Definition
The celestial equator is the great circle where the plane of the Earth's equator meets the celestial sphere. It lies exactly halfway between the north and south celestial poles, divides the sky into a northern and a southern hemisphere, and is the line of declination 0° in equatorial coordinates.
As the Earth rotates, every star moves along a circle parallel to the celestial equator. Stars on the equator itself follow the largest possible circle: from anywhere on Earth, except the poles, they rise due east, set due west and stay above the horizon for exactly half of the sidereal day.
Where to find it in your sky
The celestial equator always crosses the horizon at the east and west points. Between them, it crosses the meridian at an altitude of 90° minus your latitude, towards the south in the northern hemisphere and towards the north in the southern hemisphere.
| Place | Latitude | Altitude of the celestial equator on the meridian |
|---|---|---|
| Oslo | 59.9° N | 30.1° to the south |
| Paris | 48.9° N | 41.1° to the south |
| New York | 40.7° N | 49.3° to the south |
| Singapore | 1.4° N | 88.6°, almost at the zenith |
| Sydney | 33.9° S | 56.1° to the north |
| North Pole | 90° N | 0°: the equator lies along the horizon |
From the Earth's equator the celestial equator passes through the zenith and all stars rise and set vertically. From the poles it coincides with the horizon, and the same half of the sky is visible all year.
Stars that mark it
- Mintaka (δ Orionis), the westernmost star of Orion's Belt, lies less than half a degree south of the celestial equator. Orion's Belt is the easiest way to locate the equator in the winter sky.
- Among the constellations it crosses are Pisces, Cetus, Orion, Monoceros, Hydra, Virgo, Aquila and Aquarius.
- Procyon (+5°), Altair (+9°) and Rigel (−8°) all lie within 10° of it, so they are visible from almost everywhere on Earth.
The equator and the ecliptic
The Sun does not move along the celestial equator but along the ecliptic, which is inclined to it by the obliquity, about 23.44°. The two great circles cross at two points, the equinoxes. The Sun crosses the equator northward at the March equinox and southward at the September equinox. On those dates it rises due east and sets due west everywhere, and day and night are of nearly equal length.
The vernal equinox, where the ecliptic crosses the equator northward, is the zero point of both right ascension and ecliptic longitude.
An equator that moves
The celestial equator is tied to the Earth's axis, and that axis precesses with a period of about 25,800 years. The equator therefore slides slowly among the stars, carrying the equinoxes with it, by about 50″ per year along the ecliptic. This is why the coordinates in star catalogues are always given for a reference epoch such as J2000.0.
Explore it on AstroAlgos
Frequently asked questions
What is the celestial equator?
The projection of the Earth's equator onto the sky: a great circle halfway between the celestial poles, where declination is 0°.
How high is the celestial equator in my sky?
It crosses your meridian at 90° minus your latitude. From 49°N it is 41° above the southern horizon, and it always meets the horizon at the east and west points.
Is the celestial equator the same as the ecliptic?
No. The ecliptic is the Sun's apparent path, inclined about 23.44° to the celestial equator. They cross at the two equinoxes.
