PlayYour voyagesTraining · play it with a coachTheory · read at your own paceSettings
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WhereSky
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Drag to look aroundScroll to zoomClick a star to take a sightSpace / Enter: sight from the keyboard
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Reading this sky
Latitude is the easy half
The altitude of the celestial pole above your horizon is
your latitude. Find Polaris — or the south celestial pole — and
measure how high it sits.
Longitude needs the clock
The same star field appears at every longitude, just at different
times. A sky alone carries zero longitude. The chronometer
in the rail is what turns it into one.
The meridian is the clock face
The star crossing your meridian has a right ascension equal to
your local sidereal time. Look it up in the almanac, subtract the
Greenwich sidereal time on the chronometer, and multiply by 15:
that is your longitude.
Instruments & hints
Where are you?
You are standing somewhere on Earth, at night, looking up. The sky is
real and it is correct for that place and that instant. Work out where
you are.
The two halves of the problem
Latitude is easy. The altitude of the celestial pole above your
horizon is your latitude. Find Polaris — or the south celestial
pole — measure how high it sits, and you have it.
Longitude is the hard one. The same sky appears at every longitude
on Earth, just at different times. A star field alone contains
zero longitude information. That is why you are given a
chronometer reading UTC. This was the great navigational problem of the
18th century, and the clock is how it was solved.
Taking a fix
Drag the sky to look around. Click a star to record its altitude
and the sidereal time.
Latitude: the altitude of the pole. Or, for a star on your
meridian, its declination plus or minus its zenith distance.
Longitude: find a star on your meridian — advance the clock
until one crosses it — and look up its right ascension in the
almanac. That is your local sidereal time. Subtract the Greenwich
sidereal time and multiply by 15.
Open the chart full screen, place your position and commit.
Deep-sky objects are landmarks, not sight targets. Seeing the Magellanic
Clouds at all tells you which hemisphere you are in.
Credits
Star positions, brightnesses and motions from the
HYG database
v4.1 by David Nash (astronexus), licensed
CC BY-SA 4.0;
the star data files derived from it are shared under the same licence.
Alpha Centauri's motion from SIMBAD, operated at CDS, Strasbourg.
Coastlines from Natural Earth
(public domain). More, and what the game keeps, in
About & privacy.
Place your fix
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Tutorial
Reasoning, not buttons · every number comes from a real sky in the game
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Latitude—
Longitude—
Logbook
WhereSky
Where on Earth are you?
You will be put under a real night sky — the right stars, for one place
and one instant — and asked to say where you are standing, using only
the stars and a clock.
If that sounds impossible, start with training: three short skies, each
played for real, with a coach beside you. North, south, then on your own.
Training, the tutorial and the briefing are always in the menu (☰).