Definition

1. Noun:
   • (Astronomy): The angular distance measured westward along the celestial equator from the observer's local meridian (the line from the north celestial pole through the zenith to the south celestial pole) to the hour circle of a specific celestial body. It indicates how much time has passed since the object crossed the meridian.
   • (Astronomy): A coordinate used in the equatorial coordinate system, representing the angular distance of a point on the celestial sphere measured westward from the observer's meridian along the celestial equator.

Usage Examples

• Noun:
  • The astronomer calculated the hour angle of the star to determine its position relative to the local meridian.
  • Knowing the hour angle and the local sidereal time allows you to find the right ascension of a celestial object.

Advanced Usage

• Sidereal Hour Angle (SHA): A specific measurement where the angle is measured westward from the vernal equinox to the hour circle of a body, often used in navigation and astronomy.
  • The navigator used the star's sidereal hour angle for celestial navigation.
• Local Hour Angle (LHA): The angle measured westward from the observer's local meridian to the meridian (hour circle) of a celestial body.
  • Calculating the local hour angle is a key step in determining a line of position from a sextant sight.

Variants and Related Words

• Hour Circle: A great circle on the celestial sphere that passes through the celestial poles and a given celestial body. The hour angle is measured along the celestial equator to this circle.
• Right Ascension: A celestial coordinate measured eastward along the celestial equator from the vernal equinox. The hour angle of an object is related to the local sidereal time and its right ascension.

Synonyms

• Angular distance west of the meridian (descriptive synonym).
• HA (common abbreviation).

Related Phrases (Technical Contexts)

• Greenwich Hour Angle (GHA): The angular distance west of the Greenwich celestial meridian.
  • The nautical almanac lists the Greenwich Hour Angle of celestial bodies for each hour.
• To compute the hour angle: A standard phrase describing the calculation process in astronomy and navigation.
  • The first step was to compute the hour angle from the observed local time.

Noun

1. (astronomy) the angular distance of a celestial point measured westward along the celestial equator from the zenith crossing; the right ascension for an observer at a particular location and time of day
2. the angular distance along the celestial equator from the observer's meridian to the hour circle of a given celestial body