Reckoning / Sky & sea / The celestial sphere
Sky & sea · coordinates of the sky
The celestial sphere
Seen from Earth, the stars sit on a vast sphere turning once a day. Extend the Earth’s equator and poles outward and you get the sky’s coordinates: declination is latitude on the sky, right ascension is longitude, counted east in hours from the point where the Sun crosses the equator in March. The Sun’s own yearly path, the ecliptic, is tilted 23.4° to that equator.
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The Sun on
Blue: celestial equator and the hour circles every 2 h. Red: the ecliptic, marked every 30°. Yellow: the Sun. The small globe at the centre is the Earth.
Signs and constellations
Two thousand years ago the ecliptic was divided into twelve equal signs of 30°, each named after the constellation that lay in it. The equinox, the starting point, slides westward along the ecliptic by about 50.3″ a year, a full circle in about 25 800 years, because the Earth’s axis wobbles like a top (precession). The signs moved with the equinox; the stars did not. Today the March equinox lies in Pisces, and the Sun spends most of each sign in the constellation before it. Since 1930 the constellations also have official boundaries, and the ecliptic crosses thirteen of them, including Ophiuchus.
| Constellation on the ecliptic | Sun enters (this year) | Days |
|---|
| Sign | Ecliptic longitude | Sun enters (this year) | |
|---|---|---|---|
| ♈︎ | Aries | 0°–30° | |
| ♉︎ | Taurus | 30°–60° | |
| ♊︎ | Gemini | 60°–90° | |
| ♋︎ | Cancer | 90°–120° | |
| ♌︎ | Leo | 120°–150° | |
| ♍︎ | Virgo | 150°–180° | |
| ♎︎ | Libra | 180°–210° | |
| ♏︎ | Scorpius | 210°–240° | |
| ♐︎ | Sagittarius | 240°–270° | |
| ♑︎ | Capricornus | 270°–300° | |
| ♒︎ | Aquarius | 300°–330° | |
| ♓︎ | Pisces | 330°–360° |
Dates computed in your browser for the current year from the Sun’s position (NOAA solar calculator) and the official boundaries (Roman 1987 (CDS VI/42), precessed with the routine published with that table). The build checks the boundary lookup against the table’s own test positions and ten bright stars. Constellation names: IAU constellations.
Reckoning treats the zodiac as astronomy: the signs are a coordinate system, 30° steps of ecliptic longitude from the equinox, and that is how they appear in old almanacs and navigation tables. They carry no meaning beyond position.