| Julian Day (d) | |
|---|---|
| Julian epoch (100y) | |
| Solar mean anomaly (M°) | |
| Solar mean Longitude (L°) | |
| Solar Equation of Center (C°) | |
| Solar true Longitude (°) | |
| Solar apparent Longitude (λ°) | |
| Solar true Anomaly (ν°) | |
| Earth orbit eccentricity | |
| Solar radius vector (AU) | |
| Obliquity of the ecliptic (ε°) | |
| Solar Right ascension (α°) | |
| Solar declination (δ°) | |
| Equation of time (min) | |
| Greenwich hour angle (°) | |
| Local hour angle (°) | |
| Solar zenith angle (°) | |
| Solar elevation (°) | |
| Solar azimuth (°) |
| Solar rise & set times | |||
|---|---|---|---|
| Sunrise | Solar noon | Sunset | |
| Local time | |||
| UTC time | |||
| Length of day | |||
| Azimuth | Angular distance between the north point on the observer horizon and the intersection of the object's meridian with the horizon, measured westwards |
|---|---|
| Declination (δ°) | Equivalent of terrestrial latitude on the celestial sphere. Angular distance north or south of the celestial equator. |
| Eccentricity | The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptical orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. |
| Equation of Center (C) | For body in an elliptical orbit, the difference between the actual angular position in the elliptical orbit and the position the orbiting body would have if its angular motion was uniform. It arises from the ellipticity of the orbit, is zero at periapsis and apoapsis, and reaches its greatest amount nearly midway between these points. When this is calculated using a series expansion of Kepler's equation, the result is known as the equation of center. |
| Elevation | Angular distance between a body in the sky and observer horizon along the meridian of the body. Also called altitude |
| Equation of time | Quantifies the discrepancy between mean solar time, which tracks a fictitious "mean" sun with noons 24 hours apart, and apparent solar time,
which directly tracks the motion of the sun.
The equation of time is the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth.
It is the consequence of non-uniformity in the apparent daily motion of the Sun relative to the stars due to: (1)The obliquity of the ecliptic (the plane of the Earth's annual orbital motion around the Sun), which is inclined by about 23.44° relative to the plane of the Earth's equator. (2)The eccentricity of the Earth's orbit around the Sun, which is about 0.0167. The equation of time is constant only for a planet with zero axial tilt and zero orbital eccentricity. |
| Greenwich Hour Angle (GHA°) | In the equatorial coordinate system, the angular distance westwards from the Greenwich (prime) meridian to the meridian of a celestial body |
| Local Hour Angle (LHA°) | In the equatorial coordinate system, the angular distance westwards from the observer's meridian (the meridian passing from north to zenith) to the meridian of a celestial bodyLHA = GHA + observer longitude |
| Mean anomaly (M) | The fictious angular position that an orbiting body would have relative to its periapsis if its orbit were the auxiliary circle of its Keplerian orbit.
It is based on equal areas being swept in equal intervals of time by a line joining the focus and the orbiting body (Kepler's second law).
The mean anomaly increases uniformly from 0 to 2π radians during each orbit.
M = 2π(t-τ)/τ, where τ is the orbital period. |
| Mean Longitude (L) | The angular position that an orbiting body would have relative to the ascending node (vernal equinox) if its orbit were a circle. Mean longitude equals mean anomaly plus longitude of periapsis. |
| Nutation | A periodic variation in the uniform precession of the axis of any spinning body about the horizontal. The principal cause of the Earth's nutation is that the plane of the Moon's orbit around the Earth is tilted by about 5 degrees from the plane of the Earth's orbit around the Sun. This superimposes a small oscillation, with a period of 18.6 years and an amplitude of 9.2 seconds of arc |
| Obliquity of the Ecliptic (ε) | Angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. It is not the same as orbital inclination, which is the angle between a reference plane (usually the ecliptic) and the orbital axis. Therefore Earth's inclination is, by definition, zero. |
| Radius vector (R) | Instantaneous distance between orbited and orbiting bodies, usually expressed in AU |
| Right ascension (α°) | Equivalent of terrestrial longitude on the celestial sphere. Right ascension is measured eastward from the vernal equinox or the first point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north. Terrestrial spring occurs at solar α = 0°, autumn at solar α = 180° |
| True Anomaly (ν°) | It is the angle between the direction of periapsis and the current position of an orbiting body, as seen from the main focus of the ellipse (the point around which the object orbits). |
| True Longitude (°) | Angular position of an orbiting body relative to the ascending node. True longitude equals mean longitude plus equation of center. |
| Zenith Angle (Z°) | Angular distance from the zenith (the point directly overhead from the observer) and the observed position of a body in the sky along its meridian. The zenith angle is complementary to the elevation. |