Using Sky Coordinate Systems: Horizontal Vs Equatorial in Stargazing

What coordinate system is used in astronomy? How do you decipher these celestial points to locate an object with your telescope? What do all these different terms mean? It can be confusing when you’re starting out. Here I look at what you need to know about sky coordinate systems, what they mean, and how to use them.

sky coordinate systems, horizontal coordinate system vs equatorial coordinate system, local v global, alt-az

This is important because: Once you get the hang of using celestial coordinates, you’ll find navigating around the night sky much easier and less frustrating.

Why have different types of celestial coordinate systems?

How does Alt-Azimuth, altitude and azimuth, fit in?

You’re probably already familiar with the Alt-Az system in terms of how a simple telescope moves on its mount, where Alt is short for altitude and Az stands for azimuth. Let’s now look at what else there is to know about pinpointing objects in the sky.

Local vs Global relevance

There are two main celestial coordinate systems: Horizontal and Equatorial. Of the two, one has use on the local level, e.g. from your backyard, and the other has global relevance for no matter where you are on the planet.

Local: Horizontal coordinate system

The use of local coordinates introduces us to the horizontal coordinate system which involves altitude and azimuth. Altitude is the height of the object above the horizon (0º; zenith 90º), likened to parallels of latitude. Azimuth is the placement along the horizon (0º North; 90º East; 180º South; 270º West), much like lines of longitude.

Astronomers express horizontal coordinates in degrees and this coordinate system is the easiest of the two to understand. If you’re a beginner who has taken the advantage of using a Dobsonian, you may already recognize this system. You’ll also notice that these coordinates change constantly while observing, as the Earth is rotating.

While the Horizontal Coordinate System helps describe the night sky to local viewers, it has little relevance for directing observers who are situated in different locations around the world. In which case, the Equatorial System is best.

Global: Equatorial coordinate system

Equatorial is the most common coordinate system used in astronomy and it involves two values: right ascension (RA) and declination (Dec). The use of these coordinates helps us find constellations and planets no matter where we are in the world. They are values you’re likely to find in a book.

How are right ascension and declination used?

Right ascension and declination are coordinates of the sky similar to the latitude and longitude system for Earth’s surface. They are universal markers used to pinpoint objects in the celestial sphere — the sky conceived as a globe — anywhere in the world. For example, the super hot giant star, Rigel’s coordinates of 5h 14m, -8º 12′ apply at all locations on Earth.

Right ascension is analogous to longitudinal lines

Right ascension (RA) is the angular distance of an object from the celestial meridian (0h), which is an imaginary line. It differs to the local meridian line, which runs through the zenith from north to south above your observation point.

The 0h line for RA runs through the vernal equinox, a point in the celestial sphere (presently in the constellation of Pisces). It goes from celestial north to celestial south pole, much like the meridian running longitudinal through Greenwich on Earth’s surface.

How do you convert RA to degrees? A right ascension is expressed as hours, minutes, and seconds (where 60s = 1m / 60m = 1hr). You can convert these units to degrees by first using decimal hours and then converting the result ( where 24hr = 360º ∴ 1hr = 15º). Example: Changing Rigel’s 5h 14m to decimal hours gives us 5.2333 hours (5 + 14m÷60) and timing 5.2333 by 15º gives us an RA in degrees of 78.5º.

Declination (Dec) is analogous to latitude lines

Declination (Dec) is measured in degrees, arcminutes, arcseconds. The reference point, or 0, for Dec is the celestial equator, a projection of Earth’s equator, while the North Celestial Pole is +90º and the South Celestial Pole is -90º.

Declination is the celestial counterpart to the latitude lines on Earth’s surface. The following video helps you understand this as well as knowing how to read right ascension and declination.

Practical use: How do you connect these two systems

How do you use these coordinates in a practical sense? If you have a GoTo telescope, like those I compared for viewing planets, you simply input the right ascension and declination of a target celestial body. Having the location and time, it’ll calculate the horizontal coordinates and point the telescope towards the target object.

With an altazimuth mount: If your sky map or planisphere has grids showing the RA and Dec lines, you would use them to identify an object above and then estimate your local settings of Xº above the horizon and Yº along the horizon to point your telescope. Tips: Use a compass to help with the azimuth of your viewing direction and your finderscope to pinpoint your object.

With an equatorial mount: Use the coordinates to direct your scope, providing it’s ‘polar aligned’, by adjusting the mechanical setting circles, which in theory show the right ascension and declination, to find and point the scope towards the target object.

Other astronomy coordinate systems

There are others. The third and fourth main systems are the ecliptic co-ordinate system and the galactic co-ordinate system. The third is useful in relation to the position of the Sun; the fourth is for specialized use in relation to the Milky Way. Plus there are lesser known systems, that I won’t cover here.

Bottom line

A coordinate system when finding planets or other objects in the sky provides a precise approach.

The horizon coordinate system is for local use. It is the practical one — the system you will use to know where to look in the sky. The equatorial coordinate system has global application, meaning no matter where you are in the world, it applies. It’s the one you’ll find expressed in astronomy books.