Astronomy Definition of Terms (Jargon!)

Altitude and azimuth

The altitude-azimuth (alt-az) coordinate system. Azimuth measures an object’s position (in degrees) along the horizon, starting at north (0°) and passing through east, south, and west. Altitude measures height above the horizon (0°). An object overhead has the maximum altitude — 90°. So, an object halfway up in the sky due west has an altitude of 45° and an azimuth of 270°.

Apparent magnitude (m) - a number that is a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value (i.e. inverse relation), with the brightest astronomical objects having negative apparent magnitudes: e.g., Sirius at −1.46.

The dimmest object visible with the naked eye is Magnitude 6; visible with binoculars Magnitude 10.

Conjunction - an alignment of objects at the same celestial longitude. The conjunction of the moon and planets is determined with reference to the Sun. A planet is in conjunction with the Sun when it and the Earth are aligned on opposite sides of the Sun.

Declination - how high an object will rise in the sky. Like Earth’s latitude, Declination (Dec) measures north and south, in degrees, arcminutes (60 in a degree) and arcseconds (60 in an arcminute).

The ecliptic is the mean plane of the apparent path in the sky that the Sun follows over the course of one year; it is the basis of the ecliptic coordinate system. This plane of reference is coplanar with Earth's orbit around the Sun (and hence the Sun's apparent path around Earth).

Opposition - when a celestial body is in line with the Earth and Sun. During Opposition, an object is visible for the whole night, rising at sunset and setting at sunrise. At this point in it’s orbit, the object is closest to Earth making it appear bigger and brighter.

Right Ascension (RA) - is to the sky what longitude is the the surface of the Earth, corresponding to east and west directions. Measured in hours, minutes and seconds since we see different parts of the night sky throughout the night as the earth rotates.

Magnitude - see Apparent Magnitude and the AM scale, above. How bright the object appears from Earth, represented in a numeric scale; the lower the number the brighter the object. So a magnitude of -1 is brighter than an object with a magnitude of +2

Greatest elongation - when the inner planets, Mercury and Venus, are at their maximum distance from the Sun. During greatest elongation, distant planets can be observed as evening stars at greatest eastern elongations, and as morning stars during western elongations

Precession - any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the Earth, known as the precession of the equinoxes. Think of a wobbling spinning top.

The perihelion of any orbit of a celestial body about the Sun is the point where the body comes closest to the Sun. It is the opposite of aphelion, which is the point in the orbit where the celestial body is farthest from the Sun.

Wikipedia glossary of astronomy: https://en.wikipedia.org/wiki/Glossary_of_astronomy