Astronomy is like any other specialised area of interest in that it comes with its own language and describing the position of celestial objects can be rather confusing for the novice astronomer. This article covers what some of the terminology is and how to describe where things are in the night sky.
The Celestial Sphere
The best way that we have figured out to explain where things are in the night sky is based around what is called the Celestial Sphere. This is the concept that you are at the centre of the universe and the universe is depicted on a giant sphere. We all know people who think they are at the centre of the universe and in the case of the celestial sphere, they really are – and so you can be as well. There is a big difference between what we see, how we try to explain it and what is truly there. The aim being to describe where things are so that you can tell other astronomers where to go looking.
Long before we knew much about the universe someone looked up and considered that the stars looked like they were painted on a rounded ceiling. This is what people saw and as they didn’t know any different they just assumed that’s what it was – some kind of 2 dimensional cover with lights on it.
The celestial sphere is a way of mapping those lights onto the inside of a really big imaginary sphere, with the Earth at its centre. To find things on that sphere, we use the same concept for the coordinate system we use to map the surface of the Earth – except that’s on the outside of a sphere (an oblate spheroid to be more accurate). The Earth has poles, equator and a prime meridian, this gives us the ability to map a grid over our model of the planet and develop a coordinate system based on longitude and latitude. A Global Positioning System (GPS) will give you your distance from the equator as latitude and your distance from the prime meridian as longitude. The Celestial Sphere has the same poles, equator and meridian, they are just extended onto the inside of the sphere that we are conceptually siting in.
The equivalent on the Celestial Sphere of latitude is declination and that of longitude is right ascension. The whole sphere looks like it is rotating westward as it goes through one revolution each day, and based on observations from Earth stars return to the same position after 23 hours and 56 minutes. The system doesn’t include a distance so the Celestial Sphere is kind of like using a GPS without altitude. Starmaps tend to last a long time – in fact they don’t really change except over thousands of years, as stars don’t tend to move that much over our lifetimes. Knowing where things are in the sky will help you to impress your family and friends.
A trick that is good to remember for astronomy is that unlike on Earth, rather than considering the distance between objects we consider the angular distance betwen them.
Zenith and Nadir
If you look straight up (90 degrees up from the horizon) that is the zenith on the Celestial Sphere and the opposite is called the nadir, straight down. To locate islands in the Pacific, Polynesian navigators had to remember the stars at the zenith for each island (at a given time and date). It’s important to remember that zenith and nadir are not the poles, they are just the points that are straight up and straight down from where you are on the surface of the Earth.
The Meridian is like a big circle painted on the inside of the Celestial Sphere that goes from the zenith through the cardinal point of North and then through the nadir and through the South cardinal point and back up to the zenith. When the sun crosses the meridian we call it midday.
These are the extension of the North and South poles of the Earth onto the Celestial Sphere. Stars that are always visible in the sky are called circumpolar and they cross the meridian above and below the relevant celestial pole. When they cross the pole on the meridian at the lowest point it is called the lower culmination.
This is like latitude and describes the distance from the equator (being the celestial equator, the extension of the Earth’s equator on the Celestial sphere) in angular terms.
This is the measurement like longitude and it is in time from a position on the celestial equator when the sun crosses the equator at the March Equinox, from 0 seconds to 12 hours both East and West. So the position is described as a time either East or West.
Azimuth is measured in degrees from the North cardinal point to the right.
This is the measurement in degrees from the horizon to the zenith. Both azimuth and altitude are relative to where you are standing. Whereas right ascension and declination are on the Celestial Sphere.