Glossary
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This manual is written for people who run weather stations, not for astronomers. Where it uses a term of art, this page explains it.
Altitude — how high something is above the horizon, in degrees. 0° is the horizon, 90° is straight overhead. Not to be confused with your station’s elevation, which WeeWX also calls altitude.
Analemma — the figure-eight the sun traces if you photograph it at the same clock time all year. It is the sum of Earth’s axial tilt and its elliptical orbit.
Antitransit — the moment a body crosses the lower meridian: the midnight counterpart of transit, when it is as far below your horizon as it will get.
Apogee / perigee — the moon’s farthest and closest points from Earth. Collectively, apsides.
Aphelion / perihelion — Earth’s farthest and closest points from the sun (early July and early January). For a comet, perihelion is its closest approach to the sun — the moment it is likely to be brightest.
Astrometric position — a body’s position corrected for light travel time but not for the aberration and refraction that affect what you actually see. Contrast apparent and topocentric.
Azimuth — compass direction, in degrees: 0° north, 90° east, 180° south, 270° west.
Circumpolar — never sets, from your latitude. Its opposite here is neverup.
Colongitude — the selenographic longitude of the sunrise line on the moon, the standard way of saying which part of the lunar surface is lit.
Culmination — the highest point of a pass; for satellites, this manual’s word for transit.
Declination — celestial latitude: how far north or south of the celestial equator a body sits. Paired with right ascension.
Ecliptic — the plane of Earth’s orbit, and so the line the sun appears to follow through the year. The planets stay near it.
Elongation — the angle between a body and the sun as seen from Earth. Small elongation means the body is lost in the sun’s glare; large elongation means it is well placed.
Ephemeris — a table (here, JPL’s DE421 file) from which the positions of the sun, moon and planets are computed.
Equation of time — the difference between what a sundial reads and what a clock reads, swinging roughly ±16 minutes across the year. See the tag.
Geocentric — measured from Earth’s center. Contrast topocentric.
Heliocentric — measured from the sun’s center. The orrery panel is a heliocentric view.
Hipparcos — the ESA satellite mission whose catalog of 118,218 stars ships with this extension. A star’s catalog number is its HIP number.
Hour angle — how far a body is from your meridian, measured along its daily path: 0 at transit, negative before, positive after.
Libration — the moon’s slight rocking, which lets us see about 59% of its surface over time rather than exactly half.
Lunation — one complete cycle of moon phases, new moon to new moon, about 29.5 days.
Magnitude — brightness, on a backwards scale: smaller is brighter, and each step of 1 is about 2.5×. Sirius is −1.4; the naked-eye limit under a dark sky is about 6.5.
Meridian — the imaginary north–south line passing directly overhead. A body transits when it crosses it.
NORAD catalog number — the number that identifies a satellite: 25544 is the ISS.
OMM (Orbit Mean-Elements Message) — the standard format for satellite orbital elements that replaces the TLE, with room for catalog numbers past 99999. This extension fetches it as CSV, one header line and one row per satellite, and caches each satellite’s in its own file.
Parallactic angle — the angle between “up” as seen by an observer and celestial north, which is what tells you how a chart’s orientation relates to the view through a telescope.
Parallax — the tiny shift in a star’s apparent position as Earth orbits, and the only direct measure of its distance. A star without a measured parallax has no known distance, which is why this extension reports N/A rather than a fictitious number.
Plate — this manual’s word for one of the Sky page’s two color schemes, borrowed from printed star atlases: the night plate (night, the default dark theme) and the light plate (light, a paper-atlas look). A plate is chosen at generation time by the theme option and sets the page’s colors; an embedding skin can override them from its own stylesheet (see the role classes). See also the two plates.
Radiant — the point on the sky meteors in a shower appear to stream away from.
Refraction — the atmosphere bending light near the horizon, which makes bodies appear slightly higher than they are and shifts every rise and set time. It varies with the real atmosphere in ways no model captures — see Accuracy.
Right ascension — celestial longitude, the east–west companion to declination.
Sidereal time — time kept by the stars rather than the sun. A sidereal day is about four minutes shorter than a solar one, which is why the same stars rise four minutes earlier each night.
SGP4 — the standard model that turns a satellite’s published orbital elements into a position. Its input ages quickly, which is why this extension refuses elements more than seven days old.
Solar time — time as a sundial keeps it, running ahead of or behind clock time by the equation of time.
Supermoon — a full moon falling within about a day of perigee, so it appears slightly larger and brighter. This extension’s tag uses exactly that rule.
TLE (two-line element set) — the older text format for satellite orbital elements, which this extension fetched before 2.10 and still reads from a file you supply. Its catalog-number field holds five characters, so numbers past 99999 need the Alpha-5 form or OMM.
Topocentric — measured from where you stand on Earth’s surface, rather than from its center. This is what you actually see, and what most of this extension’s position tags report.
Transit — the moment a body crosses your meridian, which is when it is highest in your sky.
Twilight — the time between sunset and full darkness, in three defined stages: civil (sun 6° below the horizon; you can still read outside), nautical (12°; the horizon is still discernible at sea), and astronomical (18°; the sky is as dark as it will get).
ZHR (zenithal hourly rate) — the number of meteors an ideal observer would see per hour under a perfectly dark sky with the radiant directly overhead. Real counts are always lower, which is why the Sky page reports the moon’s illumination beside it.