Historical Astronomy: Concepts: Terminology

This is just an alphabetical list of (I think) the most confusing or specific things in this unit. Please let me know if you would like terms added to this list. It is not meant to be full-blown explanations of each item.

Almagest This was the Arabic name for Ptolemy's geocentric model of the universe, and is the common way of refering to his work, as the early influential Latin translations of the book were based on the Arabic versions of the 9th century. (The Greek name was Mathematical Syntaxis.)

Aphelion Aphelion is the point on an orbit that is farthest from the sun. It lies on the Major Axis of the orbit.

Apogee Apogee is the point on a satellite's orbit that is farthest from the earth. It lies on the Major Axis of the orbit.

Astronomical Unit The Astronomical Unit, or AU, is simply the average distance between the earth and the sun, which is about 1.5 x 1011 meters. This is commonly used when comparing planets and asteroids. It wasn't accurately measured until telescopic observations of the nineteenth century.

Celestial Equator The equator of the earth mapped onto the celestial sphere. It is the line that is perpendicular to both of the celestial poles.

Celestial Sphere It is easy to think of the stars as being attached to a big sphere with the earth at the center of the sphere.

Commentariolus This was the little manuscript written by Copernicus that we looked at in class. It gave an overview of the key features of his heliocentric model and was circulated around 1511. It wasn't until 1543 the he finally published the full mathematical description of his model.

Conjunction A term used to describe when astronomical bodies are lined up (or nearly lined up.) A planet is in conjuction with the sun when they have the same longitude along the ecliptic. (Put another way, a planet is in conjunction when its angle of elongation is 0.) Superior planets are in conjunction with the sun when they are "behind" the sun from the earth's point of view, so that they appear lined up behind the sun. Inferior planets have 2 conjunctions: Superior, when the sun is between the earth and the planet, and Inferior, when the planet is bewteen the earth and the sun.

Deferent Both Ptolemy and Copernicus used two different circles to carry a planet in its orbit. The main circle was called the deferent. The radius of the deferent was (almost) the average distance from the planet to either the sun or the earth (depending on the center of the solar system.)

Eccentric In the epicyclic model of the solar system, the orbits of the planets were circular, but the earth was not at the exact center of that circular orbit. The small distance between the earth and the center of the circular orbit was called the eccentric.

Ecliptic The ecliptic is the apparant path of the sun on the celestial sphere, as seen from the earth. (It is the plane of the earth's orbit projected onto the celestial sphere.) The sun moves roughly 1 degree a day along the ecliptic.

Elongation The angle between the sun and a planet along the ecliptic is called the angle of elongation. For example, if the sun is located at 20 degrees along the ecliptic and Mars is located at 50 degrees along the ecliptic, then the angle of elongation is 30 degrees: Mars is 30 degrees away from the sun. (Opposition is when the angle of elongation is 180 degrees. Conjunction is when the angle of elongation is 0 degrees.)

Epicycle The epicycle was the smaller secondary circle that carried the planet in its orbit around the earth or the sun (for Ptolemy and Copernicus.) The center of the epicycle was on the deferent, and the center of the deferent was near the earth or the sun.

Equant In Ptolemy's model of the solar system, the planets traveled in a circular orbit around the earth. Instead of making the planet travel around the orbit at constant angular speed as seen from the center of the orbit, Ptolemy had the planet travel with a constant angular speed as seen from a special spot he called the equant point. This has the effect of making the planet travel around the orbit with a varying linear speed. The Equant was Ptolemy's big "improvement" to the Greek epicyclic model of the solar system. While it greatly improved the accuracy and predictive power of the model, it was controversial as many thought it violated the Aristotelian idea of the Principle of Uniform Circular Motion.

Equinox There are two equinoxes - one in the spring and one in the fall. The equinoxes are the two times of year when the sun is at the point along the ecliptic that crosses the celestial equator. If there was no atmosphere on the earth, there would be exactly equal amounts of day and night. (Due to the atmospheric bending of sunlight, there is actually more than 12 hours of daylight on the equinoes.

Geocentric Literally "earth-centered." A geocentric model of the solar system is one with the earth as the center of the solar system. The culmination of geocentric models was the work of Ptolemy.

Heliocentric Literally "sun-centered." A heliocentric model of the solar system is one with the sun as the center of the solar system, which is basically what is true.

Hippopede The figure-8 curve that is the essential ingrediant in Eudoxus' planetary model, which was based on concentric spheres with non-parallel axes rotating in opposite directions. (Look it up on Wikipedia if you want a more official math definition.)

Homocentric Literally, one-centered. The very first geometric model of the solar system was the homocentric model of Eudoxus. All the orbits of the planets and the earth had the exact same center. Note that the geocentric model of Ptolemy didn't actually have the earth at the exact center of every planet's orbit.

Latitude To specify the position of an object on the celestial sphere, we need two angles. Latitude is the degrees above or below the ecliptic, and so goes from 90 degrees to -90 degrees. (We did not worry about the latitiude of the Earth and Mars when we did the orbits.)

Longitude To specify the position of an object on the celestial sphere, we need two angles. Longitude is its angle along the ecliptic, and goes from 0 (the vernal equinox) to 360. (These were the angles we used in plotting the orbits of the Earth and Mars and for which conjunction, opposition etc are all defined.)

Inertia Inertia is the resistance of a body to changes in its motion. This means that an object at rest will tend to stay at rest, and a moving object tends to keep moving the same way (constant velocity - neither speed nor direction change.) The only way to change the motion of an object is to apply a net force on the object.

Noon The time of day when the sun is at its highest point in the sky. (Note that this is not true with the standardized time zones we have now.)

Opposition A planet is in opposition when it is in the opposite part of the sky compared to the sun. Put another way, it is when a planet and the sun are 180 degrees apart along the ecliptic (which means the angle of elongation is 180 degrees.) Put yet another way, it is when the earth is right in between the sun and the planet.

Parallax Parallax is the apparent change in position of an object due to the motion of the observer. For example, when you walk down the road and look at a tree, it appears that the tree is moving backwards, but this is only because you are moving forwards.

Perigee Perigee is the point on a satellites orbit that is closest to the earth. It lies on the Major Axis of the orbit.

Perihelion Perihelion is the point on an orbit that is closest to the sun. It lies on the Major Axis of the orbit.

Polaris Polaris is the name of the North Star. It is a realtively bright star located almost at the North Celestial Pole, so that it doesn't move that much over the course of a night, and so is always located in the "northern" part of the sky.

Precession of the Equinoxes First noticed by Hipparchus, this is the roughly 1 degree per century advancement of the vernal equinox along the ecliptic. The earth rotates once per day on its axis - and its axis is almost always pointed in the same direction, which is currently very close to Polaris. The rotation does rotate in 26,000 year cycle due to a very slight seasonal net torque on the earth due to the fact that the earth isn't a perfect sphere.

Quadrature The point in a superior planet's orbit when it is 90 degrees from the sun. Another way of saying that is when the angle of elongation for the planet is 90 degrees. (Note that an inferior planet can never be in quadrature, because it is always inside the orbit of the earth, and the angle of elongation is always less than 90.)

Retrograde Motion The apparent backwards motion of a planet along through the zodiac. (If one follows the motion of a planet against the Celestial Sphere, the planets usually move in one direction, moving east through the Zodiac. However, on a regular interval, each planet will travel backwards, meaning west, for a few weeks. (The intervals and the amount of time vary for each planet.) This brief time of backwards motion is called "retrograde" motion.)

Sidereal The sidereal period of a planet is the time it takes the planet to orbit the sun. (This applies to anything orbiting the sun.) "Sidereal" means with respect to the stars, and this is because when we talk about the time it takes to go around the sun, we are using the fixed stars as the reference frame.

Sidereus Nuncius If you don't know what this is, and the test is tomorrow, you have set new standards for not paying attention in class. This was Galileo's book in which he announces his first astronomical discoveries with the telescope.

Solstice There are two solstices - one in winter and one in summer. The two solstices are the two times of year when the sun is at the point along the ecliptic that is farthest from the celestial equator that it ever is. This is why the summer solstice has the longest day of the year and the winter solstice has the shortest day of the year. This is also why the sun gets to its highest position in the sky on the summer solstice. In the southern hemisphere, this is the opposite - and not really true at all close to the equator.

Synodic Period The synodic period of a planet is the time it takes for the planet to return to the same position, relative to the sun, as seen from the earth. Put another way, if we go out and measure the angular separation between the planet and sun, the synodic period is the time it will take to return to this same angular separation. An easy way to think about this is the time between oppositions, or the time between conjuctions. "Synodic" means with respect to the sun. While this may seem a strange way to measure periods, it is the synodic period that is directly measurable from the earth. (The sidereal period of a planet cannot be directly measured because the earth itself is moving.)

Zodiac The band of constellations that goes along the ecliptic. By convention, the ecliptic is divided into twelve equally spaced regions, each 30 degrees wide.