Navigating the night sky

Which stars do you recognise when you look up at the night sky, and how do you find them?

I’ve been planning to re-introduce shorter, lighter blog posts on random topics, and one topic that seems fit for that purpose is to describe how I personally find my way around the sky at night. This is not meant as advice on how other people should navigate the sky; it’s just a description of how I do it, and you are welcome to reciprocate in the comments. It is also only a summary and not a guide, so please look things up if you need more information. Bear in mind that I’m in the southern hemisphere.

When the sky is not yet dark and only a few stars are visible, I like to identify those stars as early as I can. This requires being familiar with the shapes formed by only the brightest stars, without relying on dimmer ones for context.

I start by looking for the triangle formed by Sirius, Rigel, and Canopus, which is back-to-back with the triangle formed by Sirius, Rigel and Procyon. All of these stars are among the brightest in the sky, and Sirius is the brightest of all, so once you’ve identified Sirius you know that anything brighter is not a star. The angles between these stars can look quite different depending on whether they’re overhead or near the horizon, but I can usually identify them quickly. (From Sirius, the line to Canopus is longer than the line to Procyon, so I find it helpful to remember that this is the opposite of what you’d expect given that pro- invokes words like prolonged.)

Once I’ve identified Rigel I can easily find Betelgeuse, and thus predict where the belt of Orion will appear even before it is visible (good party trick). If the sky is dark with little light pollution, I also look nearby for the Hyades (a.k.a. the horns of Taurus) and identify Aldebaran.

My usual next step is to follow the zigzag line from Procyon to Sirius to Canopus and then take one more zag to Achernar. Achernar is the only bright star anywhere near its vicinity, so you don’t need much precision to find it. The line of best fit through these four bright stars I have nicknamed the false ecliptic, because if you didn’t know your way around the night sky you might assume they were planets.

We’ll come back to Achernar in a moment, but let’s now turn our attention to the stars of the Southern Cross (featured on the flags of many Southern Hemisphere countries) and the adjacent Pointers. These are easy to find, and not hard to identify individually. The Pointers are Alpha and Beta Centauri (also known as Rigel Kentaurus and Hadar, but I prefer the former names) and the stars of the Southern Cross are Alpha, Beta, Gamma, Delta and Epsilon Crucis. Telling them apart is easy when you know that the sequence from brightest to dimmest proceeds clockwise, and that they are named in that order.

The classic way to find the South Celestial Pole is to draw a line through the long axis of the Southern Cross and another line passing between the Pointers at 90 degrees, and to find the point where those two imaginary lines meet. An alternative method — slightly more accurate but less often practical depending on what clouds and the horizon obsure — is to identify Beta Centauri (the lesser Pointer) and Achernar (which I promised we would return to), and find the point halfway between the two. It’s no bad thing to have two different methods for finding the Pole, because then you can verify one against the other and also use this as an alternative way to find Achernar.

In total, I can point to and name 14 stars in the night sky — Sirius, Rigel, Canopus, Procyon, Betelgeuse, Aldebaran, Achernar, plus the seven stars of the Pointers and Southern Cross. I’ve never made the effort to memorise any more. It’s perhaps worth noting that I live in a fairly light-polluted city, but spent most of my childhood on a farm with many more stars visible.

That’s about as much as I can say from a personal perspective; most other details are things you can look up. If you have a perspective you’d like to share, it is now your turn.

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