In mapping the night sky astronomers have created a bunch or arbitrary points and lines over the last few centuries. These have been developed by observing the night sky (and day time sky) and watching repeating patterns. The result is now a collection of features that astronomers talk about that can be daunting to people who are just getting acquainted with stargazing. This post looks at these features, explains what they are and how they can be useful – especially for decoding what astronomers are talking about.
The Celestial Sphere
First we have to use a bit of imagination and cast our minds back to when we thought the Earth was the centre of the universe. Back then we didn’t know that the stars were all different distances and it looked like they were lights on the inside of some huge sphere that the Earth, Sun and planets were inside. This huge sphere was known as the Celestial Sphere. You might think that it’s a waste of time thinking about the Celestial Sphere now because we know the stars are all different distances but it is a useful model for describing where objects are in the night sky and it forms the basis of a coordinate system.
The Celestial Equator
If you imagine a the Earth’s equator extending out from the Earth and intersecting with the Celestial Sphere then you’ll find the Celestial Equator. This is a huge imaginary line going all the way around the Celestial Sphere, the top is North and the bottom is South. If you live on the equator then the Celestial Equator is straight up, at the poles it is on the horizon.
The Celestial Poles
90 degrees out from the equator and the top and bottom of the sphere bisected by the Celestial Equator is the Celestial Poles. In the Northern Hemisphere this is almost marked by a star called Polaris – or the North Star. In the South there’s no such handy star marking the position of the South Celestial Pole.
The path that the Sun appears to take across the sky is called the Ecliptic and this is key for mapping the night sky. It is the plane of the Earth’s orbit going around the Sun, imagine a big flat almost round plate with the Earth on the edge and the Sun in the middle. The Sun gives the appearance of passing in front of background stars along the ecliptic, these stars are arranged into the zodiac constellations. Because there’s an ecliptic plane then there is a North Ecliptic Pole and a South Ecliptic Pole (also called orbital poles).
The Ecliptic is also the general path that the planets take across the sky. So it’s a good way to find the planets is to look where the Sun was. The Ecliptic is also where you’ll find the constellations that make up the zodiac. These were basically used as a calendar, way back before people had iPhones.
Mapping The Night Sky With The Equinoxes
Where the two great circles of the Celestial Sphere, the Celestial Equator and the Ecliptic, intersect is where the Northward and Southward Equinox points are. The terms Northward and Southward Equinoxes are not widely used but they make the most sense to me so I’m sticking with them. They are also known as the Vernal and Autumnal Equinoxes, though which one depends on the hemisphere. These two points are quite important as the reference system for defining where objects are in the night sky starts from the position of the Northward Equinox. Basically when the path of the Sun crosses the Celestial Equator into the Northern Hemisphere of the Celestial Sphere. This is the zero point on the Right Ascension measure (kind of like longitude). Because the Earth’s axis wobbles like a kids spinning top the position of Equinox points changes very slowly over a period of 24,000 years which messes up the coordinate system over time.
The equivalent of latitude is the angle between the Celestial Equator and the Celestial Pole and it is measured in degrees. The Right Ascension value, just to be helpful, is measured in hours rather than degrees. So zero hours is the Northward Equinox (Vernal Equinox), which is in the round bit of Pisces. The 2 hour point is near the pointy bit of Pisces, and it continues all the way around until 24 hours, which is back to zero. That is essentially how the coordinate system works.
These terms are really important for mapping the night sky and being able to convey the positions of objects. At stargazing evenings we point these features out.