What Is an Almucantar?

A circle of equal height

An almucantar is a circle on the sky parallel to the horizon, and every point on it has the same altitude.

Stand in the open and the 30° almucantar runs right round you, a third of the way from the horizon to the point overhead. The horizon itself is the almucantar of 0°. Climb toward the zenith and the circles shrink, until at 90° there is nothing left of them but the zenith. Two stars on the same almucantar are equally high, whatever their compass bearings.

Where the word comes from

From the Arabic al-muqanṭarāt, by way of medieval Latin.

Al-muqanṭarāt is the plural of muqanṭara, a word built on qanṭara, an arch. Astronomers writing in Arabic used it for the circles of altitude on the astrolabe, and Latin translators kept it whole, which is how English came to have almucantar, and sometimes almacantar.

Its neighbours on the same instruments crossed over too. Zenith is a medieval copyist's misreading of the Arabic samt, short for samt ar-ra's, the way over the head. Azimuth is as-sumūt, the ways or bearings. Nadir is naẓīr, the point opposite. Most of the words on an astronomer's compass card are Arabic, and almucantar is the one that kept its article.

Geoffrey Chaucer, writing a guide to the astrolabe for his young son Lewis around 1391, spelled them almykanteras.

Why an astrolabe needs them

Because they let you turn a star's height into the time.

An astrolabe is a sky you can hold. A brass plate, engraved for one latitude, carries the horizon, the almucantars and the lines of azimuth. Over it turns the rete, an open fretwork map of the bright stars, each one marked by a small pointer.

At night you sight a known star along the rule on the back and read its altitude from the scale on the rim. Then you turn the rete until that star's pointer sits on the matching almucantar. The whole sky on the instrument now stands as the real one does, and the hour can be read off the rim.

ProjectionStereographic, described by Ptolemy in the Planisphaerium, 2nd century.

On an astrolabe plate the almucantars are nested circles crowding toward the zenith mark, each a little off-centre, because the plate is a stereographic projection centred on the celestial pole rather than on the point overhead. A large instrument might carry one for every degree or two; a pocket one, every five or ten.

In the 1880s the American astronomer Seth Carlo Chandler gave the name to an instrument of his own, which timed stars as they crossed a fixed almucantar rather than the meridian.

Why this chart is named after them

Because it's drawn the astrolabe's way, with the zenith at the centre instead of the pole.

Radiusr = R tan((90° − h) / 2)
30°: 0.577 R
60°: 0.268 R

The chart on the front page uses the same projection the astrolabe does, stereographic, which keeps every constellation's shape true wherever it falls. The difference is the centre. Ours sits on the zenith, so the almucantars come out as true concentric circles: the horizon is the ring, the dashed circles are 30° and 60°, and the zenith is the dot in the middle. The circles aren't evenly spaced, and shouldn't be: the projection opens out toward the horizon, which is why the 30° ring sits a little over halfway out.

That makes them the easiest marks on the chart to read. A star on the 30° circle is a third of the way up the sky, about three fists held at arm's length above the horizon. A star inside the 60° circle is high enough that you'd tip your head right back to see it.

The name also says what kind of thing this is. It's an instrument first, drawn to be read, and the prints keep that: the graduated horizon and the 30° and 60° almucantars are drawn on every view, so the sky on your wall can still be read the way an astrolabe's could.

Where can I see them?

On the chart, for wherever you are, right now.

Set your place on the chart on the front page and look for the dashed circles; any star you click reports its altitude, and you can check it against them. About the Stars has the rest of the method, with every chapter of Meeus it uses, and The Colours of a Clear Dusk is a good next read if it's the evening sky you're after. When you'd like your own sky drawn this way, the studio is where you set it.