How this is computed
Where every number on this site comes from, what it was checked against, and what is approximate.
Nothing here is copied from another prediction service. The positions of the Moon, the Sun and the planets come from NASA JPL's DE431 ephemeris; from them the site works out where on Earth each event can be seen and when, city by city, and your browser repeats the calculation for any place you choose. Other people's predictions are used to check the results, never as input.
The inputs
| What | Source |
|---|---|
| The Moon, the Sun and the planets | NASA JPL's DE431 planetary ephemeris, read with the Skyfield library |
| Jupiter's and Saturn's moons | JPL's satellite ephemerides jup365 and sat441 |
| The Moon's mountains and valleys | Topography from NASA's Lunar Reconnaissance Orbiter laser altimeter (LRO LOLA), LDEM_64: a height every 1/64° of lunar longitude, about 470 m |
| Which way the Moon faces (libration) | NAIF's lunar orientation kernels (the DE421 principal-axes frame) |
| Stars | ESA Gaia DR3 to magnitude G 9.5 in the band the Moon crosses, Hipparcos-2 for the brightest stars and those Gaia cannot solve, names from the Yale Bright Star Catalogue — 30,166 stars, each carried to the date with its proper motion, parallax and radial velocity |
| Planets' poles and rotation | The IAU Working Group on Cartographic Coordinates and Rotational Elements |
| Earth's rotation (ΔT) | Skyfield's tables and their projection forward: ΔT ≈ 69 s in 2026 |
| The Great Red Spot's longitude | An observation, not an ephemeris: 91° (System II) on 1 June 2026, drifting 1.75° a month — Sky & Telescope, from the JUPOS drift record |
| Crater names and sizes | The IAU Gazetteer of Planetary Nomenclature (USGS), for the outlines on the Moon diagrams |
| Maps and cities | Natural Earth outlines with India's official borders; city positions from GeoNames and a curated list of Indian cities |
How each page is made
Lunar occultations
The 25 events from 2026 to 2029 are chosen by hand from a scan of DE431 for every time the Moon covers a planet or a bright star. For each one the engine maps the part of Earth that sees it and traces the northern and southern graze limits. For every listed city it finds the moments of contact, then refines them against the real, uneven edge of the Moon — LOLA terrain turned to that city's view — which moves them by up to a few seconds. Each page also carries a compact description of the event, about 2 KB, from which your browser solves any latitude and longitude, including light-time and aberration.
When the Moon rises or sets during an occultation, a place sees only half of it; the page says which half.
The Moon and the stars
Each month from September 2026 to December 2028 is one file: the Moon and the Sun as smooth day-by-day curves, and the few thousand stars the Moon can cover somewhere that month. Your browser finds every occultation for your location. Whether you'll see one follows a stated rule of thumb for each instrument — how faint a star can be at the dark and at the bright edge of the Moon, less in moonglare and twilight, with the Moon at least 5° up. The rule is printed on each page.
Jupiter's and Saturn's moons
Eclipses, occultations, transits and shadow transits of Jupiter's four large moons and seven of Saturn's are searched in JPL's satellite ephemerides, with the planet as a flattened globe and its shadow as a cone — 3,882 events for Jupiter and 5,302 for Saturn from 2026 to 2028. These happen at one instant for the whole Earth, so the page only works out whether the planet is up and the sky dark where you are.
The Great Red Spot
A storm, not a body with an orbit. The site computes when Jupiter's central meridian, in the System II longitude system, reaches the spot's longitude, using DE431 and the IAU pole. The longitude itself is measured by observers and drifts, so the site carries it forward only until 31 December 2026 and shows no transits after that until it is updated.
Calendar feeds
The same calculations for each of 516 cities: the chosen lunar occultations the city can see, and the star occultations visible there in binoculars over the next twelve months.
Checked against
Comparisons with independent predictions, and the checks that tie the browser's calculations to the engine's. Those marked automated test run again whenever the data is rebuilt.
| What | Compared with | Agreement |
|---|---|---|
| The Moon hides Aldebaran, 9 January 2017: disappearance at Jaipur, New Delhi and Silchar | in-the-sky.org's published times | Within 5 s at each city; through the Moon-and-stars pipeline +0.5, −1.4 and +1.2 s. Automated test. |
| Which planets the Moon hides, 2026–2029 | Fred Espenak's Sky Event Almanac | Every occultation it lists is found; the scan's few extras are within about 12° of the Sun, where no one can see them. |
| Jupiter's moons, 1–3 October 2026 | Project Pluto's 2026 tables | Every Io, Europa and Ganymede event to the minute. |
| Jupiter's moons eclipsing and hiding each other | IMCCE and BAA mutual-event predictions | Io occults Europa 23 September 2026 04:29 UT; Io eclipses Ganymede 18 January 2027 04:56 UT. |
| Saturn's moons, 12 September 2026 | Astronomy.com's listing | Dione's shadow on Saturn, Dione crossing the north pole and Tethys entering eclipse, at the listed times. |
| The Moon's libration and the tilt of its pole | JPL Horizons | 0.02° and 0.3°. Automated test. |
| Jupiter's central meridian, for the Great Red Spot | JPL Horizons | 0.01°. Automated test. |
| The star-occultation solver your browser runs | Direct Skyfield searches at random places | Within 0.5 s. Automated test. |
| The any-location solver on each event page | The engine's own contact search for every listed city | The compact description sits on the Moon's edge to within 0.05″ at every contact; the build refuses to publish otherwise. |
What is approximate
- The Moon's edge. City tables and the solid graze lines use the real lunar terrain. Times for “your location”, the shaded map and the world map treat the Moon as a smooth sphere, which is good to about 2 seconds.
- The horizon. Altitudes are geometric, for sea level, without atmospheric refraction or hills and buildings. Near the horizon the Moon looks about half a degree higher than the number given, and what you can see depends on your skyline.
- Earth's rotation. ΔT for future dates is a projection, a small and slowly growing uncertainty — well under the few seconds that matter here through 2029.
- Grazes. Near a graze limit a few hundred metres north or south changes what you see. The graze profile on each event page shows how sensitive your spot is.
- Seeing a star. Visibility is a rule of thumb. Haze, light pollution, your instrument and your eyes decide.
- Jupiter's and Saturn's moons. The brightness drop in a mutual event assumes evenly lit discs. Saturn's rings are not modelled, so “behind Saturn” means behind the globe. The configuration diagrams are pictures, good to about a tenth of a planet's radius; the times come from JPL.
- The Great Red Spot. Its times are only as good as its assumed longitude: each degree off moves them by 1.65 minutes.
- The list. The lunar occultations are a hand-picked selection, not every one that happens.
About
Occult is a static site built by Alok Mandavgane: no accounts and nothing to sign in to, with basic page analytics only. Prediction services such as IOTA's Occult and in-the-sky.org are used to check the results here and are never pasted in — their output is their own work, and a copied time couldn't be recomputed for your location anyway. Corrections are welcome through alokm.com.