Capricornus
What is Capricornus?
Constellations· Late summer–autumn (N) · a triangle southeast of Altair. The sea-goat: a goat's head on a fish's tail. It is the smallest constellation of the zodiac, yet it has a place in history as the area where Neptune was first seen in 1846. With no bright stars, it is found by its blunt triangular outline.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Deneb Algedi | 2.9 |
| Dabih | 3.1 |
| Algedi | 3.6 |
| Nashira | 3.7 |
The 4 brightest of the 8 stars used in the figure (8 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | late summer to autumn evenings, August–October |
|---|---|
| Brightest star | Deneb Algedi (2.9) |
| How to find | a blunt triangle southeast of Altair |
| Area rank | 40th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Algedi — two stars the eye can split, 600 light-years apart
Alpha Capricorni, Algedi, can be split into two stars by a keen naked eye. The two look tightly paired in the sky but are not partners: the brighter one is about 110 light-years away and the fainter one about 690. Nearly 600 light-years lie between them, and they simply happen to sit in the same direction.
Before star distances could be measured, such chance pairs were hard to tell from true double stars. Once the first stellar distances were measured in the late 1830s, it became clear that stars that look close in the sky can be very far apart in space. Algedi is a naked-eye reminder that the night sky is not a flat picture but a space with depth.
The constellation in depth
Capricornus has been drawn as a goat with a fish's tail since Babylonian times. A Greek story links it to the god Pan, who leapt into a river to escape the monster Typhon and changed into a fish only below the waterline. The Sun once stood here at the December solstice, which is why the southern tropic is called the Tropic of Capricorn.
On the night of 23 September 1846, Johann Galle at the Berlin Observatory searched the position that the French mathematician Urbain Le Verrier had calculated, and found the new planet Neptune near Delta Capricorni, also called Deneb Algedi. Le Verrier had noticed that Uranus kept drifting from its predicted path and reasoned that an unseen planet was pulling on it. Neptune was found by mathematics before any telescope saw it.
On late-summer evenings, finding Altair and then dropping to the south-east leads to a blunt triangle of stars, which some see as a boat: that is Capricornus. Its stars are only third to fourth magnitude, so the shape is hard to trace from a city and needs a moonless night away from lights. With the Sky view set to a September night, the outline appears.
Frequently asked
- When can I see Capricornus?
- Capricornus is best seen in late summer to autumn evenings, August–October.
- How do I find Capricornus?
- To find Capricornus, look for a blunt triangle southeast of Altair.
- What is the brightest star in Capricornus?
- The brightest star in Capricornus is Deneb Algedi, at magnitude 2.9.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.