Troubleshooting

Glossary

Plain definitions. Where a term has a common misunderstanding, it is flagged.

Time

UTC: Coordinated Universal Time. The global reference. Not a timezone; the thing timezones are offsets from.

Local civil time: the clock on your wall, including daylight saving.

Julian Day (JD): days elapsed since 1 January 4713 BC noon, as a decimal. Turns date arithmetic into ordinary subtraction. J2000.0 = JD 2451545.0.

Sidereal time: time measured against the stars rather than the Sun. A sidereal day is 23h 56m 04s.

Equation of time: the difference between sundial time and clock time, swinging ±16 minutes over the year. Caused by Earth's elliptical orbit and axial tilt.

Tropical year: 365.2422 days, equinox to equinox. Why leap years exist.

Leap second: an occasional extra second keeping UTC aligned with Earth's slightly irregular rotation.

Position

Latitude: degrees north (+) or south (−) of the equator.

Longitude: degrees east or west of Greenwich. Moonfield uses east-positive, so New York is −74, Tokyo is +140. Getting this sign wrong is the single most common setup error.

Altitude: angle above your horizon. 0° = horizon, 90° = overhead.

Azimuth: compass bearing, clockwise from true north. 0° N, 90° E, 180° S, 270° W.

Zenith: the point directly overhead.

Meridian: the north-south line through your zenith. Objects are highest when crossing it.

Right ascension / declination: the sky's own coordinates, fixed to the stars. Declination is celestial latitude; right ascension is celestial longitude.

Ecliptic: the plane of Earth's orbit, and so the Sun's apparent path.

Hour angle: how far an object is past your meridian, in degrees or hours. Zero at transit.

The Moon

Phase: how much of the lit half faces you. A consequence of geometry, not a property of the Moon.

Elongation: the Sun-Earth-Moon angle. 0° new, 180° full.

Synodic month: 29.53 days, new Moon to new Moon.

Sidereal month: 27.32 days, one true orbit against the stars.

Perigee / apogee: closest / furthest points of the Moon's orbit.

Libration: the Moon's apparent rocking, letting us see 59% of its surface.

Terminator: the line between lit and unlit. Curved on a crescent, straight at half Moon.

Earthshine: sunlight reflected off Earth faintly lighting the Moon's dark part.

Waxing / waning: growing / shrinking.

Gibbous: more than half lit, less than full.

Tides

Spring tide: largest range, at new and full Moon. Nothing to do with the season.

Neap tide: smallest range, at the quarters.

Lunar day: 24h 50m. The tidal clock.

Lunitidal interval: the lag at your port between the Moon crossing your meridian and high water. A property of the place; must be measured, cannot be computed.

Semidiurnal / diurnal / mixed: two highs a day / one / two unequal.

Chart datum: the reference level tide heights are measured from.

Harmonic analysis: how real tide prediction works: decompose a long measured record into sine waves at known astronomical frequencies.

Storm surge: sea level change from weather, not astronomy. Can exceed the astronomical tide.

Seasons

Axial tilt / obliquity: 23.44°. The cause of seasons.

Solstice / equinox: instants when the Sun's longitude reaches 90°/270° and 0°/180°.

Perihelion / aphelion: Earth's closest / furthest approach to the Sun. Early January and early July, note that these do not line up with the seasons.

Modelling

Model: a simplification that predicts something. All models are wrong; the question is whether they are wrong in a way that matters for your purpose.

Approximation: a deliberate simplification with a known cost.

Prediction: what a model says will happen. Not an observation.

Observation: what actually happened. Also has error bars.

Validation: comparing prediction against observation.

Calibration: adjusting a model using local measurements, as with the lunitidal interval.

Ephemeris: a table or algorithm giving positions of celestial bodies over time.