Glossary
Every term this site uses, in alphabetical order. Each has its own page, with a diagram and the traps that make it easy to confuse with its neighbour.
A
- All-sky — An all-sky view shows the entire hemisphere above you at once, horizon to horizon. It distorts shapes near the edge in exchange for showing everything.
- Altitude — Altitude is the angle of an object above the horizon — 90° overhead, negative once it has set. A total eclipse at 3° may be hidden behind the skyline.
- Angular separation — The angle between two directions in the sky. Every eclipse calculation here comes down to comparing it against the Sun's and Moon's apparent radii.
- Annular eclipse — In an annular eclipse the Moon crosses the middle of the Sun but is too far away to cover it, leaving a ring of photosphere. Never safe to view unaided.
- Annularity — Annularity is the interval when the Moon sits wholly inside the Sun's disc and a full ring shows. Bounded by second and third contact, and never safe unaided.
- Antumbra — The antumbra is the umbra's continuation past the point where it converges to nothing. From inside it the Moon looks smaller than the Sun: an annular eclipse.
- Apparent radius — The apparent radius is the angular size of a body as seen from here. That the Sun's and the Moon's overlap is the coincidence total eclipses depend on.
- Azimuth — Azimuth is the compass bearing of an object, measured in degrees clockwise from north. With altitude it fixes a position in the sky.
B
- Baily's beads — Baily's beads are the points of sunlight streaming through valleys on the Moon's edge in the seconds around second and third contact.
C
- Central eclipse — A central eclipse is one whose shadow axis strikes the Earth, so somewhere there is a track to stand in. It is the dividing line this app is built around.
- Centre line — The centre line runs down the middle of the path of totality, where the umbra passes most squarely overhead and totality lasts longest.
- Contact — The four contacts are the moments the discs of the Sun and Moon appear to touch. They are the skeleton of any eclipse timetable, and every one is local.
- Corridor — The corridor is the path of totality drawn as a region rather than a line, bounded by the northern and southern limits. It is a term of this app's own.
E
- Eclipse season — An eclipse season is the roughly 35-day window, twice a year, during which the Sun is near enough to a node for eclipses to happen at all.
- Ecliptic — The ecliptic is the plane the Earth orbits the Sun in, and so the path the Sun appears to trace against the stars. It is named after eclipses.
F
- First contact — First contact is the moment the Moon's limb first touches the Sun's and the eclipse begins. Nothing is visible to the unaided eye for some time afterwards.
- Fourth contact — Fourth contact is the moment the discs separate and the eclipse is over at your location. Like first contact, it is undramatic and easy to miss.
G
- Greatest eclipse — Greatest eclipse is the moment the shadow axis passes closest to the Earth's centre — one instant for the whole world, and what an eclipse is dated by.
L
- Limb — The limb is the apparent edge of the Sun's or the Moon's disc. Every contact is defined against it, and the Moon's is rough enough to matter.
- Lunation — A lunation is the 29.53 days from one new moon to the next. Solar eclipses can only happen at new moon, so eclipses are counted in lunations.
M
- Magnitude — Eclipse magnitude is the fraction of the Sun's diameter the Moon covers. It is a different number from obscuration, and means something else for a star.
N
- Node — The nodes are the two points where the Moon's orbit crosses the ecliptic. Eclipses happen only near them, which is why there is not one every month.
- Northern and southern limits — The limits are the two edges of the path, where the central phase shrinks to nothing. A prediction near one is far more sensitive to your exact position.
O
- Obscuration — Obscuration is the fraction of the Sun's area hidden by the Moon. It is the number that matches how dark an eclipse feels, and it is not the same as magnitude.
P
- Partial eclipse — A partial eclipse is one where the Moon covers only part of the Sun. It is what most of the world sees at any eclipse, and for about a third it is all there is.
- Path of totality — The path of totality is the strip the Moon's umbra traces across the Earth. Inside it there is a total eclipse; a kilometre outside it there is not.
- Penumbra — The penumbra is the region around the umbra from which part of the Sun is hidden. It spans continents, which is why partial eclipses are common.
S
- Saros — The saros is a period of about 18 years 11 days after which a very similar eclipse recurs, a third of the way west. A specific interval, not a generic cycle.
- Second contact — Second contact is the moment the Moon's disc moves fully onto the Sun's and totality or annularity begins. The most time-critical instant of an eclipse.
- Shadow axis — The shadow axis runs from the Sun's centre through the Moon's and onwards. Whether it strikes the Earth separates a central eclipse from a partial one.
- Sublunar point — The sublunar point is the place on Earth with the Moon at the zenith. Its distance from the subsolar point decides whether an eclipse is happening at all.
- Subsolar point — The subsolar point is the place on Earth with the Sun exactly overhead. During a central eclipse it sits within about 100 km of the sublunar point.
- Syzygy — A syzygy is an alignment of three bodies. Every new moon is a rough one; only the close ones produce an eclipse, and this app scans for the close ones.
T
- Terminator — The terminator is the boundary between the sunlit and dark sides of the Earth. Nothing to do with anything ending; an eclipse near it is one at a low Sun.
- Third contact — Third contact is the moment the Moon's trailing edge reaches the near limb of the Sun and the total or annular phase ends. Sunlight returns instantly.
- Total eclipse — A total solar eclipse happens where the Moon's umbra touches the ground — the only kind at which it is ever safe to look at the Sun unaided, and only briefly.
- Totality — Totality is the interval when the Sun's disc is entirely covered, bounded by second and third contact. The only time it is safe to look unaided.
U
- Umbra — The umbra is the part of the Moon's shadow within which the Sun is completely hidden. Where it touches the Earth there is a total eclipse, and nowhere else.
- UTC — UTC is the single time scale this site computes in. Eclipse times are quoted in it because an event crossing a dozen zones needs one common reference.