How we calculate

The eclipse times and durations

We compute the eclipse for each place from the positions of the Sun and the Moon in the JPL DE440 ephemeris, the planetary and lunar ephemeris published by NASA’s Jet Propulsion Laboratory. The calculation runs in Python with the Skyfield library. For each town we take the apparent positions of the Sun and the Moon as seen from that exact point on the WGS-84 Earth ellipsoid, at sea level. Then we search, to a fraction of a second, for the four contacts, the moment of maximum eclipse, the Sun’s altitude and azimuth, and the share of the Sun’s disc that is covered.

Why k = 0.2722810

This is the lunar radius that NASA (Fred Espenak) and most eclipse calculators use for the umbra. It describes the Moon’s mean edge, below its highest mountains. A slightly larger value would make every total eclipse a little shorter and the path a little narrower.

ΔT: the clock of the Earth

The Moon and the Sun move on a uniform time scale. Clocks follow the Earth’s rotation, which is slightly irregular. The difference is called ΔT. We use 69.1 seconds, from the IERS data built into Skyfield. The latest IERS Bulletin A points to about 69.3 seconds for 2 August 2027. The gap moves every time by about 0.2 seconds, less than our rounding. NASA’s eclipse pages for 2027 were prepared years ago with 71.7 seconds. That is why NASA’s clock times are about four to five seconds earlier than ours.

How we checked it

• We ran the same code on the total eclipses of 2017 and 2024. It reproduces NASA’s published greatest durations to within 0.3 seconds.
• For 2027 we compared 13 places from Spain to Saudi Arabia with the calculator of Xavier Jubier, the reference used by many eclipse chasers. Durations agree within half a second and contact times within one second.
• For Spain we also compared with the Instituto Geográfico Nacional. Durations agree within about two seconds. IGN’s clock times are about nine seconds earlier than ours, a constant shift that points to a different ΔT.

The limits of the calculation

• The Moon is treated as a smooth sphere. Its real edge has mountains and valleys. They move second and third contact by one to three seconds in most places, and by up to six or seven seconds close to the edge of the path. If you plan to stand within a few kilometres of a limit, use a limb-corrected prediction such as Jubier’s.
• Times are for sea level and for the coordinates shown on each page. A hill or a move of a few kilometres changes the seconds, most of all near the edges.
• Magnitude is the fraction of the Sun’s diameter covered at maximum. Obscuration is the fraction of its area.

Click anywhere on the interactive map and your browser computes that point with NASA’s Besselian elements and the same ΔT as our dataset. It agrees with the town pages to about a second, and to three or four seconds within a few kilometres of an edge. Town markers on the map show the town page numbers exactly.

Drive times and distances

Drive times in our trip planning tools come from OSRM, a routing engine that runs on OpenStreetMap road data. They are estimates for normal traffic. On eclipse morning the roads near the path will be much busier, so allow far more time. Distances to the central line and to the path limits on the town pages are straight lines, measured to our own DE440 path.

Official and reference sources

The eclipse

• NASA Eclipse Web Site (Fred Espenak, NASA/GSFC), with the interactive map for 2 August 2027.
• NASA JPL: the DE440 and DE441 ephemerides and the JPL ephemeris files.
• Xavier Jubier’s 2027 eclipse map, with lunar limb corrections for any point.
• Instituto Geográfico Nacional (IGN), Spain: the official Spanish eclipse site, with times for every Spanish municipality, and the IGN main portal.
• IERS, the service that measures the Earth’s rotation and ΔT.

Weather

• NASA POWER (documentation).
• Egyptian Meteorological Authority, the official forecast for Egypt (site in Arabic).
• AEMET, the Spanish State Meteorological Agency, and its forecast by municipality.
• Direction Générale de la Météorologie (Morocco), Institut National de la Météorologie (Tunisia) and the National Center for Meteorology (Saudi Arabia).

Roads

• DGT, the Spanish traffic authority, and its live traffic map.