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Early models of the solar system, In defense of the fly

Q: Concerning your question & answer on Rrmer's determining the speed of light in 1672, how did he know the diameter of Earth's orbit? Even today, without using sophisticated equipment, it would be difficult. (Steven, Oklahoma City, Oklahoma)

A: The distance to the Sun presents a problem. We can’t (without intricate gear) look at the Sun from two far-apart positions on Earth and estimate its distance from its parallax jump. The Sun’s glare defeats us.

Rrmer had the same problem. So, how did he determine the distance? By fitting together two puzzle pieces.

Giovanni Domenico Cassini was an Italian-born French astronomer who (with Hooke) discovered Jupiter’s Great Red Spot, was the first to observe Jupiter’s four moons, and — critical to our discussion — measured the distance from Earth to Mars.

Mars. In 1672, Cassini and a friend measured how far Mars is from Earth using parallax. That measurement defined the scale of the solar-system model astronomers used then.  (By the way, the image shows Valles Marineris, the solar system’s largest canyon. It goes a fifth of the way around Mars.) [NASA]

In 1672, Cassini stayed in Paris while friend Jean Richer traveled almost a quarter a world away to Cayenne, French Guiana in South America. They observed Mars, simultaneously, but against two different star backgrounds from their individual perspectives. Knowing the distance between Paris and Cayenne (4500 miles (7200 km)), they used plane trigonometry to calculate the distance to Mars. See the parallax example for their method.

Cassini’s estimate turned out to be only 7% less than the value we now use.

Over the ages, astronomers had built a model of the solar system from knowing

(Physicist James Schombert of the University of Oregon relates the history of the model. )

The model gave the relative distance between all solar-system large bodies. To fix the scale for the whole model, the modelers needed only the distance of Earth to any other body in the solar system. Cassini determined the scale when he measured the distance to Mars.

Knowing the scale, Rrmer then knew the distance to the Sun and, therefore, the diameter of Earth’s orbit.

This work followed in the footsteps of Kepler, Galileo, and Newton. "The advances they made allowed Cassini to calculate the distance to Mars and Rrmer to calculate the speed of light — all within a few decades," says astronomer Robert Massey of the Greenwich Royal Observatory in London. "Earlier in the same century it had been heretical to believe that the Earth traveled around the Sun."

* * *

Geometry, moon phases, and eclipses provided much of the early basis for the model. In fact, Greek astronomer Aristarchus of Samos first modeled the Sun-Moon-Earth system back in about 250 BC. Nobody, however, believed him. But his geometric argument, based on observations, was brilliant. His conclusion (the Sun is 20 times farther from Earth than the Moon) was off by a factor of 20 but that was due only to inaccurate instruments.

The figure illustrates his geometric argument: The moon shines by light reflected from the Sun. So, if we can measure the angle between the Moon and the Sun when the Moon appears half illuminated, then we can compute the ratio of their distances.

Figure.  Aristarchus developed the first model of the Earth, Sun, and Moon. [From University of St Andrews]

He measured the angle (87°) between the Moon and the Sun when the Moon appears half illuminated. That gave him the ratio (sin 3°) of the distances from Earth to the Sun and from Earth to the Moon. Trigonometry hadn’t been invented yet but he approximated this value nevertheless. Clever.

Further Reading:

Royal Observatory Greenwich: Mars

Wikipedia: Giovanni Domenico Cassini

University of St Andrews, Scotland: Aristarchus of Samos by JJ O’Connor and EF Robertson

Q: I am doing a speech on "in defense of the fly" and wonder if you could tell me a couple of good things about flies. (Courtney, Rotorua, New Zealand)

A: Here are some ideas.

Figure.  A greatly magnified housefly mouth ready to suck up liquid food (fruit, sewage, candy, whatever — liquefied by its spittle). [©1998-2005 by Michael W. Davidson, Mortimer Abramowitz, Olympus America Inc., and the Florida State University, used with permission]

Flies — along with bees, butterflies, and moths — help pollinate plants.

The larvae (maggots) are bait for catching many kinds of "coarse fish" (British shorthand for freshwater creatures not a trout or a salmon — the kind of fish Americans call "white fish" or "suckers"). British fishermen call the maggots "gentles." A whole industry exists —"gentle farms" — that breed immense numbers of gentles.

Blowfly maggots (Cochliomyia macellaria) aid healing wounds. In World War I, soldiers whose wounds teemed with maggots healed faster than otherwise because the maggots ate the rotten flesh before infection could set in. In fact, doctors still use maggots.

By the way, those are the good blowflies. Another species (C. Hominivorax) — the bad guys — infest and eat living tissue as well.

Maggots clean up the environment by eating decaying rotting material.

Flies feed insectivorous plants such as the Venus flytrap and a multitude of birds, spiders, insects, and frogs.

Their amazing agility inspires scientists at the University of California, Berkeley. These people are building a tiny robot (robofly) that can (eventually) hover, dart, rapidly change direction, dodge, walk upside down on ceilings, and fly or creep through tiny spaces — just like a fly.  See images of robofly.

Good luck with your speech!

Further Reading:

Maurice Burton and Robert Burton. The International Wildlife Encyclopedia. New York: Marshall Cavendish Corporation, 1969.

Washington State University: Farming with beneficial organisms

Biosurgical Research Unit: Introduction to myiasis

Natural History Magazine: The Venus Flytrap

Sfgate.com: Spy Fly — tiny winged robot to mimic Nature’s fighter jet

(Answered Aug. 19, 2005)