon moons loom (or do they?)
Q: Are the Marsí Rovers still active or did they finally say
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) ó 25 times
farther than the last (1997) mission rover and 5 times farther than planned this
Meanwhile, down south, Opportunity finished the roversí
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 theyíll find... More geological clues
to Marsí 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 craterís 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.
Mars Exploration rover mission: latest news
NinePlanets.org: Mars by Bill Arnett
Horizon moons loom (or do
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 donít 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 itís about 50% to 75% larger. A
few donít think it looks larger at all. For most people, however, the effect is
The Moon doesnít change size as it rises. The atmosphere
doesnít make it look bigger. Weíve checked this out by comparing photographs.
The Full Moonís apparent size is constant 0.5į
as it moves from horizon to horizon across the sky ó about the size of a pea
held at armís length.
The bending of the Sunís rays through the atmosphere does
distort the low Moon: a slight 1.7%. Thatís 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 canít 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.
Itís something about how our brains process information ó
filling in details from memories or allocating the brainís resources.
"Many visual illusions result from the brainís 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.
Lock Haven University: The Moon illusion, an unsolved mystery
by Donald E. Simanek
(Answered June 25, 2004)