The star sign Cancer: Cancer in the night sky

Every world map has a line of latitude named after Cancer, and yet in the sky you hardly ever lay eyes on it. The constellation is the faintest in the whole zodiac; not a single conspicuous star belongs to it. On the character of people born under Cancer you find something everywhere, so what follows is about the other half: Cancer in the night sky, where its name comes from, and the line of latitude that is called after it. Your star sign, incidentally, does not hang on these stars but on where the Sun stands over the course of the year; it is the segment that is interpreted, and what comes out of that is set out under the zodiac signs.

A constellation that has to be looked for

Cancer lacks the bright star by which the others make themselves noticed, and in a town with ordinary street lighting you see nothing of it at all. That is not your eyes; there is nothing bright there.

It is found by way of its two bright neighbours. On one side stands Gemini with its conspicuous pair of stars, on the other Leo with Regulus. Draw a line in your mind from Pollux across to Regulus: it runs straight through Cancer. On spring evenings it stands at its highest in central Europe; February to April is the time to look for it.

The Manger: the one sight worth having

In the middle of this dark field there is something that has earned its reputation: an open star cluster, known in antiquity as the Manger, in Latin Praesepe. In the catalogues it carries the number M 44.

Under a really dark sky and with no Moon you pick it out with the naked eye as a small milky patch — not as stars, as a little cloud. A simple pair of binoculars breaks the patch up into dozens of separate points. Several hundred stars belong to it in all, some 600 light years away from us; that makes the Manger one of the nearest open clusters there is.

Because it only shows up when the air is clear, its disappearance long counted as a weather sign — not astrology, but a sensible observation about haze coming in. And a mix-up to deal with straight away: the often-cited Crab Nebula does not lie in Cancer at all, but in Taurus.

Where the name comes from

The Greek myth tells it briefly. While Heracles was fighting the many-headed Hydra, Hera sent him a crab, which nipped his foot. Heracles crushed it. It was set in the sky all the same, for the attempt, as it were; it did not get any brighter for that.

Older than the myth are the figures themselves. The zodiac goes back to Babylonian star lists, and the Greeks hung their stories on images they already found in place. With a figure made up entirely of faint stars this is particularly plain: no one would have read an animal out of those points unprompted.

The Tropic of Cancer

At about 23.5 degrees of northern latitude a line runs around the Earth: the northern tropic, also the Tropic of Cancer. It marks the northernmost place at which the Sun stands vertically overhead once a year. It takes its name from the sign the Sun enters on that day.

The name is old, and it did not go along with a shift. The Earth's axis wobbles very slowly — a single circuit takes it thousands of years — and that moves the points of the year against the constellations. When the line was named, the start of summer coincided with the constellation Cancer; today the Sun stands in front of quite different stars on that day. Nothing was renamed on that account: in every atlas the line is still called after Cancer, and in many other languages just the same.

What you do with this

For the next clear spring evening you have a task: find Pollux and Regulus, draw the line between them, hold the binoculars on it. Not much more is needed, and the milky patch that falls apart into separate points as you do stays in the memory longer than any list of traits. And if the other half interests you afterwards — what is read out of the Sun's position, and with what craft — you start with astrology.