Quick definitions for the astronomical terms that show up across the app and the rest of the site.
The point in the Moon's orbit farthest from Earth, around 405,000 km away. When a full Moon coincides with apogee, it appears about 14% smaller — a Micromoon.
Lunar Cycles lets you alarm at each apogee for the smallest full moons of the year.
A total lunar eclipse, where the full Moon enters Earth's umbra and turns red from sunlight refracted through Earth's atmosphere. The dustier the air, the darker the red.
Lunar Cycles computes every Blood Moon and rings an alarm before totality.
The celestial equivalent of latitude — how far north or south a body appears in the sky relative to the celestial equator. The Moon's declination shifts daily as it moves through the ecliptic.
Lunar Cycles uses declination to compute moonrise direction and azimuth.
The apparent path the Sun traces across the sky over the year, also the plane of Earth's orbit. The Moon's orbit is tilted about 5° from the ecliptic, which is why eclipses don't happen every month.
The eclipse detector in Lunar Cycles uses Moon-to-ecliptic distance at every syzygy.
A table or function giving the position of a celestial body at a series of times. Modern lunar ephemerides like VSOP87 and ELP2000 reach minute-level precision for civilian use.
Lunar Cycles ships a reduced VSOP87 + ELP2000 ephemeris and runs it locally on your device for every alarm.
One of the two days each year when day and night are roughly equal everywhere on Earth: around March 21 and September 22. The Harvest Moon is the full Moon closest to the autumn equinox in your hemisphere.
Lunar Cycles computes the Harvest Moon for both hemispheres each year.
The slight rocking of the Moon as seen from Earth, due to the tilt of its orbit and rotation. Over a month, libration lets us see about 59% of the lunar surface — 9% more than the pure-near-side view.
Read the full article on libration in Lunar Cycles, narrated aloud.
One of the two points where the Moon's orbit crosses the ecliptic plane: the ascending node (Moon going north) and the descending node (Moon going south). Eclipses only happen when a syzygy lines up near a node.
Lunar Cycles measures Moon-to-node separation in degrees for every full and new moon to flag eclipses.
Short for mass concentration: a region under the lunar surface where rock is denser, creating local spikes in gravity. Mapped by the GRAIL mission. Mascons mess with orbital trajectories of lunar satellites.
Read the full piece on mascons inside Lunar Cycles.
A full Moon that coincides with apogee. The Moon appears about 14% smaller and dimmer than average. The opposite of a Supermoon.
Lunar Cycles tracks both Micromoons and Supermoons each year.
The partial outer shadow of Earth (or Moon, for solar eclipses). In a penumbral lunar eclipse, the Moon only enters the penumbra and the dimming is subtle, often hard to spot.
Lunar Cycles doesn't alarm for penumbral-only events; only partial and total reach the detection threshold.
The point in the Moon's orbit closest to Earth, around 363,000 km away. The Moon's elliptical orbit brings it to perigee roughly once a month. When a full Moon coincides with perigee, you get a Supermoon — up to 14% larger in the sky.
Lunar Cycles tracks every perigee and lets you alarm before each one.
An 18-year-11-day cycle after which the geometry of Sun, Earth, and Moon nearly repeats — meaning eclipses recur in similar patterns. The Babylonians were already using saros tables to predict eclipses centuries before Christ.
Lunar Cycles tracks each eclipse individually; saros membership is a future feature.
The time for the Moon to return to the same position relative to the fixed stars: 27 days, 7 hours, 43 minutes. Shorter than the synodic month because the Sun's apparent position keeps shifting.
Lunar Cycles uses synodic time (phases) for alarms; sidereal time matters more for sky observation.
The two days each year when the Sun reaches its extreme north or south position in the sky: around June 21 and December 22. Solstices mark the longest and shortest days, and frame seasonal lunar visibility.
Solstices don't trigger app alarms directly but they anchor the seasonal context.
A full Moon that coincides with perigee. The Moon looks up to 14% larger and 30% brighter than average. Tides run higher on these nights.
Lunar Cycles flags every supermoon and lets you set an alarm before it.
The time from one new moon to the next: 29 days, 12 hours, 44 minutes on average. This is the cycle most people refer to as "a month" in lunar terms. Slightly longer than the sidereal month because Earth keeps moving around the Sun.
Lunar Cycles uses 29.530588853 days as the canonical synodic month for all phase math.
An alignment of three celestial bodies in a straight line, especially Sun, Earth, and Moon. Full and new moons are syzygies. When syzygy lines up near a lunar node, an eclipse happens.
The eclipse detector in Lunar Cycles tests every syzygy against the lunar node.
The line dividing the lit and unlit sides of the Moon. It's the best place to observe lunar craters because the low-angle sunlight casts long shadows that reveal surface relief.
Lunar Cycles renders the terminator on every glyph, with soft shading like the real Moon.
The dark inner shadow cone of Earth or Moon. When the Moon fully enters Earth's umbra, you get a total lunar eclipse — the Blood Moon. When the Moon's umbra touches Earth, total solar eclipse.
Lunar Cycles classifies an eclipse as total when the Moon-to-node separation falls below the umbra threshold.
Lunar Cycles applies every concept here — perigee, syzygy, node — to compute precise alarms on your device. Free, offline.