The angle between the horizon and an object in the sky

The angle between due north on the horizon and a point on the horizon directly

below an object in the sky.

An imaginary circle on the Celestial Sphere that is the projection of the earth’s

equator up onto the Celestial Sphere. The CE divides the Celestial Sphere into a northern

hemisphere (where Declination measures are positive) and a southern hemisphere (where

Declination measures are negative). The Declination of the CE is 0°.

An imaginary sphere concentric with the earth but infinitely far away. The entire

Celestial Sphere rotates around the earth from east to west once per day. Objects on the Celestial

Sphere have coordinates of Right Ascension and Declination. The RA and Dec of the “fixed” stars

do not change significantly over short timescales. The RA and Dec of the Sun changes over the course of a year due to the orbit of the earth around the sun.

An object that never sets (i.e., crosses the horizon), but just circles the Celestial

Pole in the sky.

(Dec, or sometimes δ) The angle between an object on the Celestial Sphere and the

Celestial Equator. Declination is typically measured in degrees, arcminutes, and arcseconds.

Declination can be thought of as a kind of Celestial Latitude, since it gives the north-south position

of an object on the Celestial Sphere.

An imaginary circle on the Celestial Sphere that shows the path of the sun over the

course of a year. The Ecliptic is tilted 23.5° relative to the Celestial Equator.

A spherical coordinate system on the Celestial Sphere with the

Celestial Equator and the Vernal Equinox as reference points. The coordinates of the Equatorial

System are Right Ascension (measured east-west relative to the Vernal Equinox) and Declination

(measured north-south relative to the Celestial Equator).

Local Mean Solar Time at Greenwich Observatory, near London,

where longitude = 0°. 

An imaginary circle on the Celestial Sphere that shows where a plane tangent to the

earth at the point where the observer is standing intersects with the Celestial Sphere.

A coordinate (usually measured in hours, minutes, and seconds) that describes how

far east (negative HA) or west (positive HA) an object is on the Celestial Sphere relative to the local

Meridian. The hour angle of the sun plus 12 hours is equal to Local Apparent Solar Time.

A time system based on the rotation of the earth and the orbit

of the earth around the sun which references the sun relative to the local Meridian. When the sun

is on the Meridian, Local Apparent Solar Time is 12:00:00. Sundials measure Local Apparent Solar

Time.

A spherical coordinate system with the Horizon and the

Meridian as reference circles. The angle above the horizon is Altitude, and the Angle relative to

where the Meridian crosses the north horizon is Azimuth.

Time measured relative to a fictitious “mean” sun that moves

around the Celestial Sphere from west to east at a constant rate. The difference between mean

solar time and apparent solar time is called the Equation of Time.

A time system based solely on the rotation of the earth relative to the

stars. The reference point for LST is the Vernal Equinox on the Celestial Sphere. When the Vernal

Equinox crosses the Meridian, the LST is 00:00:00. A sidereal day is 4 minutes shorter than a

solar day, due to the orbit of the earth around the sun.

An imaginary circle in the local coordinate system that passes from the northern horizon,

through the North Celestial Pole, through the Zenith, and through the southern horizon. The

Meridian is a local circle that does not remain fixed on the Celestial Sphere—in fact, the Meridian

passes through all of the lines of Right Ascension in one sidereal day.

A reference point on the Celestial Sphere directly above the earth’s north

pole.

(RA, or sometimes α) A coordinate (usually measured in hours, minutes, and

seconds of time) that describes how far east an object is on the Celestial Sphere relative to the

Vernal Equinox. RA goes from 0 to 24 hours, increasing towards the east on the Celestial Sphere.

Right Ascension can be thought of as a kind of Celestial Longitude, since it specifies the east-west

position of an object on the Celestial Sphere.

The average length of time between new moons, i.e., the average time it takes

for the moon to come back to the same point relative to the sun. The synodic month is equal to

29.531 days.

The average length of time it takes the moon to orbit the earth relative to the

stars. The sidereal month is equal to 27.322 days.

The Local Mean Solar Time of your time zone, a roughly 15° wide swath of

longitude centered on longitude lines that are an integer number of hours east or west of

Greenwich, England. You generally set your watch by Standard Time, unless you happen to be

observing Daylight Savings Time.

A time system based on not on astronomy, but on atomic

clocks. UTC is the international standard for civil time. In the past, UTC has always been kept with

0.9 seconds of GMT (which is based on astronomy) by the occasional insertion of “leap seconds”

to account for the gradual slowing of the rotation of the earth.

The point on the Celestial Sphere where the sun appears to be on the spring

equinox. The Vernal Equinox is the (arbitrary) reference point for Right Ascension.

A reference point in the local coordinate system which is directly overhead, at altitude=90°.

The 13 constellations on the Celestial Sphere through which the Ecliptic passes. The

soon moves into a different constellation of the Zodiac approximately every month as it moves

along the Ecliptic.