Hour angleIn astronomy and celestial navigation, the hour angle is the angle between two planes: one containing Earth's axis and the zenith (the meridian plane), and the other containing Earth's axis and a given point of interest (the hour circle). It may be given in degrees, time, or rotations depending on the application. The angle may be expressed as negative east of the meridian plane and positive west of the meridian plane, or as positive westward from 0° to 360°. The angle may be measured in degrees or in time, with 24h = 360° exactly.
Astronomical nutationAstronomical nutation is a phenomenon which causes the orientation of the axis of rotation of a spinning astronomical object to vary over time. It is caused by the gravitational forces of other nearby bodies acting upon the spinning object. Although they are caused by the same effect operating over different timescales, astronomers usually make a distinction between precession, which is a steady long-term change in the axis of rotation, and nutation, which is the combined effect of similar shorter-term variations.
Hour circleIn astronomy, the hour circle is the great circle through a given object and the two celestial poles. Together with declination and distance (from the planet's centre of mass), it determines the location of any celestial object. As such, it is a higher concept than the meridian as defined in astronomy, which takes account of the terrain and depth to the centre of Earth at a ground observer's location. The hour circles, specifically, are perfect circles perpendicular (at right angles) to the celestial equator.
Equatorial mountAn equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis, called polar axis, parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any celestial object with diurnal motion by driving one axis at a constant speed. Such an arrangement is called a sidereal drive or clock drive.
TimocharisTimocharis of Alexandria (Τιμόχαρις or Τιμοχάρης, gen. Τιμοχάρους; c. 320–260 BC) was a Greek astronomer and philosopher. Likely born in Alexandria, he was a contemporary of Euclid. What little is known about Timocharis comes from citations by Ptolemy in the Almagest. These indicate that Timocharis worked in Alexandria during the 290s and 280s BC. Ptolemy lists the declination of 18 stars as recorded by Timocharis or Aristillus in roughly the year 290 BC.
Angular distanceAngular distance or angular separation, also known as apparent distance or apparent separation, denoted , is the angle between the two sightlines, or between two point objects as viewed from an observer. Angular distance appears in mathematics (in particular geometry and trigonometry) and all natural sciences (e.g., kinematics, astronomy, and geophysics). In the classical mechanics of rotating objects, it appears alongside angular velocity, angular acceleration, angular momentum, moment of inertia and torque.
