Sky and Telescope - February 2016 - 27

Under the Stars
To check the night's parade of the heavens, I ﬁ rst
wheeled the rete so that my date on the edge of the
ecliptic circle touched the west side of the horizon arc:
sunset! Using the rule to point from there to the rim told
me the sunset time.
Or rather, it told me sunset's local apparent solar time
(sundial time). That's what everyone used in the centuries before accurate clocks and fast communication.
Since I want modern clock time instead (which runs with
uniform speed and applies across an entire time zone),
adjustments are needed. Custom-printed on the back is
my permanent correction in minutes for the longitude of
my home. Thanks, Janus! Also on the back is a graphical scale for ﬁnding the Equation of Time correction for
any date, using the alidade as a rule. The Equation of
Time adjusts for the fact that Sun time runs as much as
16 minutes fast or slow around the year, due to the tilt of
Earth's axis and the ellipticity of its orbit. Thanks again.
And when daylight-saving time is in eﬀect, I need to
add an hour. For astronomers who want to track the Sun
and stars, rather than catch a train or arrive on time for
a meeting, modern time is complicated!
The astrolabe, I learned, also reads past and future.
Aligning diﬀerent dates on the ecliptic to the horizon,
I could read the time of sunrise or sunset for any day of
the year. Or I could pick a star - Arcturus, say - and
spinning to other dates and times, see where it would
stand above or below the horizon. By the time I stepped
out on that chilly November night I was a 21st-century
Hipparchus, ready to take the stars.
To check the time, I spotted Aldebaran and
held the astrolabe aloft by its two-ring chain, letting gravity align the vertical. On the back side,
I carefully nudged the alidade to point exactly
at Aldebaran. I then read, where the alidade
crossed the protractor scale on the outermost
rim, that Aldebaran was 41°° above the horizon.
I turned over my astrolabe, found Aldebaran on
the clear plastic rete, and positioned the star at
41° in the east on the grid of almucantar circles.
Holding the rete in place, I found my date
(November 15th) on its ecliptic circle and aligned
the rule to that date. Where the edge of the rule
met the outer rim, I read the time: a hair short of
9:55. Call it 9:54. Apply my longitude correction,
read the Equation of Time correction for November
15th from the scale on the back, and I had 9:31 p.m.
Eastern Standard Time.
I checked my phone. My astrolabe was a minute slow.
Not bad for a paper-and-plastic replica of a timepiece
older than Ptolemy. Maybe I was a bit lucky on my ﬁrst
try, but to check and improve my accuracy I can take
several star-sightings and average them.
It seems like a lot of work to get the time, but imagine

if it was the only way you could. Or if you wanted to know
when the Sun would rise on the date of an impending
battle, or how the stars were arrayed at the time of your
patron's birth. Early sailors used the astrolabe for navigation, especially after a streamlined mariner's version
made it easy to "shoot the Sun" and the pole star. The sextant (recently reintroduced in the U.S. Navy as a backup
in case of cyberattack) is merely a further reﬁnement.
Ancient astrolabes performed still more wonders.
A shadow square on the back enabled calculations of
distance or height. The degree scale around the rim
served as a surveyor's protractor. Arab astrolabes found
the direction of Mecca. But my modern version seemed
wondrous enough. ✦
Bruce Watson is a freelance writer in Massachusetts and the
author of Light: A Radiant History from Creation to the
Quantum Age (Bloomsbury, February 2016).

T
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Sky and Telescope - February 2016

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