zig-astro #

Astronomical calculations: where bodies in the sky are, how they look from the Earth, and when notable configurations happen. The Earth's Moon comes first.

Language #

Time #

Instant: A single moment, given as a Julian Day together with the time scale it is counted in. Avoid: Timestamp, date, epoch (except for a theory's reference instant)

Time scale: The clock an instant is counted in: TT (Terrestrial Time), the uniform scale the theories are written in, or UT (Universal Time), which follows the Earth's rotation and is what civil time approximates. UTC, which never differs from UT by more than 0.9 s, is counted as UT. Avoid: Timezone

ΔT: The difference TT − UT at an instant, measured for the past and predicted for the future. Avoid: Delta-T offset, clock correction

Positions #

Apparent position: Where a body appears to be: corrected for light time, aberration and nutation, and referred to the true equator and equinox of the instant. It is either geocentric or topocentric, depending on the viewpoint. Avoid: Actual position, true position

Geocentric: Seen from the Earth's center.

Topocentric: Seen by an observer on the Earth's surface. A topocentric position differs from the geocentric one by the body's parallax.

Observer: A point on the Earth's surface, given by its geodetic latitude, its longitude counted positive eastwards from Greenwich, and its height above the reference ellipsoid. Avoid: Location, site (in the API), west longitude

Geodetic latitude: The angle between the equator and the perpendicular to the reference ellipsoid at a point: the latitude on maps. The geocentric latitude, measured at the Earth's center, differs from it by up to about 0.19°. Avoid: Latitude (where the two could be confused)

Parallax: The shift in a body's direction between the Earth's center and an observer on its surface: up to about 1° for the Moon, under 9″ for the Sun. Avoid: Diurnal parallax (say parallax)

Sidereal time: The hour angle of the vernal equinox. Greenwich sidereal time is measured at longitude 0°, local sidereal time at the observer. The mean form leaves out nutation; the apparent form includes it.

Hour angle: How far west of the observer's meridian a body is, measured along the equator: local sidereal time minus right ascension.

Horizontal coordinates: Azimuth and altitude, as an observer sees them. Azimuth is counted from north through east: 0° north, 90° east, 180° south, 270° west. Altitude is measured from the horizon, positive above it. Avoid: Elevation (say altitude), azimuth from south

Airless altitude: A body's altitude above the horizon as it would be with no atmosphere. Avoid: True altitude, geometric altitude

Observed altitude: A body's altitude after refraction: where the body is actually seen. It is higher than the airless altitude, by about 0.6° at the horizon. Avoid: Apparent altitude

Refraction: The bending of light by the Earth's atmosphere, which raises a body above its airless altitude.

Nutation: The short-period nodding of the Earth's axis, given as a shift in longitude and a shift in the obliquity.

Obliquity: The angle between the Earth's equator and the ecliptic at an instant. The mean obliquity leaves out nutation; the true obliquity includes it, and is the one that belongs with apparent positions. Avoid: Axial tilt (in calculations)

Ecliptic coordinates: Celestial longitude and latitude, measured along and from the ecliptic, with longitude counted eastwards from the vernal equinox. Avoid: Ecliptical coordinates

Equatorial coordinates: Right ascension and declination, measured along and from the celestial equator, with right ascension counted eastwards from the vernal equinox.

The Moon's phase #

Principal phase: One of the four instants in each lunation when the Moon's apparent geocentric longitude exceeds the Sun's by exactly 0°, 90°, 180° or 270°: new Moon, first quarter, full Moon and last quarter. Avoid: Quarter (for all four), moon phase (for the instant), third quarter (say last quarter)

Lunation: The interval from one new Moon to the next. Avoid: Lunar month, synodic month (for one particular interval)

Elongation: The angle between the Moon and the Sun as seen from the Earth's center, from 0° to 180°.

Elongation in longitude: The Moon's apparent ecliptic longitude minus the Sun's, from 0° to 360°: 0° at new Moon, 90° at first quarter, 180° at full Moon, 270° at last quarter. Under 180° the Moon is waxing, over 180° waning. Avoid: Elongation (for this quantity), age angle

Phase angle: The angle between the Sun and the Earth as seen from the Moon; 0° at full Moon and 180° at new Moon.

Illuminated fraction: The fraction of the Moon's disk, as seen from the Earth, that is lit by the Sun; 0 at new Moon and 1 at full Moon. Avoid: Illumination, percent illuminated

Bright limb: The edge of the Moon's disk that faces the Sun. Its position angle is measured from the north point of the disk towards the east.

Named phase: One of eight names for the Moon's appearance at an instant, each covering a 45° sector of elongation in longitude centered on a principal phase or midway between two: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, waning crescent. It is a property of the sky, not of a calendar day.

Lunar age: The time elapsed since the most recent new Moon. Avoid: Moon age (in days only)

Libration #

Libration: The apparent rocking of the Moon that brings parts of its far side into view from the Earth, given as a selenographic longitude and latitude of the point at the center of the visible disk. Avoid: Libation, wobble

Optical libration: The part of libration that comes only from the geometry of the Moon's orbit and the tilt of its axis, with the Moon assumed to rotate uniformly.

Physical libration: The part of libration that comes from real irregularities in the Moon's rotation; a few hundredths of a degree.

Topocentric libration: Libration as seen by an observer on the Earth's surface rather than from the Earth's center; it differs by up to about a degree.

Position angle of the axis: The direction of the Moon's north rotational pole on the sky, measured from the north point of the disk towards the east.

Selenographic coordinates: Longitude and latitude on the Moon's surface, measured from its mean Earth-facing meridian and its equator, with longitude positive towards Mare Crisium (selenographic east). Avoid: Lunar coordinates

Subsolar point: The point on the Moon's surface where the Sun is directly overhead, given in selenographic coordinates. Avoid: Selenographic Sun (except as shorthand for its coordinates)

Terminator: The line on the Moon's surface dividing day from night; the morning terminator is where the Sun is rising.

Colongitude: The selenographic longitude of the morning terminator, measured as 90° minus the subsolar point's longitude.