Popular Misconceptions:

Popular Misconceptions in Astronomy

Instructions: MISCONCEPTIONS are in capital letters followed by a colon.
Scientific explanations or discussions follow.




TRUTH: ASTRONOMY is the science which investigates all matter-energy in the universe. It is based upon the scientific method, which states that theories must be grounded upon observational facts and endure repeated testing as new observational information is acquired. ASTROLOGY is the pseudoscience which entertains how the relationships of the sun, moon, planets, and stars influence the attitudes and lives of humans. The predictions purported by astrologers have been shown to have no scientific basis in themselves and were the synthesis of Claudius Ptolemy, a Greek astronomer. About 140 AD Ptolemy wrote a series of four books called the Tetrabiblios which summarized all of the principles of astrology which are still practiced today. Astrology began about 3,000 years ago in Babylon with what we today call mundane astrology. Predictions were applied to world or national events. To meet these needs, Babylonian astronomers were required to keep a continuous record of accurate planetary movements which in themselves were good astronomy. They did not seek answers to questions about the physical universe, but attempted to understand the motions of celestial objects in more of a mechanistic fashion. By the sixth century BC astrology had spread as far east as India where it still flourishes today. Meanwhile, the Egyptians modified Babylonian concepts by developing a more personal version of astrology which was later synthesized into natal astrology by the Greeks after the conquests of Alexander the Great. The Greeks believed that our lives were preordained by the precise configuration of the sun, moon, and planets in the sky at the moment of our birth. This astrology is still practiced today by Europeans and Americans along with horary astrology which provides the daily horoscopes found in newspapers and tabloids alike.

-- THE MOON --

TRUTH: The phases of the moon result from the revolution of the moon around the Earth, and our observations of the near hemisphere, cycling through a day and night sequence. Only during a lunar eclipse will the moon intersect the Earth’s shadow. The transition from the uneclipsed moon to the eclipsed moon takes about one hour. The phase cycle of the moon occurs in approximately 29-1/2 days.
TO DEMONSTRATE THE MOON’S PHASES, a light bulb (preferably clear) and a small ball attached to a stick are all that are necessary. In a darkened room, allow the light source to represent the sun, the ball to stand for the moon, and your head will become the Earth. Hold the moon in your left hand so that it is positioned between your eyes and the sun. You will notice the hemisphere facing you is in complete darkness. This is called the new moon. By revolving the moon in a counterclockwise direction around your head and observing the moon as it orbits, you will see the moon progress through the waxing crescent phase (less than half lit), first quarter phase (half on, half off, light to the right), the waxing gibbous phase (more than half lit—both limb and terminator appear convex), and finally the full moon phase (the hemisphere facing Earth is totally illuminated). These phases will repeat themselves in reverse order (waning gibbous, last or third quarter, and waning crescent moons) as the moon continues in its orbit around the Earth, eventually moving completely into its own shadow to renew the new moon phase.

TRUTH: Nothing could be further from the truth, but the term probably originated from the fact that we had little information about the hemisphere facing away from us. The moon has a portion of its surface (41%) that is never visible to earthbound observers, but all places on the moon experience a day and night cycle. When the moon is new and the hemisphere facing us is not illuminated, the other hemisphere is in full illumination. A better term for the hemisphere not visible from Earth would be the moon’s far side.

TRUTH: The moon’s orbital period or sidereal period is 27 1/3 days, while the time the moon takes to complete a cycle of phases, is its synodic period, 29 1/2 days. The discrepancy arises because as the moon revolves around the Earth, the Earth is also revolving around the sun. In one revolution of the moon (27 1/3 days), the Earth has moved about 27 degrees in its orbit around the sun. While the moon has completed one orbit of Earth, it is no longer in the same phase orientation. It will take the moon an extra 2 1/6 days of orbital motion on average to align itself in the same manner to be able to repeat the same phase. Therefore, a lunar phase period equals 27 1/3 days, plus 2 1/6 days, or about 29 1/2 days, on the average. Rather than compromising between these two periods as most educators continue to do, it is better to emphasize the lunar phase period of 29 1/2 days. This is what we observe in the sky as we watch the moon cycling through its phases, and it is the major “beat” in the rhythmic cycle of predicting eclipses. Solar eclipses can only occur when the moon is new, while lunar eclipses can only happen at the time of a full moon. The etymology of the word month is from the Middle English. In Old English, month was monath. The Old English word for the moon was mona.

TRUTH: The moon completes exactly one rotation about its axis in the same period of time that it takes to complete one revolution around the Earth. This period is equal to 27 1/3 days. It is only because of the synchronous motion of rotation and revolution that we observe the same hemisphere of the moon always facing us.
DEMONSTRATION-1: To illustrate this concept, orbit around a person with your face looking in the direction of that person and one hand extended toward the person at all times. As you revolve around your partner, you will notice your arm also making a complete sweep around the room during the same interval of time that it takes to make one revolution. By remaining in the same location and completing one rotation, the room will also appear to sweep around you in the same manner as before. This conclusively proves that the moon does rotate. If the moon did not rotate, then your hand would always point in the same direction as you revolved around your partner. At first, your face might be visible, but in a half revolution your back would now be observed. If the moon did not rotate, all locations on the lunar surface would face the Earth during the time period of one revolution and the concept of a dark side or unknown region of the moon would never have originated.
DEMONSTRATION-2: This illustration was the result of an insightful question which was asked at a 2008 in-service program at the ASD Planetarium about the tilt of the moon’s axis. A rotating Earth globe with a long wooden dowel which represented its axis, was substituted for the moon. The lunar axis, similar to Earth’s axis, is tilted (only by about six degrees to its orbital plane) and points in the same direction. With the lunar axis pointing in the same direction, the only way it is possible to keep the same lunar hemisphere pointed towards Earth is if the globe representing the moon rotates.
DEMONSTRATION-3: Here is another very effective way of authenticating the rotation of the moon. Drill holes at the poles of a moon globe through which a wooden dowel can be inserted to act as a visible axis. A smaller hole should be drilled at right angles to the axis through which a paper clip or other smaller rod can be inserted or glued. This will allow the globe to rotate freely around its axis. At the North Pole a large arrow should be fastened to the axial rod so that it is perpendicular to the axis and rotates with the axis. At the equator secure an eye screw though which a string can be fastened. Loosely tie the other end of the string to a post or have someone hold it. Use both hands to grasp firmly the globe by its axis. Revolve the globe around the strut keeping the string taunt. This will insure the same face of the moon globe is always pointing towards the strut. Keeping the arrow always pointing in the same direction, it is a no brainer to see that the globe is rotating underneath the arrow. Like the moon, the globe will always keep its same face pointed towards the center and complete one rotation (spin) in the same time period that it takes to make one complete revolution (orbit).

TRUTH: Any object which possesses mass (matter) has the force of gravity as a condition of its being. That goes for all matter: cars, baseballs, battleships, the moon, sun, planets, and all other objects found everywhere, regardless of whether they are large or small in volume. The force of attraction felt by one object for another is directly proportional to the product of their masses and inversely proportional to the square of the distance to which they are separated. Isaac Newton put it this way: the force of attraction between two objects, F, is equal to G m1 m2/r2, where G represents the universal constant of gravity; m1 and m2, the respective masses of the two attracting bodies; and r2, the square of the distance between the centers of the two bodies. When comparing the acceleration of a less massive object, m2, at the surface of a more massive body, m1, the equation simplifies itself to a = m1/r2 because
F = m2 a and F = Gm1 m2/r2 therefore, m2 a = Gm1 m2/r2 or a = Gm1/r2 To show the relative attraction or acceleration at the moon’s surface in relation to the Earth’s surface, substitute the relative mass of the moon and the relative radius of the moon in Earth units in the equation a = Gm1/r2. Since G is a constant which equates F in specific units of measurement, it is no longer needed to solve the problem. Keep in mind that the answer will be in relative units of Earth’s acceleration at the surface of m1.
The mass of the moon is 0.0123 of Earth’s mass, while the radius of the moon is 0.272 of Earth’s radius. The acceleration of an object at the surface of the moon is 0.0123/(0.272)2 = 0.166, or about 1/6th that of Earth. The moon attracts objects at its surface with a force of 1/6 of Earth’s gravity. Therefore, the moon definitely has a gravitational field which is noticeable when an astronaut performs activities on its surface.
LUNAR LYMPICS: Here is a superb way of combining astronomy, mathematics, and athletic prowess all into one game of several fast-paced lessons with everybody looking like a winner. Sponsor an athletic competition with events comprised of throwing or kicking balls and calculate their lunar equivalents if the games were held on the moon. Measure the straight-line ground distance from where the event initiated (the ball was thrown) to where it landed. Multiply this number by six for the moon. If some estimate of height can be made, this number can generally be multiplied by six also. Other interesting competitions are the broad jump or the running broad jump, as well as a contest to see how high a student can leap from a standing position. In the latter case, measurements should be made from the tops of the students' heads, since they have a tendency to lift their feet in the jumping process. Beware of Frisbee and high jump contests, for they will not give accurate results when translated into the lunar environment. Air currents will affect the path of a Frisbee, and, of course, the moon has no atmosphere. In the high jump, the individual may elevate himself or herself five or six feet before clearing the bar. This does not accurately reflect the true height that the person has been lifted off the ground. It is the elevation of the center of mass, the balance point of the various parts of the body that is of concern. The center of mass for the human body corresponds to the pelvic area which is already elevated about two and a half to three and a half feet above the ground for an adult. When a successful jump is made, the athlete clears the bar in a horizontal position, lifting his or her center of mass from the three foot level to just slightly above the height of the bar. If the bar was at the six foot level, and an athlete had his or her center of mass at three feet, the lunar equivalent would place the bar at 21 feet, not at 36 feet, as might be expected from the other examples—(3 feet x 6) + 3 feet = 21 feet. Mrs. Ruth Erie of Ritter Elementary provided the inspiration behind this idea.

TRUTH: This is called the moon illusion, and it is a perceptual difficulty not based upon any measurable clues. The same illusion can be created on a two dimensional surface as seen in the drawing below. According to the intuitive brain, the moon on the horizon is considered to be more distant than the moon which appears high in the sky. This is because the sky nearer to the horizon is perceived to be more distant than a position which is much higher in the heavens (see drawing). The analytical brain knows that the moon on the horizon or the moon high in the sky is the same angular diameter. The brain accepts that the moon near to the horizon which is perceived to be farther away must therefore be larger than the moon high in the sky. The moon illusion is most likely not created by comparing the moon against a background of objects which appear smaller, but which the brain knows would appear larger if they were closer.
DEMONSTRATION: Use a dime which is about the same angular diameter as the full moon when held at arm’s length to prove the fallacy of this misconception. Hold a dime at arm’s length and cover the moon when it is in a rising position. Repeat the experiment several hours later when the moon has moved to a much higher position above the horizon. It will be obvious that the moon has maintained its same apparent size in the sky.

TRUTH: The blue moon is not blue at all since the light which we see reflected from its surface is only sunlight. Astronomically speaking, the blue moon is really the second full moon which can occur during a month’s time. Since the phase cycle of the moon is 29 1/2 days, and a month has normally 30 or 31 days, blue moons occur very infrequently, about once every two to three years. The astronomical definition for the blue moon has its origins firmly rooted in the twentieth century. It appears as if our love affair with blue moons began as a mistake, over a half century ago in Sky and Telescope magazine. The use of the blue moon was traced to the 1937 edition of the Maine Farmers’ Almanac. However in that year, the blue moon date occurred on August 22, clearly not the second full moon of the month, since it takes 29 ½ days for the moon to complete its cycle of phases. What had happened? The Maine Farmers’ Almanac used a seasonal scheme for determining blue moons based upon a uniformly moving sun which made the seasons of equal length and occurring on fixed dates. There were normally three full moons for each season, and each of these moons was given a name. In 1937 summer full moons occurred on June 23 (after the summer solstice), July 23, August 21 (different from almanac), and September 20 (before the autumnal equinox). When a season contained four full moons, the rule was to designate the third full moon of that particular season as the blue moon. This allowed the other three named full moons to occur in better step with the seasons. In other words, the blue moon to the Maine Farmers’ Almanac acted as a sort of “leap moon” to reset the seasonal calendar back into step with the full moon cycle. The blue moon, as we use it today, resulted from an interpretative error based upon a July 1943 Sky and Telescope article which referenced the 1937 Maine Farmers’ Almanac. The birthing of the blue moon as the second full moon of a month was in March of 1946 also in Sky and Telescope magazine. The idea stuck and a new definition was born.

TRUTH: Look at the full moon in the night sky. It appears bright, in fact, brilliant enough to read the headlines of a newspaper by its light. The true tonal gradations of the lunar surface are actually dark gray to nearly black, with an average reflectivity of only 7%. The extremes of reflectivity range from 3% to 30%. The lunar surface appears similar to a macadamized roadway. With about 93% of the sunlight absorbed by the lunar surface, the vivid brightness of the moon is really a contrast difference between its dark surface and an even darker sky. That’s why the moon appears so bright at night. At first or last quarter, the moon is about 10% the brightness of a full moon. The moon when it first becomes visible as an exceptionally thin waxing crescent is about 1/1000th as bright as a full moon.

TRUTH: There is no statistical proof from hospital or police records that people are crazier, or that more crimes are committed, or that more babies are born when the moon is full. However, every nurse or police officer will swear that this simply cannot be so. The misconception probably arises from the condition that the moon appears to be full for a period of four or five days around the actual full moon date. If something weird happens during this interval when the moon looks full, then the blame can be attributed to the full moon. If there really would be a physiological or tidal effect on humans associated with the alignment of sun, Earth, and moon, then there should be similar occurrences happening during the new moon phase when the alignment of these three bodies are similar. In both cases the moon and sun are pulling in tandem causing the greatest tidal strain to occur on the Earth. No such phenomenon is reported for the new moon phase.


TRUTH: The Earth’s axis does slowly wobble like a top, completing one cycle in approximately 26,000 years. This motion is called precession, and it is responsible for a gradual change in the location of the position in the sky where the Earth’s axis points. During the present epoch the Earth’s axis is aimed toward Polaris, the North Star, in the constellation of Ursa Minor. It is around this star that the sky appears to pivot once each day as the Earth rotates on its axis. Nearly five thousand years ago, when the Great Pyramid at Giza was constructed, the Earth’s axis pointed towards Thuban in the constellation of Draco the Dragon. Ironically, if the Earth’s axis did flip back and forth during the interval of six months, it could be demonstrated that every position on Earth would experience the same seasonal effects throughout the year.


TRUTH: Most people believe this statement to be true. The seasons are caused in reality by the 23.5o tilt of the Earth’s axis and the axis always pointing in the same direction. During the course of a year, this one phenomenon changes the duration of time in which the sun is visible in the sky, the rising and setting positions of the sun along the horizon, and the noontime altitude of the sun. The Earth’s orbit is slightly elliptical (oval) in shape. During a year’s time the Earth’s distance from the sun varies from approximately 91.5 to 94.5 million miles. We are actually closest to the sun around January 2nd of each year and farthest from the sun around Independence Day, exactly the opposite of what one might expect. CONSIDER THE FACT that the Southern Hemisphere experiences summer when the Earth is closer to the sun. Are the summers in the Southern Hemisphere warmer? The answer is no. The Southern Hemisphere has less land. The excess heat is absorbed by the water which has an extremely high heat capacity. This moderates the summertime temperatures in the Southern Hemisphere.

TRUTH: The Earth rotates on average in a period of 23 hours, 56 minutes, 4 seconds. This is defined as the sidereal day, and it is about four minutes shorter than the solar day which is used to regulate our clocks. During this interval, the Earth’s revolution carries us about one degree along in our orbit. This causes the sun to be shifted by one degree to the east. Since we regulate our daily activities by the sun, we want the sun to return to its same noontime position of due south after a uniform interval of time has elapsed. It takes an extra four minutes of Earth rotation added to the sidereal day to accomplish this, creating the solar day which contains 24 hours. DEMONSTRATION: Each day, the stars shift about one degree to the east as we continue to revolve around the sun, but we maintain the consistency of returning the sun to its original position in the sky in a period of 24 hours. This daily shift in the positions of the stars over the course of a year causes the seasonal changes of the constellations that we observe.

TRUTH: Earth’s tropical year, the interval of time between two successive solar crossings of the spring equinox, happens every 365.2422 days. Anyone familiar with the calendar knows that we give the Earth only 365 days to complete this task. Every year, our planet lags approximately 1/4-day behind schedule in completing its orbital duties. Four years of under correcting add up to approximately one full day, so a leap-day is added to the calendar to bring the Earth’s orbital position back into general agreement with the sun and the seasons. If the calendar did not contain leap years, then the dates of the year would cycle backwards through the seasons in approximately 1500 years. Christmas would still be celebrated on December 25, but gradually this date would slide into the autumn part of Earth’s orbit and then the summer, while the dates of the solstices and equinoxes would become later by about one day every four years.

TRUTH: This is almost true. The exception occurs with century years which are not divisible by 400. The year 2000 was a leap year, but the century years of 1700, 1800, and 1900 were not leap years because they did not produce a whole number when divided by 400. This is the main difference between the older Julian calendar (45 BC) and our modern Gregorian calendar (1582) that is used for civil purposes. The reason for the change arose because the Julian calendar overcorrected for the leap year by approximately 11 minutes, 14 seconds per year. From the adoption of the Julian calendar on January 1, 45 BC to the onset of the Gregorian system in 1582, the first day of spring had slipped backwards by 14 days, from March 25 to March 11, causing the observance of Easter to fall on earlier and earlier dates. If this were not corrected, eventually Easter and Christmas would be celebrated at the same time of the year. The Gregorian calendar was adopted by the Catholic nations of Europe in 1582 during the papacy of Gregory XIII. During that year 11 days were dropped from the calendar so that the vernal equinox would occur on March 21. The day following October 4, 1582 became October 15. The calendar was now corrected to about 1 day in 3300 years. To correct the calendar to an even a greater extent, the years 4,000 AD and 8,000 AD will not be leap years.

TRUTH: The aurora is really an electrical discharge which occurs high in the Earth’s atmosphere. Trapped protons and electrons from the Van Allen radiation belts, situated thousands of miles above the Earth’s surface, follow lines of magnetic force down into the Earth’s atmosphere where they strike gas molecules causing them to glow. The Earth’s magnetic field creates two oval-shaped areas where the charged particle intensity is highest, and the auroras are most frequently observed. Auroral activity takes place at altitudes between 60 miles to 350 miles above the surface of the Earth. In the locations of greatest frequency, auroras can occur on about 250 nights during a year.

TRUTH: Since the north magnetic pole is located approximately 700 miles south of the Earth’s true geographic North Pole, a compass needle will tend to point in the general direction of north. At positions where the true geographic north lies directly south of the north magnetic pole, a compass needle will point directly north. In any other location there will always be some deflection of the needle either to the east or to the west of the true north position. This is called magnetic declination. In the Lehigh Valley a compass needle points approximately 11 degrees to the west of true north.

TRUTH: WHEN COLUMBUS DISCOVERED THE NEW WORLD: This misconception is generally true for the uneducated masses, but not so for anyone who had received a formal education and who could read. Columbus could read, and he was familiar with Greek texts which spoke of a spherical Earth as well as the circumference of the Earth. It was the Earth’s circumference that was in disagreement among European scholars of Columbus’s time. Columbus thought that the Earth was about 18,000 miles in circumference, and that the East Indies were only about 3000 miles away. Despite this misconception, Columbus was a keen observer who had sailed widely and who had witnessed the changes in the sky which were consistent with a spherical Earth. Convincing a superstitious crew that the Earth was round and not inhabited by monsters was more difficult.
DEMONSTRATION: There are many observational proofs that will show that the Earth is spherical. A favorite of mine, when starting a long east-west road or air trip, is not to change my watch. If going west, against the rotation of Earth, the sun will set much later according to my watch. My watch must be set back (earlier) if I am headed west to compensate for the spherical Earth. If going north or south, the altitude of Polaris, also called the North Star, will change a degree for every degree of change that is made traveling north or south. The shadow of the Earth projected onto the moon during every lunar eclipse is always round. Since lunar eclipses occur in different parts of the sky, at different times of the year, and at different times of the night, the Earth must be spherical in shape. The shadow cast by a sphere must always be round.

TRUTH: The same formula F = Gm1m2/r2 that was used to demonstrate the force of attraction between the Earth and the moon is the key to understanding this concept. A variant of that formula is a = Gm1/r2, where a = acceleration, G = the constant of gravity, m1 = the mass of the attracting body, and r = the radius separating the two objects. Note that the mass of the falling object is not even taken into consideration in the calculation of its acceleration under the force of gravity. Neglecting air resistance, two objects of differing masses will accelerate at the same rate. Even though the force of attraction is directly related to the mass, so is the inertia, or resistance to change of the falling body. The two balance each other perfectly so that the rate of acceleration remains the same.
DEMONSTRATION: Take a dime and a quarter or half dollor and stand on a chair. Hold the two coins at the same height above the floor and drop them. They will strike the ground simultaneously. Galileo, purportedly, was the first to perform this experiment in 1590 by dropping weights of differing masses from the Leaning Tower of Pisa in Pisa, Italy.


TRUTH: There is a type of celestial fireworks that has been witnessed by humans since the dawn of their existence. They are called meteors, or more commonly, shooting stars. A meteor is the flash of light left by a falling meteorite which hits the ground to end its flight. Whenever you observe a shooting star piercing rapidly through the black night sky, you are most likely witnessing the final chapter in the existence of an extremely small chunk of dross spewed from the nucleus of a comet. The flash of light is created by air molecules which are set aglow (ionized) as the meteoroid slams into our protective atmosphere. Entry velocities range between 7 and 45 miles per second. If the air is unable to stop or consume the meteoroid completely, a piece of it will reach the Earth’s surface, now to be known as a meteorite. Meteorites most likely originate from the asteroid belt as iron, rock, or ice-rock fragments formed through the collisions of asteroids and comets. Meteoroids are these same objects, plus cometary dust, which can be found in space orbiting the sun. Meteor, meteoroid, and meteorite are three of the most commonly maligned words in astronomy. Part of the problem stems from a lack of commitment by the professional community to use these words correctly in their own writings and discussions. Astronomers, most of whom are educators, seem to use the words “meteor” and “meteorite” interchangeably. Another inconsistency with the correct usage of these terms surely must stem from Meteor Crater, a nearly one mile-in-diameter METEORITE crater located just off I-40, 58 miles east of Flagstaff, Arizona near the town of Winslow. It was formed about 49,000 years ago by a small 100 yard-in-diameter nickel-iron asteroid which struck our planet. The impact produced a crater that was over 4000 feet across and 700 feet deep. Had this event occurred in modern times, it is believed that the “kill zone” for all living creatures would have reached a radius of two to three miles from the center of impact. The shock wave would have produced hurricane force winds in Flagstaff. Make no mistake, Meteor Crater is incredibly spectacular, and well worth the visit, but it was made by a meteorite.

TRUTH: In cases where meteorites have been witnessed to fall to the ground and then, in a short time period, were examined or handled, they were cold to the touch or were coated with a layer of frost. During their swift flights, the surfaces of small meteorites are heated to incandescence, several thousand degrees F., and their surfaces ablated (removed) rapidly by Earth’s atmosphere. This process is happening to only a very thin layer of the meteorite’s surface, and its duration would be short, under 15 seconds. The rest of the body remains at its space temperature, hundreds of degree below zero. In a small meteorite the atmosphere eventually slows the object until it is in a free fall with a terminal velocity of about 200 miles per hour. This could occur dozens of miles above the Earth’s surface, giving the meteorite five to ten minutes of free fall before it strikes the ground. The scenario is quite different when a massive meteorite strikes the Earth because it does not lose a substantial part of its space velocity before hitting the ground. Entrance velocities of 10-20 miles per second are the norm. The meteorite’s interior is cold upon impact, but 50 percent of its kinetic energy (energy of motion) goes into producing heat. Within a second or so after impact, the area around ground zero, which includes most of the meteorite, is compressed, pulverized, melted, and vaporized constructing a crater that is much larger than the impacting body.


TRUTH: Hans Lippershey (1570-1619), a maker of eyeglasses in Middelburg, Netherlands is generally credited with creating and publicizing designs for the first telescope in 1608. Although his patent for the telescope was never granted by the Dutch government, Lippershey was rewarded generously for his construction of several binocular telescopes. Credit goes to Lippershey because he actually had to have a working instrument to accompany the patent application. His invention was contested by Jacob Metius of Alkmaar, a city in the northern part of Holland in 1608, and several decades later by Sacharias Janssen, also a spectacle-maker in Middelburg. Telescopes became available for sale in 1608. It is probably a myth that Lippershey’s son or someone employed in his workshop actually made the discovery of the telescope.
Most people believe that Galileo (1564-1642) was the inventor of the telescope. In reality, Galileo was not even the first person to point a telescope towards the heavens, but he did publish his findings in March of 1610 in a book entitle Sidereus Nuncius. During his first telescopic observations made in 1609-10, Galileo discovered that Jupiter had four satellites, Saturn looked like three planets (Galileo’s telescopes could not resolve the rings), Venus went through phases, the sun had spots and rotated, the moon was a rough and cratered place, and the glow of the Milky Way Galaxy was being created by innumerable unseen stars. Galileo also recognized the military significance of the telescope, but he also comprehended its scientific importance as a tool for expanding humankind’s understanding of the universe. In this sense, he accelerated the growth of astronomy as a modern science and gave to it its most important research tool.

TRUTH: Telescopes are an extension of the human eye. Taken in this sense, a telescope’s most important function is to gather light. An object too faint to be seen by the unaided eye must first be made bright enough to be seen before it can be studied. This has always been the main purpose of constructing any telescope—to gather light, not to magnify an image. Every time the magnification of a particular telescope is doubled, the field of view and image brightness decrease to 1/4 of their original values. It is possible to magnify an image into invisibility, or to produce empty magnification, a condition where the image is enlarged to a point where no further increase in the amount of detail can be revealed. The light-gathering “power” of a telescope must, therefore, reign supreme in any consideration of a telescope’s usefulness. Lower and upper limits of magnification for a reflector are between 6-50 power per inch of aperture. For a refractor the upper limit can be extended to about 60 power per inch. The useful limits of magnification of an 8-inch reflector are between 48 and 400 power. Beyond 400 power the magnification becomes empty.


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