Bad Physics
Copyright © 2001 Mikolaj "Mik" Sawicki. All rights reserved.

Misconceptions about Astronomy

Misconceptions about astronomy are fairly common in media. Here are just few examples that I've found.
For more astronomical misconceptions visit  Phil Plait's  "Bad Astronomy" web site.

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Associated Press on Christmas Eve full moon.
(Associated Press, The Southern Illinoisan, December 26, 1996)

The AP photo of a full moon is accompanied by the following caption: "A full Christmas Eve moon: The full moon rises Christmas Eve behind the Museum of Modern Art in San Francisco. The Northern Hemisphere enjoyed its first Christmas Eve full moon since 1950, and its last until 2102."

Physicist's comments: Why only Northern Hemisphere? A full moon seen from Northern Hemisphere would still be a full moon when seen from Southern Hemisphere.

A more subtle problem is that the lunar cycle is 29.3 days long, and one would expect a full moon to fall on a Christmas Eve every 29 years or so on average, so it is somewhat surprising that one has to wait 106 years for a next one to happen. A check of astronomical data reveals the following full moons on Christmas Eve:

    1996, December 24, 20:41 (shown in the AP photo)
    2007, December 24, 01:15
    2026, December 24, 01:30
    2045, December 24, 00:51
    2083, December 24, 03:54
    2102, December 24, 11:28
    2121, December 24, 01:09

The time above is the astronomical Universal Time (UT1), commonly known as the GMT. So there will be 4 full moons on Christmas Eve before 2102.

One could perhaps argue that the author had in mind a full moon in San Francisco, and since UT is 8 hours ahead of the PST (Pacific Standard Time), the December 24 full moon in 2007, 2026, 2045 and 2083 will in fact fall on December 23 in San Francisco!  But then we also have to check if there is a full moon on December 25, that will be still on December 24 in San Francisco. And indeed, there is one:

    2072, December 25, 07:18 = December 24, 23:18 PST.

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US News and World Report on elliptical orbit.
(US News and World Report, August 18/August 25, 1997, page 60)

The article "What Causes Ice Ages" discusses hypothesis that the advance and retreat of glaciers may be tied to slight changes in Earth's orbit, including tilt, wobble and the shape of the orbit. In particular, the shape of Earth's path around the Sun is claimed to change  from circular to more elliptical over 100,000 years. This is illustrated by a drawing, showing circular and elliptical orbit of the Earth, with the Sun sitting right at the center of the orbit in each case.

Physicist's comments: For elliptical orbit, the Sun is not at the center of the ellipse, but at one of the foci.
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The Southern Illinoisan on Oort cloud.
(The Southern Illinoisan, June 1996)

"Most comets form in the Oort cloud, home to more than a trillion comets and located just beyond the orbit of Neptune"

Physicist's comments: The author confused the Oort cloud with the Kuiper belt. Neptune is some 30 AU (astronomical units) from the Sun. It is the  Kuiper belt that begins just beyond the orbit of the Neptune end extends up to some 1000 AU. Some comets probably originate from the Kuiper belt, but the majority of comets probably originate in the Oort cloud, which begins some 40,000 AU and extends to some 100,000 AU from the Sun.
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The Daily Egyptian on the Solar system.
(The Daily Egyptian, 22 September 1998)

The Daily Egyptian is a student paper of the Southern Illinois University in Carbondale. It published a column by a senior in the speech communication, who wrote: "As the earth travels at the terrific rate of 1037 & one-third mph, we don't feel a bump on this smooth ride. All of the planets travel at the same speed - 1037 & one-third mph."

Physicist's comments: It's the points along the Earth equator that have a speed of 1040 mph due to the Earth spinning on its axis. The orbital speed of Earth around the Sun is some 67,000 mph. And each planet has a different orbital speed, a fact discovered some 400  years ago by Johannes Kepler. Mercury, which is closest to the Sun, has the highest orbital speed of some 108,000 mph, while Pluto, the most distant  planet, has the lowest orbital speed of some 10,700 mph on average.
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Planetary motion in Micheal Crichton's "Sphere".
(Michael Crichton, "Sphere", Alfred A. Knopf, Inc., New York, 1987, page 77.)

"Space around the sun is curved like this bowl."
"Okay..."
"And if your ball had the right speed, it wouldn't escape from the bowl, but instead would just spiral around endlessly inside the rim of the bowl. And that's what planets are doing. They are endlessly spiraling inside the bowl created by the sun."

Physicist's comments: The word "spiraling" is misleading. Planetary orbits are elliptical, not spiral.
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Associated Press on a solar eclipse.
(Associated Press, The Denver Post, June 6, 2002)

The AP provided the following explanation of the partial solar eclipse of June 10, 2002: "The event is called an annular, or ring-shaped, eclipse. Because the moon will be closer to the Earth than during total eclipses, it will only partially cover the distant sun."

Physicist's comments: Try it yourself. Take a coin (the moon), hold it at arm's length, and try to block a distant object, say a car (the Sun). Move the coin closer to your eye and observe how much of the car will it block now. Do you see what's wrong with the AP explanation?
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Where's Mars, usually?
(Associated Press, The Southern Illinoisan, August 28, 2003)

The AP filed the following dispatch explaining event of August 27, 2003: "JOHANNESBURG, South Africa (AP). Mars is usually about 140 million miles away from Earth, but on Wednesday its orbit brought it about 34.6 million miles away."

Physicist's comments: You can't say that Mars is usually 140 million miles away. Here's why: Earth orbits Sun in 365.26 days, while Mars, orbiting Sun at larger distance, moves with smaller speed and completes one revolution in 686.98 days (sidereal year). Hence every 779.74 days (synodic year), Earth catches up with Mars and the distance between them is smallest. When this happens, Mars is said to be in opposition, because at this point Mars is in the part of the sky opposite the Sun and is highest in the sky at midnight. This is also when Mars appears to be brightest, because it is closest to us. If you imagine Mars orbit around Sun as a perfect circle with a radius of 141.6 million miles, and Earth's orbit as a circle with a radius of 93.0 million miles, then a distance between Mars and Earth at opposition would be always 48.6 million miles.

Note that 390 days later, fast moving Earth will leave Mars so far behind, that Mars will be located exactly behind Sun. When this happens, Mars is said to be in conjunction, and the distance between Earth and Mars is about 234.6 million miles. Averaging Mars-Earth distances at opposition and conjunction one obtains 141.6 million miles, but certainly Mars is not usually 141.6 million miles away from Earth.

As Johannes Kepler had discovered, Mars' orbit is elliptical, with closest point (perihelion) only 128.3 million miles away from Sun, and farthest point 154.8 million miles from Sun (aphelion). Earth's orbit is also slightly elliptical. Occasionally, opposition of Mars coincides with Mars passing through the perihelion (perihelic opposition) and Earth passing through the aphelion. Furthermore, Mars orbit is tilted with respect to Earth's orbit, the position of its perihelion slowly rotates in space, and the shape of the ellipse is also variable. In a very exceptional situation, a favorable combination of these circumstances may reduce the Mars-Earth distance to 34.6 million miles, and this is what we were lucky to witness on August 27, 2003.

For more information about Mars orbit and how it changes over time see "The Planet Mars: A History of Observation and Discovery" by William Sheehan.


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