1 Telling Time by the Stars
Telling time by the stars is not really very useful.
However, learning how to do it is educational. It teaches you a few
things about the constellations, and a few things about spherical
This discussion is limited to northern temperate latitudes.
I apologize to those who live elsewhere.
The task requires more skill than you might think. I just checked
with Google and found a googol of sites that describe what I call the
“standard hokey” technique – namely the one that depends 100% on
the “pointer” stars: Alpha and Beta Ursae Majoris (Dubhe and Merak).
This has the slight problem that it doesn’t work. It has apparently
been devised by people who spend too much time looking at star-charts
and not enough time looking at the real sky. It works OK at, say,
10PM in late April, when the pointers are high overhead, but it gives
the wrong answer 3 months later and/or 6 hours later (because of
spherical geometry) and it gives no answer at all 6 months and/or 12
hours later (because you’ll have trouble seeing the pointers from most
of the northern temperate latitudes).
Typical charts of the circumpolar stars use a polar projection, which
doesn’t accurately portray how things actually look. Suppose you are
standing at latitude 40 degrees North, and the 0-hour circle (marked
by e.g. Beta Cassiopeiae) is overhead. That does not mean that
the 18-hour circle (marked by e.g. Gamma Draconis) is a horizontal
line 40 degrees above the horizon. It is locally tangent to the
horizontal near the pole, but then it dips quite markedly on both
sides. If you extend it far enough it dives through the western
horizon. Gamma Draconis is halfway along the great-circle route from
the pole to the horizon, so its elevation above the northwestern
horizon is less than 3/4ths of the elevation of the (since the sine of
45 degrees is 0.7).
I’ve found several on-line star-chart generation sites that get this
wrong, but I’ve been unable to find one that gets it right. Can
anybody recommend something that works?
Anyway, here is how I do it. This is just a quick overview; you will
have to fill in many details on your own. Also note that tradeoffs
have been made between convenience and accuracy: there are simpler
methods that are grossly inaccurate, and more-accurate methods that
are more complex (using equatorial rather than circumpolar stars).
Memorize four landmarks (skymarks?)
The 0-hour circle. This is marked by Beta Cassiopeiae (Caph)
which is the star at the bright end of the famous W in the
constellation Cassiopeia, i.e. the end with the acute angle.
Continuing along the 0-hour circle, we come to Alpha Andromedae
(Alpheratz) and Gamma Pegasi (Algenib) which together constitute the
trailing (eastern) edge of the Great Square – hard to miss.
- The 6-hour circle. This is marked by Delta Aurigae, Beta
Aurigae (Menkalinan), and Theta Aurigae.
- The 12-hour circle. This is marked by point halfway between
Delta and Gamma Ursae Majoris, the two non-pointer stars in the bowl
of the Big Dipper. (The pointer stars are excellent
markers for the 11-hour circle.)
- The 18-hour circle. This is marked by Chi, Phi, Xi, and Gamma
Draconis, i.e. the hind feet, chin, and nose (Eltanin) of the Dragon.
Also remember that the 12-hour circle is the continuation of the
0-hour circle, and that the 18-hour circle is the continuation of the
- Remember that the 12-hour circle is overhead at midnight at the
spring equinox. The 18-hour circle is overhead 6 hours later, and/or
3 months later in the year. And so forth. This gives you four
“primary” reference pictures, where one of these four circles is
- You can then construct four “secondary” reference pictures,
halfway between the primaries. These correspond to the situation
where the primary circles form a giant V shape that is symmetrical
with respect to the vertical. Do not try to judge the angle that the
circles form relative to horizontal, because the perception of
horizontal is distorted by the spherical geometry. The perception of
vertical is OK, and the perception of symmetry is OK. Anything else
you need can be judged by interpolation between the symmetrical
(secondary) picture and the vertical (primary) picture.
- As you face north, the great clock in the sky rotates
counterclockwise.1,2 It moves counterclockwise as you
get later in the night or later in the year. The time-of-year
contribution is 2 hours per month, or half an hour per week, or four
minutes per day.
- Therefore: Suppose it is March 22nd. If you see the 12-hour
circle is past vertical, 1/3rd of the way to the symmetrical V
position, it must be 1:00 AM. If it is two weeks later in the year,
the same picture is 12:00 midnight (standard time); the
advancement is explained by being later in the year, not later at
- Correct for daylight savings time. If DST is in effect,
official time is one hour later than star time.
- Correct for longitude.
- To a rough first approximation, time zones are 1 hour wide, and
the standard time in each zone more-or-less corresponds to the mean
solar time at the middle of the zone. Therefore, roughly speaking, if
you are near the edge of a zone, standard time could be offset by half
an hour from the local mean solar time.
- In reality, things are much more complicated than that. Time
zone boundaries follow political boundaries, not the ideal theoretical
lines of longitude. Useful diagrams can be found in
reference 1. You can easily find places (such as western
Spain) where the standard time is offset by more than 1.5 hours from
the local mean solar time. That is, the zone extends more than 1.5
hours from where the middle of the zone “should” ideally be.
If you are west of the nominal midline of your time zone, official
time is later than star time. By the same token, if you are east of
the midline, official time is earlier than star time.
The Robinson Library, “Time Zones”