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Planets are always spherical. (Dunca Daniel/Getty Images/Hemera)
Planets are always spherical. (Dunca Daniel/Getty Images/Hemera)


It’s all within gravity’s sphere of influence Add to ...

Today, Collected Wisdom turns its gaze toward the heavens to see what kind of shape the cosmos is in.

The question

Heather MacAndrew of Victoria wants to know why planets are spherical. Are there no square planets?

The answer

According to several correspondents, it’s all a question of gravity. For instance, gravity drags all of the Earth’s mass toward the planet’s centre, and only a sphere lets everything get as near to the centre as possible.

To explain this further, let’s paraphrase an example sent to us by Dave Hanes, head of the department of physics, engineering physics and astronomy at Queen’s University in Kingston.

Suppose CW wanted to create a monument to itself by building a mountain of rock into a pinnacle far exceeding the size of Mount Everest (and doubtless topped off by neon lettering thousands of metres high saying, “Collected Wisdom Rules – OK”).

Unfortunately, Dr. Hanes says, such a monument could not be built because of the “self-gravity” of the Earth and the limited strengths of ordinary matter – even solid rock.

“In simple terms,” he writes, “the enormous mass of the upper parts of the mountain, pulled down by the gravity of the Earth, would weigh down so enormously on the mountain’s base that it could not withstand the pressure.” The mountain would therefore flatten out considerably under its own weight. “In other words, the Earth itself limits the size of features on its surface and ‘relaxes’ into a nearly spherical shape over all.”

He adds that the strength of rocky materials indicates that Everest is just about as large a mountain as you could get on Earth.

Meanwhile, small objects such as asteroids are not spherical because their rocky matter is strong enough to withstand their much weaker self-gravity.

“The asteroid Toutatis that passed close by the Earth on Dec. 12 is a good example,” Dr. Hanes writes. “It is elongated and ‘potato-shaped,’ but only a few kilometres in size.”

So, CW would surmise that it is theoretically possible for an asteroid to be square. But not a planet.

The question

Shelley Nickerson of Lincoln, N.B., tells us she has always kept the heat turned off in a spare room in her house during the winter and also kept the door shut, thinking that she was saving energy. Is she right?

The answer

“You bet,” writes Sjoerd Roorda, a professor in the physics department at the University of Montreal. “As long as it is a room with an outside wall.”

The amount of fuel required to keep a house warm, he explains, depends only on the heat loss – the amount of heat lost to the cold outside environment.

“By keeping one room with an outside wall unheated,” he writes, “the heat loss across its outside wall is reduced because the temperature difference between that room and the outside is much less than for the rest of the house.

Another way to see this is to say that the room acts as an extra-thick insulation for part of the house.”

Help wanted

When a police officer stops a motorist, he always puts on his hat before approaching the motorist’s car, writes Ross Towler of Halifax. As the officer is already wearing a uniform, why the need for a hat?

Why do people cover furniture in closed-up houses? asks Cheri Warren of Toronto. If there is no one in the house, where is all the dust going to come from?

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