Terrestrial Coordinates: The system used to locate where you are on Earth. Terrestrial coordinates are latitude and longitude.

Latitude: A coordinate measured north (N) or south (S) of the equator on the Earth, in degrees. The North Pole is 90^oN. Latitudes S are sometimes (but not usually) given as negative numbers.

Longitude: A coordinate measured east (E) or west (W) of the Prime Meridian. Longitudes W are sometimes (but not usually) given as negative numbers.

Equator : A line around the Earth 90^o from the North Pole or the South Pole; the line of latitude = 0^o.

Prime Meridian: (Also called Greenwich Meridian) A line of constant longitude (=0^o) running from the North Pole through Greenwich, England, to the South Pole.

Horizon Coordinates: The system used to locate a position in the observer's sky in terms of altitude and azimuth. The horizon system coordinates of a star or other celestial object change from minute to minute as the Earth spins.

Horizon: This may refer to the observer's true horizon - where the observer's sky meets the ground - or the astronomical horizon, located 90^o from the Zenith.

Rotate:  A body that spins about an axis through itself is rotating. 

Revolve:  A body that orbits around another object or a point in space external to itself is revolving about that object or point.

Altitude: A coordinate measured up from the observer's horizon along a line (the altitude line) through the Zenith.

Azimuth: A coordinate measured around from North (0^o) through East (90^o), South (180^o), and West (270^o) along the horizon to the altitude line for an object.

Zenith: The point in the sky directly above an observer; altitude = 90^o.

Nadir: The point directly below and observer; altitude = -90^o.

North Point: The point on an observer's horizon closest to the North Pole, located directly north of the observer. Altitude = 0^o, azimuth = 0^o for the North Point.

South Point: The point on an observer's horizon closest to the South Pole, located directly south of the observer. Altitude = 0^o, azimuth = 180^o for the South Point.

Meridian: The observer's meridian is a line running from the north
point on the horizon through the zenith to the south point. When the Sun crosses the Meridian it is solar noon. The "Prime Meridian" is a line on Earth that runs from the North Pole through Greenwich, England to the South Pole.

Apparent Motion: As a result of the motions of the Earth - rotation, revolution about the Sun - the stars appear to move across the sky, and the Sun appears to move against the stars. This motion is not really something that those bodies do, but rather are a result of our motion. Hence, they are "apparent" motions.

Circumpolar: From any part of Earth except the equator, there is one point in the sky around which the rest of the sky appears to turn over the course of a day and night. This is the North Celestial Pole in the northern hemisphere, or the South Celestial Pole in the southern hemisphere. Stars that are close to the pole in the sky will make small circles around it, while stars farther away make bigger circles. Any star whose daily circle is entirely above the horizon is called circumpolar.

Equatorial Coordinates: A system used to locate stars in the sky that gives their position for a given epoch (year or date). Equatorial coordinates, declination and right ascension, for stars change very slowly due to precession of the Earth's axis and motions of the stars through space.  The Sun's right ascension progresses from 0^h at the Vernal Equinox to 12^h at the Autumnal Equinox and on to 24^h=0^h again at the next VE, and the moon and the other planets also "wander" in RA and dec.

Declination: A coordinate measured north (+) or south (-) from the Celestial Equator. The North Celestial Pole has declination +90^o; the South Celestial Pole has declination -90^o.

Right Ascension: A coordinate measured eastward from the Vernal Equinox.  Right ascension or RA is expressed in hours, minutes, and seconds and runs from 0^h to 24^h.

Vernal Equinox: (a) The date when the Sun passes the Celestial Equator moving northward, usually around March 21. (b) The point in the sky where the Sun passes the Celestial Equator moving northward. This is one of the two places where the Celestial Equator crosses the Ecliptic. The position of the Vernal Equinox in the sky moves along the ecliptic very slowly, making a complete circuit every 26,000 years, as a result of the precession of Earth's axis.

Celestial Equator: A line on the Celestial Sphere that consists of all points directly above the Earth's equator. It may also be thought of as the intersection of the Celestial Sphere with the plane of the Earth's equator. The Celestial Equator's position among the stars shifts very slowly as the Earth's axis precesses, making one circuit every 26,000 years.

North Celestial Pole: The point in the sky directly above the Earth's north pole. The declination of the North Celestial Pole is +90^o.

Diurnal:  Having to do with a day.  In particular, we use this to refer to the Earth's rotation about its axis once each day, and the apparent motion of the sky about the Earth that results.

Annual:  Having to do with a year.  In particular, we use this to refer to the Earth's revolution about the Sun once each year, and the apparent motion of the Sun against the stars that results.

Solar time:  This is the time that is given by a traditional sundial.  Noon solar time is when the Sun crosses your meridian; an hour later is 1:00 PM solar time, etc. 

Mean solar time:  This is the time given by a steady clock that on the average matches a traditional sundial.  Sundials run fast or slow by up to 15 minutes relative to a steady clock, an effect called "The Equation of Time".

Zone time (or Standard time):  This is the time chosen to apply uniformly across a geographic region.  Usually, but not always, zone time = mean time for some location within the zone.  Central Standard Time is mean solar time for 90^oW longitude.  Universal Time or Greenwich Mean Time is mean solar time for 0^o longitude.

Daylight Savings Time:  This is standard time plus one hour.  If it is 4PM standard time then it is 5 PM Daylight Savings Time.  The purpose is to put more of the daylight hours into the evening.

International Date Line:  An agreed-upon line near 180^oW = 180^oE longitude, where the date on the West side of the line is 1 day later than the date on the East side of the line. 

Equation of Time:  The difference between solar time and mean solar time.  This difference can amount to as much as 15 minutes and is the result of (a) the tilt of the Earth's axis and (b) the eccentricity of Earth's orbit.

Eccentricity:  An eccentric orbit around the Sun has the Sun off-center (eccentric) and the orbiting body has variable speed, moving faster when it is closer to the Sun.  The second course, EveningStar, presents more background about orbits.

Ephemeris Time: The Earth is not a perfect clock; changes in where the snow is deposited from season to season and other effects cause it to run a little slower or a little faster, and the effect of the tides is to gradually slow Earth's rotation. The effect is no more than one part in 10 million, but that adds (or subtracts) a second to some years. These are the "leap seconds" you may read about in the newspaper around New Year's day. Astronomers maintain ephemeris time = UT as of 1900 = what you would get without the leap seconds so that there is a uniform basis for interpreting observations.

24 Hour Clock: The Earth rotates once every 24 hours. While we usually use clocks and watches that indicate the hours as 1-12AM and 1-12PM, it is sometimes more convenient to start with 0 at midnight and run to 24 at the next midnight, with 12 = noon.

Sidereal Time: We usually mark time by observing where the Sun is in the sky, at least in principle. If we were to use any star or special point in the sky instead, we would find that our normal clocks all run 4 minutes fast. A given star rises four minutes earlier each day, as marked by our usual solar (or mean solar or zone or universal) time. Time as indicated by the stars is sidereal time; we use the position of the Vernal Equinox as the marker, with 0hours
occuring when the Vernal Equinox transits the meridian.

Transit: This is short for "transit the meridian", or cross the meridian. When the Sun transits the meridian we have solar noon.
When the Vernal Equinox transits the meridian we have 0hours sidereal time. When any star or planet transits the meridian it reaches its greatest altitude for the day. A meridian diagram is a means of showing (or working out) the altitude at which various things will transit.

Meridian Diagram: A diagram that includes the North and South Point on the horizon, the North or South Celestial Pole, the Zenith, and the Celestial Equator where it crosses the meridian. The meridian is represented as a semi-circle running from the North Point to the South Point.

Precession: In general, the tendency of a spinning body under the influence of external forces to change the direction of its axis of rotation. In particular, the precession of the Earth's spin axis resulting from gravitational forces originating in the Sun, the Moon and the other planets. Precession of Earth's axis changes the equatorial coordinates of stars, but very slowly; the effect of precession on the declination of a star is at most about 5 parts in 10,000 (or about 0.05%) per year.

Summer Solstice: The date when the Sun reaches its maximum northern declination (23.5^o) and begins to move southward, approximately June 21. Sometimes called "midsummer" or "the first day of summer".

Autumnal Equinox: The date in the fall when the Sun rises due East and is up exactly 12 hours, usually approximately September 21. Sometimes called "the first day of fall".

Winter Solstice: The date when the Sun reaches its maximum southern declination (-23.5^o) and begins to move southward, approximately December 21. Sometimes called "the first day of winter".

Seasonal delay: The delay of a few days to two months between the summer solstice and the hottest days of the year.

Arctic and Antarctic: The part of Earth's surface from 66.5^oN to 90 ^oN (the Arctic) or 66.5^oS to 90^oS (the Antarctic) where the Sun may be entirely above the horizon for 24 hours or longer ("midnight Sun"), or entirely below the horizon for 24 hours or more.

Temperate Zone: The part of the Earth's surface from 23.5^o to 66.5^oN or S.

Tropics: The part of Earth's surface between latitude = 23.5^oN (the Tropic of Cancer) and 23.5^oS (the Tropic of Capricorn). In the tropics the Sun passes through the observer's Zenith once or twice each year.