Glossary
Below you will find definitions for many of the words used throughout AWS-DIN online activities. Some are linked directly into this document, and users can search for others by using the javascripted list in the box directly below. From the Glossary prepared by Francis J. Heyden, S.J., Chief Astronomer, Manila Observatory.
ALMANAC—you
can find an example of an almanac in the weather report of most daily papers. In
the almanac you can find information about astronomical bodies, for example, the
times of sunrise-sunset, the phases of the moon and information about other
visible planets, like Mercury, Venus, Mars, Jupiter, and Saturn. This
information is contained in tables. Astronomers, navigators, surveyors, and
space scientists use this to find astronomical objects in the sky overhead, and
to make measurements of them.
ALTITUDE—is an angle, not a distance or height. It is the angle that is measured from the edge of the horizon along a great circle that passes through the zenith or point directly overhead. It is always 90 degrees or less.
ANTE-MERIDIEM (a.m.)—before noon. before the sun reaches the meridian, its highest altitude.
ANGLE—The separation of two lines or planes that intersect
APPARENT MOTION—The big difference between the Ptolmaic theory of the solar system and the Copernican is an appearance, or better still "point of view." If we watch the way the sun and planets move while standing on the earth, we see everything going around us. This was Ptolemy's viewpoint. But if we stood on the North Star we would see everything going around the sun. This is Copernicus' viewpoint.
ARC—curved line that is part of a circle held by two lines that are radii of the circle.
AXIS—when something turns, it rotates around an axis.
AZIMUTH—the compass rose goes around through the east through 360 degrees. The azimuth is the angle of the compass rose around the horizon from the north pole through the east, south, and west to the north again.
BISECT—this is pure geometry, and refers to dividing an angle in half; to bisect an angle This is done with a compass and a ruler.
CARDINAL POINT—is a particular reference point or plane that serves as a basis for other measurements.
CIRCLE—the path of a point that is at the same distance from a center. A compass is used for drawing a circle.
CIRCUMFERENCE—this is one of the oldest measurements known. The Pythagoreans prided themselves on the accuracy with which they could draw a circle free hand. It was like a secret sign in a club by which the true members of a club could be known. The aim among many was to find a circumference in which the radius of the circle would fit evenly. This was never found and from all the effort came the value of "Pi" which equals 3.1416….It is also an unique number because it will never come out even.
CLOCKWISE—standard clocks all go to the right. The sun appears to move around the earth in a clockwise direction from east to west for anyone looking toward the south. But if you were to stand on the North Pole star and look down on the solar system you find that all of the planets are going around the sun in a counterclockwise direction. Most of them are rotating also in counterclockwise direction.
COMPASS—a compass is an instrument for showing the directions on the earth. The earliest known compass was a piece of natural magnet, called "magnetite" that is found near an iron mine, fastened to a board that floated in a small bowl or tub of water The points of the compass "rose" were marked on the board, so that as a ship sailed, the prow or bow of the ship could be pointed in the direction in which the sailor wanted to go, like north, south, east, or west. These directions were determined by the magnetic field or the magnetism of the earth. The magnetism of the earth is not quite the same as the geographic latitude and longitude There is a magnetic equator and a magnetic north pole. The difference has been known for a long time, so that anyone sailing with a magnetic compass knows how far he is from the geographic or rotational coordinates of the earth.
COMPASS ROSE—a star-like drawing of the points of a compass showing north, east, south, and west with the other divisions that form the "box" of the compass. These points go: north, north-north east, northeast, east north east, east, east-southeast, southeast, south southeast, south, south-southwest, west-southwest, west, west-northwest, northwest, north-northwest. In the box there are 16 points. Each is subtending an angle of 360 divided by 16 = 22 and 1/2 degrees. Every scout and navigator must know how to "box the compass."
DEGREES—are a unit of measure derived from Geometry. Look at the face of a clock in your classroom. It is probably a circle divided in to 12 hours marking half of a 24-hour day. Some clock faces are divided into 24 hours. Notice the divisions that are made between each hour. They are separations around the circular face of the clock, and each one represents 1 minute. If you enlarge the face of the clock, the hours around the circle will be separated by more space, but each mark will still equal one hour and the smaller separations will still represent one minute. We are measuring with relative units called degrees. Circles are measured in degrees. Curved lines are measured in degrees. Longitude and latitude measuring the circular surface of the earth are measured in degrees.
DIRECTIONS—may follow a compass heading like one of the points or it may be just a straight line up, down, horizontal, or at any target.
EARTH—the planet on which we live. It is really a system of spheres each one larger than the other First there is the core at the center which is the heaviest part. It is very hot because of the weight of the solid material, rocks, metals, etc. on it. It is made mostly of iron and nickel with probably most of the gold, silver, and platinum we would like to have up here. Those metals are also very heavy. Then there is the mantel consisting mostly of hard rock like granite. The core is about 1,500 kilometers thick. The crust, or the surface where we live with the oceans and the solid land, is not more than 50 kilometers thick, like the skin on a piece of fruit. On top of it all are the gases we call the atmosphere and this layer of gas with its oxygen, its nitrogen, and other gases we need for life extends up another 1,000 kilometers, with most of it close to the earth's surface where the birds and the airplanes fly.
ECLIPTIC—is the name given to the plane of the earth's orbit around the sun.
ELLIPSE—is a well-known geometrical figure that differs from the circle because it effectively half a changing radius and two centers called foci. Johannes Kepler in the sixteenth century discovered that planets do not go around the sun in circles as Galileo and others believed but in ellipses with the sun at one of the foci. Just as you can draw a circle by using a piece of string with a loop around a pin and another loop around a pencil, you can draw an ellipse by putting one loop around one pin and the other loop around a second pin with a loose string in between. Let the string guide your pencil point as it slides past the point of your pencil. Keep the string tight all the way. Ellipses can be very nearly a circle or they can be so flat that they are nearly a straight line.
EQUATOR—a great circle that goes around the earth and is always 90 ° from the north and south poles.
EQUINOX—is the point where the orbit of the earth crosses the equator in the sky. When the sun comes to this point on its way north, the people in the northern hemisphere are having spring. But people in the southern hemisphere are having fall. We still call this the Vernal Equinox.
FIXED OBJECT—in the sky this is any star that does not shift or move with the rotation of the earth or because the earth rotates. Actually no star is really fixed in its position because we live in a universe that is never still. Everything is moving in some direction or other, and many big groups of stars are rotating. Hence if we watch stars for a long time we see that each one is moving in some direction. These motions on the face of the sky help us to study the physical motions of the Milky Way and of the Universe.
GRAPH—a piece of paper marked with squares for plotting the positions of mathematical points, physical curves, cities on a map, or stars on a chart Eratosthenes is supposed to have made the first map with latitudes and longitudes for the whole world that was then explored about 2250 years ago.
GREAT CIRCLE—any circle drawn on a sphere with its center at the center of the sphere is a great circle. Any other circle is a parallel to some great circle but must be smaller. Thus there are parallel circles for different latitudes that are parallel to the equator. But great circles may take any direction of a sphere. On the earth, a plane, a ship, a radio, or television wave or an earthquake wave must travel in a great circle The reason is that the arc of a great circle is the shortest distance between any two points of the surface of the earth. Can you show how it works with a pebble dropped into a tray of water. Imagine the tray to be the surface of the earth.
HORIZON—this is the circle we see when we stand in one spot and look around at the edge where the earth and the sky meet. Usually we have to stand in the desert or in a boat at sea to see the horizon all the way around. It is a natural circle with the observer in its center. As he moves his horizon moves with him. The distance to the horizon depends on how tall we are. An ant has an horizon of only a few inches. A six foot man sees a horizon that is 5 kilometers away. A sailor in the crow's next of a ship, 100 feet high or so meters will see a horizon about 20 kilometers. In an airplane at 130 kilometers will have a horizon of 650 kilometers and an observer in the Shuttle will see all the way across the United States.
HOUR—this is a unit of time, or one twenty-fourth of a day. It is divided into 60 minutes or 3,600 seconds.
INCLINE—is an angle. A hill is inclined to the horizontal or surface of the earth. Two lines at an angle are inclined. Two planes at an angle are inclined, like the plane of the earth's orbit and the plane of the earth's equator. This particular inclination is very famous and is called the "obliquity of the ecliptic." It is 23.5 degrees approximately and is the cause of our seasons because as the sun appears to orbit the earth it moves 23.5 north and 23.5 south every year. This changes the inclination of the sun's rays on the surface of the earth making the places hotter with less inclination than when the inclination is greater It also changes the hours of daylight, because the sun appears to stay above the horizon longer and longer when it is above the equator for northern latitudes.
INTERSECT—when two straight lines or any pair of lines cross one another the point is called an intersection. The corner where two streets meet is called an intersection. Parallel lines can have no intersection.
LATITUDE—the
angle measured with its vertex at the center of the Earth from the point of
measurement to the nearest point on the equator, with value 0 at the equator
through 90 at the north or south pole.
LONGITUDE—is the angle measured around the equator of the earth from any standard meridian, like the one at Greenwich, England. The meridian passing through the north and south poles of rotation and through the point overhead or the zenith marks the meridian of the place where an observer, two cities, or some standard like a time zone is located. Longitude is measured in degrees, minutes and seconds of time. 15 degrees makes one hour of time. Hence the rule: Divide degrees by 15.
- Multiply remainder by four to get minutes
- Do the same for minutes and seconds.
- Multiply hours by 15 to get degrees.
- Do the same for minutes and seconds of time.
- Convert all minutes and seconds over by dividing by 60.
MAGNETIC COMPASS—A
magnetic compass is used to find the direction north. It consists of a circular
face marked in the points of the compass—N, NE, E, etc. (Draw a compass face.)
Fixed to the circle is a magnetic needle. You know from handling bar magnets
that one end of a magnet will be attracted to the other end of a second magnet.
Each end is called a pole. The north pole of Magnet One is attracted to the
south pole of Magnet Two. The Earth also is a magnet, and on a magnetic compass,
the needle is attracted toward the North Pole of the earth. Once you find North,
you can determine the other directions.
MATHEMATICS—is a language for measuring and describing things—both real and imaginary. There
are many branches of mathematics. The branch that you use when you count your
fingers is called Arithmetic. When you were little you started counting on your
fingers—10 fingers, 2 hands. You counted “1, 2, 3, 4, 5, 6, 7, 8,9, 10.”
Your hands represented a set of numbers—10 fingers, 2 hands. Then you learned
to add, subtract, multiply, and divide numbers. For instance, if you and a
friend got together and counted the fingers on your hands, you would have a
larger set of numbers, which could be measured or described in a number of ways.
Four hands multiplied by five fingers on each hand equals 20 fingers [4 X 5 =
20]; or, one set of your fingers plus one set of your friend’s fingers equals
20 fingers [10 + 10 = 20]. Either way, it totals the same amount. Mathematics
simply gives you a way of sharing the measurement—the number of fingers—with
other people.
MERIDIAN—a circle drawn on the earth that passes through the north and the south poles. Places on the earth like any city or town are located by the meridian that passes through them.
On the Earth meridians are usually spaced 15 degrees apart so that they can also mark the time zones which are 15 degrees wide These are zones, or time zones, in which one hour is 15 wide with the Standard Meridian in the middle. The standard zones are numbered +1, +2, etc., for all meridians 15 degrees apart west of the zero degree meridian in Greenwich (pronounced "Grenitch"), in England near London. Washington is in the +5 Zone. This means if you have the time for Washington, you must add five hours if you want to know time at Greenwich. In the same way the zones go -2, -3, etc., all the way east of Greenwich. But there is a meridian in the Pacific Ocean that is 180 or 11 hours away from Greenwich. At this meridian the longitudes or time zones stop The meridian is marked –12 hours west and –12 hours east. It is called the International Date Line, because when you cross this meridian the day must change. If you cross it going east, you go from Wednesday to Tuesday, and if you cross it going west, you go from Tuesday to Wednesday. This International Date Line can be demonstrated with a tape on which hours and days are marked Wrapped around the earth the old end must come off as the new end goes on when the earth rotates This shows that time is not just measured in hours but in days The date line does not pass through any lands.
NOON—the time when the sun is on your meridian, or on the circle that goes through the north and south poles and the point right over your head. Noon that has the sun really on the meridian is called "Apparent Noon" and it is 12:00 o'clock by the true sun, or the sundial. Mean Noon is the time by the mean sun or the imaginary sun that moves uniformly around the equator at the same rate all year like the hands of a clock. Then there is Standard Noon which is 12:00 o'clock when the sun in on one of the standard meridians on which a country or zone may base its time. Thus in the United States we have Eastern Standard Noon when the mean sun is on the 75th meridian west of Greenwich.
OBELISK—is the monument that points straight up and has four sides It is the original style of the one used by the Egyptians for measuring time by the sun. There is an obelisk in front of the big church of St Peter in Rome and in Central Park in New York City, and the best known one, of course, is the Washington Monument.
OBLIQUITY—is the angle of the plane of the earth's orbit with the plane of its equator.
OBSERVER—is the point of origin from which we observe. When you look up into the sky at the stars, you are at your origin, which is a point on the sphere of the earth. You are in the center of the horizon, and you must refer the things you see to their altitude above the horizon and to their angle around it from the north point. This is an apparent position as seen by an observer. This position has to be changed by placing the observer at the center of the earth or in a "geocentric" position but with the same observations of altitude and azimuth. Unfortunately this apparent motion is changing as the earth, so that it depends on time. The observer then has to calculate the right ascension and declination of the body or star he is observing, so that she/he no longer fits into the observation and anyone else in the world or in time can use this same observation.
ORBIT—is the path followed by astronomical bodies that move under the pull or attraction of gravity. There are only four possible orbits for gravity. These are circles, ellipses, parabolas, and hyperbolas. Spirals and other figures cannot be orbits under gravity, but they may be combinations of circles, etc., combined with- other pulls. Old satellites fall into the earth again because they are not just pulled by gravity, but they are slowed down in their orbits by friction or drag in the air or- atmosphere of the earth. These spiral into the earth like the shuttles because they are slowing down.
PARALLEL—any two straight lines that never meet or touch are said to be parallel. Two circles may be parallel also as long as their centers are always at thee same point. Latitude 30 degrees is always parallel to the equator. Hence it is called a "parallel of latitude."
PERPENDICULAR—a line making a right or 90 degree angle with another line.
P.M.—"Post Meridiem" or after noon.
POINT—mathematically invisible because a point has no dimensions, not length, breadth, or thickness. When printed it has dimensions and is called a "dot." When a dot moves it makes a line. When a line moves at an angle it makes a plane. When a plane moves it makes a solid (like a layer cake).
PROTRACTOR—is a circular ruler for measuring angles.
RAYS—this applies mostly to light, whether a visible ray like red, yellow, blue, green, etc., or an X-ray, but not a cosmic ray, which is a bad name for an atom hitting the earth.
REVOLUTION—is a little more general than rotating. The earth rotates on its axis, but it revolves in an orbit around the sun. Planets, moons, and comets, not to forget satellites, revolve in orbits. Moons revolve around planets and planets with comets revolve around the sun.
RIGHT ANGLE—when an angle is 90 degrees exactly, it is called a right angle To construct a right angle with a ruler, you draw a line three inches long in one direction, and another four inches long nearly vertical to it. Then when you connect the ends of the two lines with a line five inches long, the angle is a right angle. This convention is useful for laying out the corners of a basketball court or football field, only you probably would want to scale it up to three, four, and five meters.
ROTATION—an astronomical body spins around an axis that runs through the body's center of mass.
SOLAR—an adjective for anything pertaining to the sun, like solar time, solar activity (sunspots. flares, prominences, filaments, plages, and eclipses, etc.
SPHERE—when you rotate a circle about any two of its poles you make a sphere, which has thc same radius from its center to its surface in all directions. The sky looks like a half sphere or a hemisphere to anyone who lies on his back and looks up. Estimates of the sky's height to the eye vary, but strange as it may sound, the most common estimate is about 300 feet or roughly about a hundred meters. That's just about the length of a football field. This shows that our eyes are not very accurate measurers of distance, and we need special instruments for that purpose. But the heavens arc called the "dome of the sky" and from earliest times of thc Greek astronomer Aristarchus, at least the stars were considered as fastened to the celestial sphere. If you are a follower of Ptolemy, you will say the earth sees the celestial sphere rotate around it everyday. If you follow Copernicus you will believe or rather know that the earth rotates inside of the celestial sphere once a day.
STELLAR MAGNITUDE—is
a scale used to measure the brightness of stars, planets, moons, and other
astronomical bodies. It ranges above and below zero, with the brightest stars
measuring in negative numbers, and the faintest stars in the positive. There are
two kinds of measurement listed: apparent and absolute. Apparent is the
brightness as measured from the surface of Earth. Absolute is the brightness as
measured from a fixed relative distance from the object. For example, the
Sun’s apparent magnitude is –26.7. It is the brightest object in the sky
over Earth, because it is so close. Spica, a star that is much bigger than the
Sun, has an apparent magnitude of +1.2. But there absolute magnitudes—the Sun
is +4.8 and Spica is –2.3—;which are measured from a fixed distance of 10
parsecs, show that Spica is actually a brighter star.
SUNRISE—as the earth is rotating around on its axis, the sun will appear to move just like the stars during the night. Like the stars, the sun is visible only when it is above the horizon. The sun rises only when the place where YOU are turns to face the sun. We speak of its "rising" because it just looks that way.
SUNSET—sunset is just the opposite of sunrise. As the place where you are turns away from the sun with the rotating of the earth, the sun disappears below the horizon. We say it is setting because it "sits" on the horizon, just as it goes down.
TIME—is a measure of the rotation of the earth. If you were to stand on the sun and watch the earth go around you would measure about 24 hours of solar time. Since we all get up and go to bed with the sun it is the time we follow. Actually we do not have to stand on the sun to watch the earth go around. We can do it with a gnomon, an obelisk, or a sundial. Unfortunately, the time by the sun is not always the same for every day. On the average the day is 24 hours long, but this is kept only by a clock or watch. So, we have clocks that keep average time for the day, and these show that the sun time that we measure with a gnomon is 15 minutes fast in July and 15 minutes slow in February. Sun time cannot be measured from the sunset or from the sunrise if we want any accuracy, because the sun does not have the same declination all year long. The clock therefore keeps time for an imaginary sun that is moving uniformly around the earth's equator. Even clock time becomes very mixed up, because the earth does not rotate at the same speed every day or every year. The spinning earth is a poor time keeper, and now astronomers are using the moon instead of the earth to keep time.
In this year (1986) the earth lost about 56 seconds as a time keeper. It may start gaining that in the future. The climate has an effect on the rotation of the earth. As the ice melts on the polar caps either at the north or south the level of the oceans (sea level) gets higher. As this happens the radius of the earth gets bigger and the earth takes more time to rotate. The science of clocks for keeping time is very old. They began in a very simple way. The "klepshydra" were pitchers of water with a hole in them, so that the water flowed out steadily. The hours were then measured by the level of the water in the pitcher. Other time keepers were candles that burned down steadily and hours were measured by the height or length of the candle.
After Galileo discovered the pendulum by watching a hanging lamp in a church people began to make clocks and pendula. These became very accurate and they are still used, but pendula are no good when one carries them around. So, people invented watches or clocks that ran with springs and had to be wound up every day. These could be carried around and used on ships, so the ship could remember the time it had when it left its home port and then by observing the sun time with a gnomon or sextant, they could find their longitude from home. These very accurate watches are called chronometers because they have special parts to keep them regular.
ZENITH—if
you lie on your back and look at the sky directly above you, you are looking at
your zenith. If you move and lie down in another place, your zenith will move
with you. This moving point in the sky is useful to you because it serves as a
“point of reference,” or a starting point, from which you can measure the
location of objects in the sky overhead. Directly under you and through
to the other side of the earth and beyond is your nadir. It also moves
with you and can serve as a point of reference.