Glossary of astronomical terms

Altitude (Alt)
The angular distance of an object to the horizon. Positive when above the horizon

The point in the orbit of a planet around the Sun which is the most distant from the Sun.

The point in the orbit of the Moon or an artificial satellite which is most distant from Earth

Astronomical twilight
The period before sunrise or after sunset when the Sun is between 12 and 18 degrees below the horizon.

Astronomical Unit (AU)
The average distance from Earth to the Sun. 1 AU = 149.6 million km. Used for measuring distances within the solar system

Azimuth (Az)
The horizonal direction to an object. Azimuth is measured clockwise around the horizon from a reference point, usually due north. Occasionally, azimuth is measured from due south leading to some confusion. AstroTools uses the convention of North = 0°

See magnitude

Civil twilight
The period before sunrise or after sunset when the Sun is between 0 and 6 degrees below the horizon.

Normally defined as the time when two objects have the same ecliptic longitude. When just one object is mentioned this means solar conjunction. The inner planets Mercury and Venus reach inferior conjunction when they are between Earth and Sun and superior conjunction when they are "behind" the Sun

The 'latitude' in the equatorial coordinate system. The angular distance from the equator. +90° is the celestial north pole, -90° is the south pole

When one object covers another object as seen from the Sun, thus leaving the second object in shadow. Solar eclipses happen when the Moon passes between Sun and Earth (at New Moon). A lunar eclipse happens when the Moon enters the shadow of the Earth (at Full Moon)

The apparent path of the Sun against the stars. Ecliptic coordinates is the coordinate system using the Ecliptic as the 'equator'. This system is practical for measuring positions of planets that in general stay close to the Ecliptic

The angular distance between an object and the Sun.

A fixed instant of time used as a reference point for a celestial coordinate system. Due to the precession of Earth's axis it is important to specify at which epoch a given set of coordinates is valid. Typically used are 'Epoch J2000.0' (1 January, 2000) and epoch of date. AstroTools uses epoch of date

Equatorial coordinates
The most common system for locating objects in the sky. In this system the celestial equator is the 'projection' of the terrestial equator onto the celestial sphere. The 'longitude' is called right ascension (RA), the 'latitude' is called declination

One of the two points where the ecliptic intersects the celestial equator and also the time when the Sun is at this point. The vernal equinox happens in Spring around 21 March and defines the reference point for both right ascension and ecliptic longitude coordinate scales. Due to Earth's precession the vernal equinox slowly moves across the celestial sky.

Full Moon
Defined as the time when the Moon reaches a difference in ecliptic longitude of 180° compared to the Sun. If the Moon is near the ecliptic at this time a lunar eclipse will happen. First Quarter or Third Quarter Moon happens when the difference in longitude is 90°

Geocentric position is the position as seen by an imaginary observer located at the center of the Earth. Coordinates of objects are always calculated for a geocentric location and then translated into topocentric coordinates (if necessary) taking parallax into account

The fraction of a planetary disk that is illuminated as seen from Earth.

Julian Day Number (JD)
The time format used in astronomical calculations (AstroTools uses JD too). The integer fraction is the day number starting at an arbitrary date a long time ago and the fraction is the fraction of a day since noon Greenwich time (12:00 UT)

Latitude (ecliptic)
Ecliptic latitude is the angular distance from the Ecliptic towards the ecliptic poles in the ecliptic system

Longitude (ecliptic)
Ecliptic longitude measures the position along the Ecliptic starting from the vernal equinox

A logarithmic brightness scale for measuring brightness of celestial objects. Higher magnitude values indicate fainter objects. A change in magnitude of 5 corresponds to a change in brightness by a factor of 100.

The vertical plan passing directly over the observer's head and intersecting the horizon due south and north. As the Earth rotates objects will reach their highest and lowest altitude when they cross the meridian. The crossing of the meridian at the highest point is called the transit

Nautical twilight
The period before sunrise or after sunset when the Sun is between 6 and 12 degrees below the horizon

New Moon
Defined as the time when the Moon reaches the same ecliptic longitude as the Sun. If the Moon is near the ecliptic at this time a solar eclipse will happen

When one object covers another object as seen from the Earth, e.g. when the Moon covers a planet or star

An outer planet is said to be in opposition to the Sun when the difference in ecliptic longitude is 180° relative to the Sun, i.e. when it is opposite the Sun. The object is then visible all night.

The angular difference in the direction to an object as seen by observers at different locations. Due to the large distances to most astronomical objects the parallax is usually very small (in the order of arcseconds or less), but for the nearby Moon the parallax can exceed one degree.

The point in the orbit of the Moon or an artificial satellite which is closest to the Earth

The point in the solar orbit of Earth or another planet which is closest to the Sun.

Position angle (PA)
The angle measured counterclockwise around a first object from north to a second object. Used for indicating the position of two planets or the components of a binary star relative to each other

The slow rotation of the celestial poles around the ecliptic poles in a circle with radius 23.5°. One rotation takes about 26000 years. This slow movement of the celestial poles against the stars also slowly changes the equatorial coordinates of a fixed object. Therefore, it is important to specify for which epoch the coordinates are valid.

A planet is said to be in quadrature with the Sun when the difference in ecliptic longitude is 90°. For the outer planets this corresponds roughly to the time when the planet shows the smallest phase (minimum illumination). Mercury and Venus cannot reach quadrature.

See 'Right ascension'

Right ascension (RA)
The 'longitude' in the sky in the equatorial coordinates system. Measured in hours, minutes and seconds, not degrees. The RA is counted from the vernal equinox, the point where the Sun crosses the equator in the Spring.

Rise, sunrise
The time of rise or set of an object does not happen when its calculated altitude is precisely 0 degrees. Atmospheric refraction causes the object to be visible slightly below the mathematical horizon. The amount of refraction depends on meteorological conditions but is normally assumed to be 34' (or 0.57°). Further, for the Sun and Moon the visibility of the upper limb defines the time of rise and set, thus the radius of the Sun or Moon (about 16') has to be taken into account. The standard definition of sunrise and set is thus when the center of the Sun is 34' + 16' = 50' (or 0.83°) below the horizon

Set, sunset
See 'Rise'

Sidereal time
The time with respect to the fixed stars. Sidereal time is equal to the right ascension of points currently transiting. Thus a fixed object will always transit at the same sidereal time of the day.

The time when the Sun reaches an ecliptic longitude of 90° (Summer Solstice) or 270° (Winter Solstice). This corresponds to the extrema in declination of the Sun.

The time when an object crosses the meridian at the highest point, i.e. when the object reaches the highest altitude in the sky.
Also: Passage of a small object in front of a larger body. E.g. transits of Mercury or Venus on the Sun or transits of Galilean Moons on Jupiter

Topocentric coordinates
The apparent coordinates of an object as seen by a real observer on the surface of the Earth. The parallax is then taken into account. The difference between geocentric and topocentric positions is insignificant for most purposes. However, for the Moon the difference becomes important because of the large parallax of the Moon (in the order of one degree). AstroTools only calculate topocentric positions for the Moon and only in altitude and azimuth (thus RA and Dec for Moon are geocentric!).

Universal Time (UT)
Time system that forms the basis for our civil time.

My web site: Ole's Astronomy Site