How Umbro works
Every number on this site is computed in your browser from first principles. There is no
eclipse database behind it, and no API call to fetch.
Why not just use an API?
Because there isn't a good one. NASA/GSFC publishes the authoritative catalogue, but as static
pages covering a fixed list of cities. Outreach maps from observatories are images. Xavier
Jubier's excellent path files aren't licensed for redistribution. timeanddate.com has the data
but no free API. None of them answer the only question that actually matters to you:
what happens at my coordinates?
So Umbro carries its own ephemeris and works the geometry out directly. That means it runs
offline, costs nothing to serve, and gives the same answer for a mountaintop in Chile as it
does for midtown Manhattan.
The method
Solar coordinates come from Meeus's abridged solar theory; lunar coordinates from the
truncated ELP‑2000/82 series — sixty periodic terms in longitude and distance, sixty more in
latitude. From those two position vectors everything else follows geometrically:
- The shadow axis is the line through the centres of the Sun and the Moon.
- Gamma is that axis' least distance from the Earth's centre, in Earth radii.
It decides whether there's an eclipse at all, and how central it is.
- The umbral and penumbral cones follow from similar triangles. Where the
umbral cone crosses the Earth's surface, its radius tells you whether you get a total
eclipse or an annular one — a negative radius means the cone's apex falls short of the
ground, and you see a ring instead.
- The central line comes from intersecting the shadow axis with the Earth
ellipsoid, stepped through the eclipse thirty seconds at a time.
- Your contact times come from solving for the instants when the two
apparent discs touch, as seen from your exact spot on a rotating, slightly flattened
Earth — not from the geocentre.
This is the Besselian construction, computed on the fly rather than read from published
tables of elements.
Percentage covered, and why it isn't magnitude
Two different numbers get called "how much of the Sun is covered", and they disagree badly.
Magnitude is the fraction of the Sun's diameter hidden.
Obscuration is the fraction of its area — the one that matches what
the sky actually looks like, and the one shown in large type on the front page.
At magnitude 0.5 only about 39% of the Sun's area is gone. Umbro computes obscuration properly
from the circle–circle lens area, so a "50% eclipse" here means half the light, not half the
width.
How accurate is it?
Checked against NASA/GSFC's published catalogue for 2017–2030:
- 32 of 32 eclipses found, with no false positives
- Every eclipse type classified correctly, including the awkward
2023‑04‑20 hybrid that is annular at both ends of its path and total in the middle
- Greatest durations within about 3 seconds
- Gamma within 0.001 Earth radii
Local circumstances, checked against published figures for individual cities:
Contact times land within roughly ±30 seconds. That is comfortably good
enough to plan a trip around. If you are timing equipment to the second, check NASA/GSFC too.
Known limitations
- Times display in your device's timezone, not the timezone of the place
you look up. The label always says which zone is in use, and there's a UTC toggle. Doing
this properly needs a timezone-boundary database.
- Path edges are approximate by a kilometre or so. The width shown ignores
terrain elevation and the Moon's ragged limb profile, both of which shift the exact
boundary. Don't plan to stand right on the edge.
- IP-based location is rough. If the header badge says "approx", you may be
tens of kilometres from where the site thinks you are — which near a path edge is the
difference between totality and a deep partial. Search for your town or click the map.
Privacy
All the astronomy runs locally. The only things that leave your browser are map tile requests,
place searches you type yourself, and — if you don't grant precise location — a single lookup
that turns your IP into an approximate city. No accounts, no analytics, no cookies. Your chosen
location is kept in your own browser's local storage and nowhere else.
Looking at the Sun safely
Never look at the Sun without certified eclipse glasses. The only moment it is
safe to look with the naked eye is during a total eclipse, between second and third
contact, when the Sun's disc is completely covered. During an annular eclipse the photosphere
is never fully hidden, so it is never safe — and the same goes for every partial phase
of every eclipse. Camera lenses, binoculars and telescopes concentrate the light and will cause
injury far faster than the naked eye.
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