Astronomy Chapter S1 Terms and Definitions

Solar Day

24 hours, which is the average time between appearances of the Sun on the meridian.

Leap Year

A calendar year with 366 rather than 365 days. Our current calendar (the Gregorian calendar) incorporates a leap year every 4 years (by adding February 29) except in century years that are not divisible by 400.

Sidereal Period

A planet's actual orbital period around the Sun.

Global Positioning System (GPS)

A system of navigation by satellites orbiting Earth.

Equation of Time

An equation describing the discrepancies between apparent and mean solar time.

Conjunction (of a Planet with the Sun)

An event in which a planet and the Sun line up in our sky.

Transit

An event in which a planet passes in front of a star (or the Sun) as seen from Earth. The search for transits of extra-solar planets is an important planet detection strategy.

Declination (DEC)

Analogous to latitude, but on the celestial sphere; it is the angular north-south distance between the celestial equator and a location on the celestial sphere.

Right Ascension (RA)

Analogous to longitude, but on the celestial sphere; it is the angular east-west distance between the spring equinox and a location on the celestial sphere.

Elongation (Greatest)

For Mercury or Venus, the point at which it appears farthest from the Sun in our sky.

Gregorian Calendar

Our modern calendar, introduced by Pope Gregory in 1582.

Universal Time (UT)

Standard time in Greenwich, England (or anywhere on the prime meridian).

Daylight Savings Time

Standard time plus 1 hour, so that the Sun appears on the meridian around 1 p.m. rather than noon.

Hour Angle

The angle or time (measured in hours) since an object was last on the meridian in the local sky; defined to be 0 hours for objects that are on the meridian.

Julian Calendar

The calendar introduced in 46 B.C. by Julius Caesar and used until the Gregorian calendar replaced it.

Tropic of Cancer

The circle on Earth with latitude 23.5 degrees N, which marks the northernmost latitude at which the Sun ever passes directly overhead (which it does at noon on the summer solstice).

Tropic of Capricorn

The circle on Earth with latitude 23.5 degrees S, which marks the southernmost latitude at which the Sun ever passes directly overhead (which it does at noon on the winter solstice).

Arctic Circle

The circle on Earth with latitude 66.5 degrees N.

Antarctic Circle

The circle on Earth with latitude 66.5 degrees S.

Celestial Coordinates

The coordinates of right ascension and declination that fix an object's position on the celestial sphere.

Analemma

The figure-8 path traced by the Sun over the course of a year when viewed at the same place and the same time each day; it represents the discrepancies between apparent and mean solar time.

Opposition

The point at which a planet appears opposite the Sun in our sky.

Synodic Period (of a Planet)

The time between successive alignments of a planet and the Sun in our sky; measured from opposition to opposition for a planet beyond Earth's orbit, or from superior conjunction for Mercury and Venus.

Tropical Year

The time from one spring equinox to the next, on which our calendar is based.

Sidereal Day

The time of 23 hours 56 minutes 4.09 seconds between successive appearances of any particular star on the meridian; essentially, the true rotation period of Earth.

Sidereal Year

The time required for Earth to complete exactly one orbit as measured against the stars; about 20 minutes longer than the tropical year on which our calendar is based.

Synodic Month (or Lunar Month)

The time required for a complete cycle of lunar phases, which averages about 29 1/2 days.

Sidereal Month

The time required for the Moon to orbit Earth (as measured against the stars); about 27 1/4 days.

Standard Time

Time measured according to the internationally recognized time zones.

Sidereal Time

Time measured according to the position of stars in the sky rather than the position of the Sun in the sky.

Apparent Solar Time

Time measured by the actual position of the Sun in your local sky, defined so that noon is when the Sun is on the meridian.

Mean Solar Time

Time measured by the average position of the Sun in your local sky over the course of the year.